[Sitemap] [Contact] [Imprint] Deutsche Version Search site 

Atmospheric Chemistry and Physics - Current Research Articles



Current research articles: Atmospheric Chemistry

The author- or copyrights of the listed research articles below are held by the respective authors or site operators, who are also responsible for the content of the presentations.

More current articles from Chemistry Journals same topic: see the navigation menu on the left.

To list your article here please contact us by eMail.

To search this web page for specific words type "Ctrl" + "F" on your keyboard (Command + "F" on a Mac). Then: type the word you are searching for in the window that pops up!




On this page considered journals:



Atmospheric Chemistry and Physics - published by Copernicus Publications

ACP is an international scientific open access journal dedicated to the publication and public discussion of high quality studies investigating the Earth's atmosphere and the underlying chemical and physical processes. It covers the altitude range from the land and ocean surface up to the turbopause, including the troposphere, stratosphere and mesosphere.




Current articles of the journal:



A molecular-level approach for characterizing water-insoluble components of ambient organic aerosol particulates using ultrahigh-resolution mass spectrometry

A molecular-level approach for characterizing water-insoluble components of ambient organic aerosol particulates using ultrahigh-resolution mass spectrometry

Atmospheric Chemistry and Physics, 14, 10299-10314, 2014

Author(s): A. S. Willoughby, A. S. Wozniak, and P. G. Hatcher

The chemical composition of organic aerosols in the atmosphere is strongly influenced by human emissions. The effect these have on the environment, human health, and climate change is determined by the molecular nature of these chemical species. The complexity of organic aerosol samples limits the ability to study the chemical composition, and therefore the associated properties and the impacts they have. Many studies have addressed the water-soluble fraction of organic aerosols and have had much success in identifying specific molecular formulas for thousands of compounds present. However, little attention is given to the water-insoluble portion, which can contain most of the fossil material that is emitted through human activity. Here we compare the organic aerosols present in water extracts and organic solvent extracts (pyridine and acetonitrile) of an ambient aerosol sample collected in a rural location that is impacted by natural and anthropogenic emission sources. A semiquantitative method was developed using proton nuclear magnetic resonance spectroscopy to determine that the amount of organic matter extracted by pyridine is comparable to that of water. Electrospray ionization Fourier transform ion cyclotron resonance mass spectra show that pyridine extracts a molecularly unique fraction of organic matter compared to water or acetonitrile, which extract chemically similar organic matter components. The molecular formulas unique to pyridine were less polar, more aliphatic, and reveal formulas containing sulfur to be an important component of insoluble aerosol organic matter.

Posted on 30 September 2014 | 12:00 am


The balances of mixing ratios and segregation intensity: a case study from the field (ECHO 2003)

The balances of mixing ratios and segregation intensity: a case study from the field (ECHO 2003)

Atmospheric Chemistry and Physics, 14, 10333-10362, 2014

Author(s): R. Dlugi, M. Berger, M. Zelger, A. Hofzumahaus, F. Rohrer, F. Holland, K. Lu, and G. Kramm

An inhomogeneous mixing of reactants causes a reduction of their chemical removal compared to the homogeneously mixed case in turbulent atmospheric flows. This can be described by the intensity of segregation IS being the covariance of the mixing ratios of two species divided by the product of their means. Both terms appear in the balance equation of the mixing ratio and are discussed for the reaction between isoprene and OH for data of the field study ECHO 2003 above a deciduous forest. For most of these data, IS is negatively correlated with the fraction of mean OH mixing ratio reacting with isoprene. IS is also negatively correlated with the isoprene standard deviation. Both findings agree with model results discussed by Patton et al. (2001) and others. The correlation coefficient between OH and isoprene and, therefore, IS increases with increasing mean reaction rate. In addition, the balance equation of the covariance between isoprene and OH is applied as the theoretical framework for the analysis of the same field data. The storage term is small, and, therefore, a diagnostic equation for this covariance can be derived. The chemical reaction term Rij is dominated by the variance of isoprene times the quotient of mixing ratios of OH and isoprene. Based on these findings a new diagnostic equation for IS is formulated. Comparing different terms of this equation, IS and Rij show a relation also to the normalised isoprene standard deviation. It is shown that not only chemistry but also turbulent and convective mixing and advection – considered in a residual term – influence IS. Despite this finding, a detection of the influence of coherent eddy transport above the forest according to Katul et al. (1997) on IS fails, but a relation to the turbulent and advective transport of isoprene variance is determined. The largest values of IS are found for most unstable conditions with increasing buoyant production, confirming qualitatively model predictions by Ouwersloot et al. (2011).

Posted on 30 September 2014 | 12:00 am


Worldwide biogenic soil NOx emissions inferred from OMI NO2 observations

Worldwide biogenic soil NOx emissions inferred from OMI NO2 observations

Atmospheric Chemistry and Physics, 14, 10363-10381, 2014

Author(s): G. C. M. Vinken, K. F. Boersma, J. D. Maasakkers, M. Adon, and R. V. Martin

Biogenic NOx emissions from soils are a large natural source with substantial uncertainties in global bottom-up estimates (ranging from 4 to 15 Tg N yr?1). We reduce this range in emission estimates, and present a top-down soil NOx emission inventory for 2005 based on retrieved tropospheric NO2 columns from the Ozone Monitoring Instrument (OMI). We use a state-of-science soil NOx emission inventory (Hudman et al., 2012) as a priori in the GEOS-Chem chemistry transport model to identify 11 regions where tropospheric NO2 columns are dominated by soil NOx emissions. Strong correlations between soil NOx emissions and simulated NO2 columns indicate that spatial patterns in simulated NO2 columns in these regions indeed reflect the underlying soil NOx emissions. Subsequently, we use a mass-balance approach to constrain emissions for these 11 regions on all major continents using OMI observed and GEOS-Chem simulated tropospheric NO2 columns. We find that responses of simulated NO2 columns to changing NOx emissions are suppressed over low NOx regions, and account for these non-linearities in our inversion approach. In general, our approach suggests that emissions need to be increased in most regions. Our OMI top-down soil NOx inventory amounts to 10.0 Tg N for 2005 when only constraining the 11 regions, and 12.9 Tg N when extrapolating the constraints globally. Substantial regional differences exist (ranging from ?40% to +90%), and globally our top-down inventory is 4–35% higher than the GEOS-Chem a priori (9.6 Tg N yr?1). We evaluate NO2 concentrations simulated with our new OMI top-down inventory against surface NO2 measurements from monitoring stations in Africa, the USA and Europe. Although this comparison is complicated by several factors, we find an encouraging improved agreement when using the OMI top-down inventory compared to using the a priori inventory. To our knowledge, this study provides, for the first time, specific constraints on soil NOx emissions on all major continents using OMI NO2 columns. Our results rule out the low end of reported soil NOx emission estimates, and suggest that global emissions are most likely around 12.9 ± 3.9 Tg N yr?1.

Posted on 30 September 2014 | 12:00 am


Development of an aerosol microphysical module: Aerosol Two-dimensional bin module for foRmation and Aging Simulation (ATRAS)

Development of an aerosol microphysical module: Aerosol Two-dimensional bin module for foRmation and Aging Simulation (ATRAS)

Atmospheric Chemistry and Physics, 14, 10315-10331, 2014

Author(s): H. Matsui, M. Koike, Y. Kondo, J. D. Fast, and M. Takigawa

Number concentrations, size distributions, and mixing states of aerosols are essential parameters for accurate estimations of aerosol direct and indirect effects. In this study, we develop an aerosol module, designated the Aerosol Two-dimensional bin module for foRmation and Aging Simulation (ATRAS), that can explicitly represent these parameters by considering new particle formation (NPF), black carbon (BC) aging, and secondary organic aerosol (SOA) processes. A two-dimensional bin representation is used for particles with dry diameters from 40 nm to 10 ?m to resolve both aerosol sizes (12 bins) and BC mixing states (10 bins) for a total of 120 bins. The particles with diameters between 1 and 40 nm are resolved using additional eight size bins to calculate NPF. The ATRAS module is implemented in the WRF-Chem model and applied to examine the sensitivity of simulated mass, number, size distributions, and optical and radiative parameters of aerosols to NPF, BC aging, and SOA processes over East Asia during the spring of 2009. The BC absorption enhancement by coating materials is about 50% over East Asia during the spring, and the contribution of SOA processes to the absorption enhancement is estimated to be 10–20% over northern East Asia and 20–35% over southern East Asia. A clear north–south contrast is also found between the impacts of NPF and SOA processes on cloud condensation nuclei (CCN) concentrations: NPF increases CCN concentrations at higher supersaturations (smaller particles) over northern East Asia, whereas SOA increases CCN concentrations at lower supersaturations (larger particles) over southern East Asia. The application of ATRAS in East Asia also shows that the impact of each process on each optical and radiative parameter depends strongly on the process and the parameter in question. The module can be used in the future as a benchmark model to evaluate the accuracy of simpler aerosol models and examine interactions between NPF, BC aging, and SOA processes under different meteorological conditions and emissions.

Posted on 30 September 2014 | 12:00 am


Linking climate and air quality over Europe: effects of meteorology on PM2.5 concentrations

Linking climate and air quality over Europe: effects of meteorology on PM2.5 concentrations

Atmospheric Chemistry and Physics, 14, 10283-10298, 2014

Author(s): A. G. Megaritis, C. Fountoukis, P. E. Charalampidis, H. A. C. Denier van der Gon, C. Pilinis, and S. N. Pandis

The effects of various meteorological parameters such as temperature, wind speed, absolute humidity, precipitation and mixing height on PM2.5 concentrations over Europe were examined using a three-dimensional chemical transport model, PMCAMx-2008. Our simulations covered three periods, representative of different seasons (summer, winter, and fall). PM2.5 appears to be more sensitive to temperature changes compared to the other meteorological parameters in all seasons.

PM2.5 generally decreases as temperature increases, although the predicted changes vary significantly in space and time, ranging from ?700 ng m?3 K?1 (?8% K?1) to 300 ng m?3 K?1 (7% K?1). The predicted decreases of PM2.5 are mainly due to evaporation of ammonium nitrate, while the higher biogenic emissions and the accelerated gas-phase reaction rates increase the production of organic aerosol (OA) and sulfate, having the opposite effect on PM2.5. The predicted responses of PM2.5 to absolute humidity are also quite variable, ranging from ?130 ng m?3 %?1 (?1.6% %?1) to 160 ng m?3 %?1 (1.6% %?1) dominated mainly by changes in inorganic PM2.5 species. An increase in absolute humidity favors the partitioning of nitrate to the aerosol phase and increases the average PM2.5 during summer and fall. Decreases in sulfate and sea salt levels govern the average PM2.5 response to humidity during winter. A decrease of wind speed (keeping the emissions constant) increases all PM2.5 species (on average 40 ng m?3 %?1) due to changes in dispersion and dry deposition. The wind speed effects on sea salt emissions are significant for PM2.5 concentrations over water and in coastal areas. Increases in precipitation have a negative effect on PM2.5 (decreases up to 110 ng m?3 %?1) in all periods due to increases in wet deposition of PM2.5 species and their gas precursors. Changes in mixing height have the smallest effects (up to 35 ng m?3 %?1) on PM2.5 .

Regarding the relative importance of each of the meteorological parameters in a changed future climate, the projected changes in precipitation are expected to have the largest impact on PM2.5 levels during all periods (changes up to 2 ?g m?3 in the fall). The expected effects in future PM2.5 levels due to wind speed changes are similar in all seasons and quite close to those resulting from future precipitation changes (up to 1.4 ?g m?3). The expected increases in absolute humidity in the future can lead to large changes in PM2.5 levels (increases up to 2 ?g m?3) mainly in the fall due to changes in particulate nitrate levels. Despite the high sensitivity of PM2.5 levels to temperature, the small expected increases of temperature in the future will lead to modest PM2.5 changes and will not dominate the overall change.

Posted on 29 September 2014 | 12:00 am


Size-resolved cloud condensation nuclei (CCN) activity and closure analysis at the HKUST Supersite in Hong Kong

Size-resolved cloud condensation nuclei (CCN) activity and closure analysis at the HKUST Supersite in Hong Kong

Atmospheric Chemistry and Physics, 14, 10267-10282, 2014

Author(s): J. W. Meng, M. C. Yeung, Y. J. Li, B. Y. L. Lee, and C. K. Chan

The cloud condensation nuclei (CCN) properties of atmospheric aerosols were measured on 1–30 May 2011 at the HKUST (Hong Kong University of Science and Technology) Supersite, a coastal site in Hong Kong. Size-resolved CCN activation curves, the ratio of number concentration of CCN (NCCN) to aerosol concentration (NCN) as a function of particle size, were obtained at supersaturation (SS) = 0.15, 0.35, 0.50, and 0.70% using a DMT (Droplet Measurement Technologies) CCN counter (CCNc) and a TSI scanning mobility particle sizer (SMPS). The mean bulk size-integrated NCCN ranged from ~500 cm?3 at SS = 0.15% to ~2100 cm?3 at SS = 0.70%, and the mean bulk NCCN / NCN ratio ranged from 0.16 at SS = 0.15% to 0.65 at SS = 0.70%. The average critical mobility diameters (D50) at SS = 0.15, 0.35, 0.50, and 0.70% were 116, 67, 56, and 46 nm, respectively. The corresponding average hygroscopic parameters (κCCN) were 0.39, 0.36, 0.31, and 0.28. The decrease in κCCN can be attributed to the increase in organic to inorganic volume ratio as particle size decreases, as measured by an Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The κCCN correlates reasonably well with κAMS_SR based on size-resolved AMS measurements: κAMS_SR = κorg × forg + κinorg × finorg, where forg and finorg are the organic and inorganic volume fractions, respectively, κorg = 0.1 and κinorg = 0.6, with a R2 of 0.51.

In closure analysis, NCCN was estimated by integrating the measured size-resolved NCN for particles larger than D50 derived from ? assuming internal mixing state. Estimates using κAMS_SR show that the measured and predicted NCCN were generally within 10% of each other at all four SS. The deviation increased to 26% when κAMS was calculated from bulk PM1 AMS measurements of particles because PM1 was dominated by particles of 200 to 500 nm in diameter, which had a larger inorganic fraction than those of D50 (particle diameter < 200 nm). A constant ? = 0.33 (the average value of κAMS_SR over the course of campaign) was found to give an NCCN prediction within 12% of the actual measured values. We also compared NCCN estimates based on the measured average D50 and the average size-resolved CCN activation ratio to examine the relative importance of hygroscopicity and mixing state. NCCN appears to be relatively more sensitive to the mixing state and hygroscopicity at a high SS = 0.70% and a low SS = 0.15%, respectively.

Posted on 26 September 2014 | 12:00 am


Submicron aerosols at thirteen diversified sites in China: size distribution, new particle formation and corresponding contribution to cloud condensation nuclei production

Submicron aerosols at thirteen diversified sites in China: size distribution, new particle formation and corresponding contribution to cloud condensation nuclei production

Atmospheric Chemistry and Physics, 14, 10249-10265, 2014

Author(s): J. F. Peng, M. Hu, Z. B. Wang, X. F. Huang, P. Kumar, Z. J. Wu, S. Guo, D. L. Yue, D. J. Shang, Z. Zheng, and L. Y. He

Understanding the particle number size distributions in diversified atmospheric environments is important in order to design mitigation strategies related to submicron particles and their effects on regional air quality, haze and human health. In this study, we conducted 15 different field measurement campaigns between 2007 and 2011 at 13 individual sites in China, including five urban sites, four regional sites, three coastal/background sites and one ship cruise measurement along eastern coastline of China. Size resolved particles were measured in the 15–600 nm size range. The median particle number concentrations (PNCs) were found to vary in the range of 1.1?2.2 × 104 cm?3 at urban sites, 0.8?1.5 × 104 cm?3 at regional sites, 0.4?0.6 × 104 cm?3 at coastal/background sites, and 0.5 × 104 cm?3 during cruise measurement. Peak diameters at each of these sites varied greatly from 24 to 115 nm. Particles in the 15–25 nm (nucleation mode), 25–100 nm (Aitken mode) and 100–600 nm (accumulation mode) range showed different characteristics at each sites, indicating the features of primary emissions and secondary formation in these diversified atmospheric environments. Diurnal variations show a build-up of accumulation mode particles belt at regional sites, suggesting the contribution of regional secondary aerosol pollution. Frequencies of new particle formation (NPF) events were much higher at urban and regional sites than at coastal sites and during cruise measurement. The average growth rates (GRs) of nucleation mode particles were 8.0–10.9 nm h?1 at urban sites, 7.4–13.6 nm h?1 at regional sites and 2.8–7.5 nm h?1 at coastal sites and during cruise measurement. The high gaseous precursors and strong oxidation at urban and regional sites not only favored the formation of particles, but also accelerated the growth rate of the nucleation mode particles. No significant difference in condensation sink (CS) during NPF days were observed among different site types, suggesting that the NPF events in background areas were more influenced by the pollutant transport. In addition, average contributions of NPF events to potential cloud condensation nuclei (CCN) at 0.2% super-saturation in the afternoon of all sampling days were calculated as 11% and 6% at urban sites and regional sites, respectively. On the other hand, NPF events at coastal sites and during cruise measurement had little impact on potential production of CCN. This study provides a large data set of particle size distribution in diversified atmosphere of China, improving our general understanding of emission, secondary formation, new particle formation and corresponding CCN activity of submicron aerosols in Chinese environments.

Posted on 25 September 2014 | 12:00 am


Modeling analysis of the seasonal characteristics of haze formation in Beijing

Modeling analysis of the seasonal characteristics of haze formation in Beijing

Atmospheric Chemistry and Physics, 14, 10231-10248, 2014

Author(s): X. Han, M. Zhang, J. Gao, S. Wang, and F. Chai

The air quality modeling system RAMS-CMAQ (Regional Atmospheric Modeling System–Community Multiscale Air Quality), coupled with an aerosol optical property scheme, was applied to simulate the meteorological field, major aerosol components (sulfate, nitrate, ammonium, black carbon, organic carbon, dust, and sea salt), and surface visibility over the North China Plain (NCP) in 2011. The modeled results in February and July 2011 were selected and analyzed to obtain an in-depth understanding of the haze formation mechanism in Beijing for different seasons. The simulation results showed that the visibility was below 10 km for most regions of the NCP, and dropped to less than 5 km over the megacities of Beijing and Tianjin, the whole of Hebei Province, and the northwest part of Shandong Province during pollution episodes in February and July. The heavy mass concentration of PM2.5 ranged from 120 to 300 ?g m?3 and was concentrated in the areas with low visibility. The haze formation mechanism in Beijing in winter was different from that in summer. The mass concentration of PM2.5 was higher, and the components more complicated, in winter. While the mass concentration of PM2.5 in summer was lower than that in winter, the mass concentrations of hygroscopic inorganic salts were comparable with those in winter, and the relative humidity was, as expected, higher. Therefore, the water uptake of hygroscopic aerosols played a key role in summer. Moreover, the analysis showed that the influence of the PM2.5 mass burden on visibility was very weak when its value was larger than 100 ?g m?3. Only when the mass burden of PM2.5 decreased to a certain threshold interval did the visibility increase rapidly. This indicates that, when emission reduction measures are taken to control haze occurrence, the mass burden of PM2.5 must be cut to below this threshold interval. The relationship between the threshold of haze occurrence and the relative humidity in Beijing was fitted by an exponential function, and the resulting fitting curves could provide a new theoretical basis to understand and control haze formation in Beijing.

Posted on 25 September 2014 | 12:00 am


Horizontal distributions of aerosol constituents and their mixing states in Antarctica during the JASE traverse

Horizontal distributions of aerosol constituents and their mixing states in Antarctica during the JASE traverse

Atmospheric Chemistry and Physics, 14, 10211-10230, 2014

Author(s): K. Hara, F. Nakazawa, S. Fujita, K. Fukui, H. Enomoto, and S. Sugiyama

Measurements of aerosol number concentrations and direct aerosol sampling were conducted on continental Antarctica during the traverse of the Japanese–Swedish joint Antarctic expedition (JASE) from 14 November 2007 until 24 January 2008. Aerosol concentrations in background conditions decreased gradually with latitude in inland regions during the traverse. The lowest aerosol number concentrations were 160 L?1 in Dp > 0.3 ?m, and 0.5 L?1 in Dp 2 ?m. In contrast, aerosol concentrations reached 3278 L?1 in Dp > 0.3 ?m, and 215 L?1 in Dp > 2 ?m under strong wind conditions. The estimated aerosol mass concentrations were 0.04–5.7 ?g m?3. Single particle analysis of aerosol particles collected during the JASE traverse was conducted using a scanning electron microscope equipped with an energy dispersive x ray spectrometer. Major aerosol constituents were sulfates in fine mode, and sulfate, sea salts, modified sea salts, and fractionated sea salts in coarse mode. K-rich sulfates, Mg-rich sulfate, Ca-rich sulfates, and minerals were identified as minor aerosol constituents. Horizontal features of Cl / Na ratios imply that sea-salt modification (i.e. Cl loss) occurred on the Antarctic continent during the summer. Most sea-salt particles in the continental region near the coast were modified with acidic sulfur species such as H2SO4 and CH3SO3H. By contrast, acidic species other than the acidic sulfur species (likely HNO3) contributed markedly to sea-salt modification in inland areas during the traverse. Mg-rich sea-salt particles and Mg-free sea-salt particles were present in coarse and fine modes from the coast to inland areas. These sea-salt particles might be associated with sea-salt fractionation on the snow surface of continental Antarctica.

Posted on 25 September 2014 | 12:00 am


On the impact of the temporal variability of the collisional quenching process on the mesospheric OH emission layer: a study based on SD-WACCM4 and SABER

On the impact of the temporal variability of the collisional quenching process on the mesospheric OH emission layer: a study based on SD-WACCM4 and SABER

Atmospheric Chemistry and Physics, 14, 10193-10210, 2014

Author(s): S. Kowalewski, C. von Savigny, M. Palm, I. C. McDade, and J. Notholt

The mesospheric OH Meinel emissions are subject of many theoretical and observational studies devoted to this part of the atmosphere. Depending on the initial vibrational level of excitation the altitude of the considered OH Meinel emission is systematically shifted, which has important implications for the intercomparison of different studies considering different transition bands. Previous model studies suggest that these vertical shifts are essentially caused by the process of collisional quenching with atomic oxygen. Following this hypothesis, a recent study found experimental evidence of a coherent seasonality at tropical latitudes between vertical shifts of different OH Meinel bands and changes in atomic oxygen concentrations. Despite the consistent finding of the above mentioned hypothesis, it cannot be excluded that the actual temporal variability of the vertical shifts between different OH Meinel bands may in addition be controlled or even dominated by other processes. It remains an open question whether the observed temporal evolution is indeed mainly controlled by the modulation of the collisional quenching process with atomic oxygen. By means of a sensitivity study which employs a quenching model to simulations made with the SD-WACCM4 chemistry climate model, we aim at assessing this question. From this study we find that the observed seasonality of vertical OH Meinel shifts is only partially controlled by temporal changes in atomic oxygen concentrations, while molecular oxygen has another noticeable impact on the vertical OH Meinel shifts. This in particular becomes evident for the diurnal variability of vertical OH Meinel shifts, which reveal only a poor correlation with the atomic oxygen species. Furthermore, changes in the H + O3 source gases provide another mechanism that can potentially affect the diurnal variability in addition. By comparison with limb radiance observations from the SABER/TIMED satellite this provides an explanation for the less evident diurnal response between changes in O concentrations and vertical OH Meinel shifts. On the other hand, at seasonal timescales the coherency between both quantities is again evident in SABER/TIMED but less pronounced compared to our model simulations.

Posted on 24 September 2014 | 12:00 am


Spatial distributions and seasonal cycles of aerosol climate effects in India seen in a global climate–aerosol model

Spatial distributions and seasonal cycles of aerosol climate effects in India seen in a global climate–aerosol model

Atmospheric Chemistry and Physics, 14, 10177-10192, 2014

Author(s): S. V. Henriksson, J.-P. Pietikäinen, A.-P. Hyvärinen, P. Räisänen, K. Kupiainen, J. Tonttila, R. Hooda, H. Lihavainen, D. O'Donnell, L. Backman, Z. Klimont, and A. Laaksonen

Climate–aerosol interactions in India are studied by employing the global climate–aerosol model ECHAM5-HAM and the GAINS inventory for anthropogenic aerosol emissions. Model validation is done for black carbon surface concentrations in Mukteshwar and for features of the monsoon circulation. Seasonal cycles and spatial distributions of radiative forcing and the temperature and rainfall responses are presented for different model setups. While total aerosol radiative forcing is strongest in the summer, anthropogenic forcing is considerably stronger in winter than in summer. Local seasonal temperature anomalies caused by aerosols are mostly negative with some exceptions, e.g., parts of northern India in March–May. Rainfall increases due to the elevated heat pump (EHP) mechanism and decreases due to solar dimming mechanisms (SDMs) and the relative strengths of these effects during different seasons and for different model setups are studied. Aerosol light absorption does increase rainfall in northern India, but effects due to solar dimming and circulation work to cancel the increase. The total aerosol effect on rainfall is negative for northern India in the months of June–August, but during March–May the effect is positive for most model setups. These differences between responses in different seasons might help converge the ongoing debate on the EHPs and SDMs. Due to the complexity of the problem and known or potential sources for error and bias, the results should be interpreted cautiously as they are completely dependent on how realistic the model is. Aerosol–rainfall correlations and anticorrelations are shown not to be a reliable sole argument for deducing causality.

Posted on 24 September 2014 | 12:00 am


Intercontinental transport and deposition patterns of atmospheric mercury from anthropogenic emissions

Intercontinental transport and deposition patterns of atmospheric mercury from anthropogenic emissions

Atmospheric Chemistry and Physics, 14, 10163-10176, 2014

Author(s): L. Chen, H. H. Wang, J. F. Liu, Y. D. Tong, L. B. Ou, W. Zhang, D. Hu, C. Chen, and X. J. Wang

Global policies that regulate anthropogenic mercury emissions to the environment require quantitative and comprehensive source–receptor relationships for mercury emissions, transport and deposition among major continental regions. In this study, we use the GEOS-Chem global chemical transport model to establish source–receptor relationships among 11 major continental regions worldwide. Source–receptor relationships for surface mercury concentrations (SMC) show that some regions (e.g., East Asia, the Indian subcontinent, and Europe) should be responsible for their local surface Hg(II) and Hg(P) concentrations due to near-field transport and deposition contributions from their local anthropogenic emissions (up to 64 and 71% for Hg(II) and Hg(P), respectively, over East Asia). We define the region of primary influence (RPI) and the region of secondary influence (RSI) to establish intercontinental influence patterns. Results indicate that East Asia is the SMC RPI for almost all other regions, while Europe, Russia, and the Indian subcontinent also make some contributions to SMC over some receptor regions because they are dominant RSI source regions. Source–receptor relationships for mercury deposition show that approximately 16 and 17% of dry and wet deposition, respectively, over North America originate from East Asia, indicating that transpacific transport of East Asian emissions is the major foreign source of mercury deposition in North America. Europe, Southeast Asia, and the Indian subcontinent are also important mercury deposition sources for some receptor regions because they are the dominant RSIs. We also quantify seasonal variation on mercury deposition contributions over other regions from East Asia. Results show that mercury deposition (including dry and wet) contributions from East Asia over the Northern Hemisphere receptor regions (e.g., North America, Europe, Russia, the Middle East, and Middle Asia) vary seasonally, with the maximum values in summer and minimum values in winter. The opposite seasonal pattern occurs on mercury dry deposition contributions over Southeast Asia and the Indian subcontinent.

Posted on 24 September 2014 | 12:00 am


Chemical mass balance of 300 °C non-volatile particles at the tropospheric research site Melpitz, Germany

Chemical mass balance of 300 °C non-volatile particles at the tropospheric research site Melpitz, Germany

Atmospheric Chemistry and Physics, 14, 10145-10162, 2014

Author(s): L. Poulain, W. Birmili, F. Canonaco, M. Crippa, Z. J. Wu, S. Nordmann, G. Spindler, A. S. H. Prévôt, A. Wiedensohler, and H. Herrmann

In the fine-particle mode (aerodynamic diameter < 1 ?m) non-volatile material has been associated with black carbon (BC) and low-volatile organics and, to a lesser extent, with sea salt and mineral dust. This work analyzes non-volatile particles at the tropospheric research station Melpitz (Germany), combining experimental methods such as a mobility particle-size spectrometer (3–800 nm), a thermodenuder operating at 300 °C, a multi-angle absorption photometer (MAAP), and an aerosol mass spectrometer (AMS). The data were collected during two atmospheric field experiments in May–June 2008 as well as February–March 2009. As a basic result, we detected average non-volatile particle–volume fractions of 11 ± 3% (2008) and 17 ± 8% (2009). In both periods, BC was in close linear correlation with the non-volatile fraction, but not sufficient to quantitatively explain the non-volatile particle mass concentration. Based on the assumption that BC is not altered by the heating process, the non-volatile particle mass fraction could be explained by the sum of black carbon (47% in summer, 59% in winter) and a non-volatile organic contribution estimated as part of the low-volatility oxygenated organic aerosol (LV-OOA) (53% in summer, 41% in winter); the latter was identified from AMS data by factor analysis. Our results suggest that LV-OOA was more volatile in summer (May–June 2008) than in winter (February–March 2009) which was linked to a difference in oxidation levels (lower in summer). Although carbonaceous compounds dominated the sub-?m non-volatile particle mass fraction most of the time, a cross-sensitivity to partially volatile aerosol particles of maritime origin could be seen. These marine particles could be distinguished, however from the carbonaceous particles by a characteristic particle volume–size distribution. The paper discusses the uncertainty of the volatility measurements and outlines the possible merits of volatility analysis as part of continuous atmospheric aerosol measurements.

Posted on 23 September 2014 | 12:00 am


Carbon balance of China constrained by CONTRAIL aircraft CO2 measurements

Carbon balance of China constrained by CONTRAIL aircraft CO2 measurements

Atmospheric Chemistry and Physics, 14, 10133-10144, 2014

Author(s): F. Jiang, H. M. Wang, J. M. Chen, T. Machida, L. X. Zhou, W. M. Ju, H. Matsueda, and Y. Sawa

Terrestrial carbon dioxide (CO2) flux estimates in China using atmospheric inversion method are beset with considerable uncertainties because very few atmospheric CO2 concentration measurements are available. In order to improve these estimates, nested atmospheric CO2 inversion during 2002–2008 is performed in this study using passenger aircraft-based CO2 measurements over Eurasia from the Comprehensive Observation Network for Trace gases by Airliner (CONTRAIL) project. The inversion system includes 43 regions with a focus on China, and is based on the Bayesian synthesis approach and the TM5 transport model. The terrestrial ecosystem carbon flux modeled by the Boreal Ecosystems Productivity Simulator (BEPS) model and the ocean exchange simulated by the OPA-PISCES-T model are considered as the prior fluxes. The impacts of CONTRAIL CO2 data on inverted China terrestrial carbon fluxes are quantified, the improvement of the inverted fluxes after adding CONTRAIL CO2 data are rationed against climate factors and evaluated by comparing the simulated atmospheric CO2 concentrations with three independent surface CO2 measurements in China. Results show that with the addition of CONTRAIL CO2 data, the inverted carbon sink in China increases while those in South and Southeast Asia decrease. Meanwhile, the posterior uncertainties over these regions are all reduced (2–12%). CONTRAIL CO2 data also have a large effect on the inter-annual variation of carbon sinks in China, leading to a better correlation between the carbon sink and the annual mean climate factors. Evaluations against the CO2 measurements at three sites in China also show that the CONTRAIL CO2 measurements may have improved the inversion results.

Posted on 23 September 2014 | 12:00 am


Summertime tropospheric ozone assessment over the Mediterranean region using the thermal infrared IASI/MetOp sounder and the WRF-Chem model

Summertime tropospheric ozone assessment over the Mediterranean region using the thermal infrared IASI/MetOp sounder and the WRF-Chem model

Atmospheric Chemistry and Physics, 14, 10119-10131, 2014

Author(s): S. Safieddine, A. Boynard, P.-F. Coheur, D. Hurtmans, G. Pfister, B. Quennehen, J. L. Thomas, J.-C. Raut, K. S. Law, Z. Klimont, J. Hadji-Lazaro, M. George, and C. Clerbaux

Over the Mediterranean region, elevated tropospheric ozone (O3) values are recorded, especially in summer. We use the thermal Infrared Atmospheric Sounding Interferometer (IASI) and the Weather Research and Forecasting Model with Chemistry (WRF-Chem) to understand and interpret the factors and emission sources responsible for the high O3 concentrations observed in the Mediterranean troposphere. Six years (2008–2013) of IASI data have been analyzed and results show consistent maxima during summer, with an increase of up to 22% in the [0–8] km O3 column in the eastern part of the basin compared to the middle of the basin. We focus on summer 2010 to investigate the processes that contribute to these summer maxima. Using two modeled O3 tracers (inflow to the model domain and local anthropogenic emissions), we show that, between the surface and 2 km, O3 is mostly formed from anthropogenic emissions, while above 4 km it is mostly transported from outside the domain or from stratospheric origins. Evidence of stratosphere-to-troposphere exchange (STE) events in the eastern part of the basin is shown, and corresponds to a low water vapor mixing ratio and high potential vorticity.

Posted on 23 September 2014 | 12:00 am


Comparison of ice cloud properties simulated by the Community Atmosphere Model (CAM5) with in-situ observations

Comparison of ice cloud properties simulated by the Community Atmosphere Model (CAM5) with in-situ observations

Atmospheric Chemistry and Physics, 14, 10103-10118, 2014

Author(s): T. Eidhammer, H. Morrison, A. Bansemer, A. Gettelman, and A. J. Heymsfield

Detailed measurements of ice crystals in cirrus clouds were used to compare with results from the Community Atmospheric Model Version 5 (CAM5) global climate model. The observations are from two different field campaigns with contrasting conditions: Atmospheric Radiation Measurements Spring Cloud Intensive Operational Period in 2000 (ARM-IOP), which was characterized primarily by midlatitude frontal clouds and cirrus, and Tropical Composition, Cloud and Climate Coupling (TC4), which was dominated by anvil cirrus. Results show that the model typically overestimates the slope parameter of the exponential size distributions of cloud ice and snow, while the variation with temperature (height) is comparable. The model also overestimates the ice/snow number concentration (0th moment of the size distribution) and underestimates higher moments (2nd through 5th), but compares well with observations for the 1st moment. Overall the model shows better agreement with observations for TC4 than for ARM-IOP in regards to the moments. The mass-weighted terminal fall speed is lower in the model compared to observations for both ARM-IOP and TC4, which is partly due to the overestimation of the size distribution slope parameter. Sensitivity tests with modification of the threshold size for cloud ice to snow autoconversion (Dcs) do not show noticeable improvement in modeled moments, slope parameter and mass weighed fall speed compared to observations. Further, there is considerable sensitivity of the cloud radiative forcing to Dcs, consistent with previous studies, but no value of Dcs improves modeled cloud radiative forcing compared to measurements. Since the autoconversion of cloud ice to snow using the threshold size Dcs has little physical basis, future improvement to combine cloud ice and snow into a single category, eliminating the need for autoconversion, is suggested.

Posted on 23 September 2014 | 12:00 am


Modeling regional aerosol and aerosol precursor variability over California and its sensitivity to emissions and long-range transport during the 2010 CalNex and CARES campaigns

Modeling regional aerosol and aerosol precursor variability over California and its sensitivity to emissions and long-range transport during the 2010 CalNex and CARES campaigns

Atmospheric Chemistry and Physics, 14, 10013-10060, 2014

Author(s): J. D. Fast, J. Allan, R. Bahreini, J. Craven, L. Emmons, R. Ferrare, P. L. Hayes, A. Hodzic, J. Holloway, C. Hostetler, J. L. Jimenez, H. Jonsson, S. Liu, Y. Liu, A. Metcalf, A. Middlebrook, J. Nowak, M. Pekour, A. Perring, L. Russell, A. Sedlacek, J. Seinfeld, A. Setyan, J. Shilling, M. Shrivastava, S. Springston, C. Song, R. Subramanian, J. W. Taylor, V. Vinoj, Q. Yang, R. A. Zaveri, and Q. Zhang

The performance of the Weather Research and Forecasting regional model with chemistry (WRF-Chem) in simulating the spatial and temporal variations in aerosol mass, composition, and size over California is quantified using the extensive meteorological, trace gas, and aerosol measurements collected during the California Nexus of Air Quality and Climate Experiment (CalNex) and the Carbonaceous Aerosol and Radiative Effects Study (CARES) conducted during May and June of 2010. The overall objective of the field campaigns was to obtain data needed to better understand processes that affect both climate and air quality, including emission assessments, transport and chemical aging of aerosols, aerosol radiative effects. Simulations were performed that examined the sensitivity of aerosol concentrations to anthropogenic emissions and to long-range transport of aerosols into the domain obtained from a global model. The configuration of WRF-Chem used in this study is shown to reproduce the overall synoptic conditions, thermally driven circulations, and boundary layer structure observed in region that controls the transport and mixing of trace gases and aerosols. Reducing the default emissions inventory by 50% led to an overall improvement in many simulated trace gases and black carbon aerosol at most sites and along most aircraft flight paths; however, simulated organic aerosol was closer to observed when there were no adjustments to the primary organic aerosol emissions. We found that sulfate was better simulated over northern California whereas nitrate was better simulated over southern California. While the overall spatial and temporal variability of aerosols and their precursors were simulated reasonably well, we show cases where the local transport of some aerosol plumes were either too slow or too fast, which adversely affects the statistics quantifying the differences between observed and simulated quantities. Comparisons with lidar and in situ measurements indicate that long-range transport of aerosols from the global model was likely too high in the free troposphere even though their concentrations were relatively low. This bias led to an over-prediction in aerosol optical depth by as much as a factor of 2 that offset the under-predictions of boundary-layer extinction resulting primarily from local emissions. Lowering the boundary conditions of aerosol concentrations by 50% greatly reduced the bias in simulated aerosol optical depth for all regions of California. This study shows that quantifying regional-scale variations in aerosol radiative forcing and determining the relative role of emissions from local and distant sources is challenging during `clean' conditions and that a wide array of measurements are needed to ensure model predictions are correct for the right reasons. In this regard, the combined CalNex and CARES data sets are an ideal test bed that can be used to evaluate aerosol models in great detail and develop improved treatments for aerosol processes.

Posted on 22 September 2014 | 12:00 am


Concentrations and fluxes of isoprene and oxygenated VOCs at a French Mediterranean oak forest

Concentrations and fluxes of isoprene and oxygenated VOCs at a French Mediterranean oak forest

Atmospheric Chemistry and Physics, 14, 10085-10102, 2014

Author(s): C. Kalogridis, V. Gros, R. Sarda-Esteve, B. Langford, B. Loubet, B. Bonsang, N. Bonnaire, E. Nemitz, A.-C. Genard, C. Boissard, C. Fernandez, E. Ormeño, D. Baisnée, I. Reiter, and J. Lathière

The CANOPEE project aims to better understand the biosphere–atmosphere exchanges of biogenic volatile organic compounds (BVOCs) in the case of Mediterranean ecosystems and the impact of in-canopy processes on the atmospheric chemical composition above the canopy. Based on an intensive field campaign, the objective of our work was to determine the chemical composition of the air inside a canopy as well as the net fluxes of reactive species between the canopy and the boundary layer. Measurements were carried out during spring 2012 at the field site of the Oak Observatory of the Observatoire de Haute Provence (O3HP) located in the southeast of France. The site is a forest ecosystem dominated by downy oak, Quercus pubescens Willd., a typical Mediterranean species which features large isoprene emission rates. Mixing ratios of isoprene, its degradation products methylvinylketone (MVK) and methacrolein (MACR) and several other oxygenated VOC (OxVOC) were measured above the canopy using an online proton transfer reaction mass spectrometer (PTR-MS), and fluxes were calculated by the disjunct eddy covariance approach. The O3HP site was found to be a very significant source of isoprene emissions, with daily maximum ambient concentrations ranging between 2–16 ppbv inside and 2–5 ppbv just above the top of the forest canopy. Significant isoprene fluxes were observed only during daytime, following diurnal cycles with midday net emission fluxes from the canopy ranging between 2.0 and 9.7 mg m?2 h1. Net isoprene normalized flux (at 30 °C, 1000 ?mol quanta m?2 s−1) was estimated at 7.4 mg m?2 h?1. Evidence of direct emission of methanol was also found exhibiting maximum daytime fluxes ranging between 0.2 and 0.6 mg m?2 h?1, whereas flux values for monoterpenes and others OxVOC such as acetone and acetaldehyde were below the detection limit.

The MVK+MACR-to-isoprene ratio provided useful information on the oxidation of isoprene, and is in agreement with recent findings proposing weak production yields of MVK and MACR, in remote forest regions where the NOx concentrations are low. In-canopy chemical oxidation of isoprene was found to be weak and did not seem to have a significant impact on isoprene concentrations and fluxes above the canopy.

Posted on 22 September 2014 | 12:00 am


Size distribution, mixing state and source apportionment of black carbon aerosol in London during wintertime

Size distribution, mixing state and source apportionment of black carbon aerosol in London during wintertime

Atmospheric Chemistry and Physics, 14, 10061-10084, 2014

Author(s): D. Liu, J. D. Allan, D. E. Young, H. Coe, D. Beddows, Z. L. Fleming, M. J. Flynn, M. W. Gallagher, R. M. Harrison, J. Lee, A. S. H. Prevot, J. W. Taylor, J. Yin, P. I. Williams, and P. Zotter

Black carbon aerosols (BC) at a London urban site were characterised in both winter- and summertime 2012 during the Clean Air for London (ClearfLo) project. Positive matrix factorisation (PMF) factors of organic aerosol mass spectra measured by a high-resolution aerosol mass spectrometer (HR-AMS) showed traffic-dominant sources in summer but in winter the influence of additional non-traffic sources became more important, mainly from solid fuel sources (SF). Measurements using a single particle soot photometer (SP2, DMT), showed the traffic-dominant BC exhibited an almost uniform BC core size (Dc) distribution with very thin coating thickness throughout the detectable range of Dc. However, the size distribution of sf (project average mass median Dc = 149 ± 22 nm in winter, and 120 ± 6 nm in summer) and BC coating thickness varied significantly in winter. A novel methodology was developed to attribute the BC number concentrations and mass abundances from traffic (BCtr) and from SF (BCsf), by using a 2-D histogram of the particle optical properties as a function of BC core size, as measured by the SP2. The BCtr and BCsf showed distinctly different sf distributions and coating thicknesses, with BCsf displaying larger Dc and larger coating thickness compared to BCtr. BC particles from different sources were also apportioned by applying a multiple linear regression between the total BC mass and each AMS-PMF factor (BC–AMS–PMF method), and also attributed by applying the absorption spectral dependence of carbonaceous aerosols to 7-wavelength Aethalometer measurements (Aethalometer method).

Air masses that originated from westerly (W), southeasterly (SE), and easterly (E) sectors showed BCsf fractions that ranged from low to high, and whose mass median Dc values were 137 ± 10 nm, 143 ± 11 nm and 169 ± 29 nm, respectively. The corresponding bulk relative coating thickness of BC (coated particle size/BC core – Dp/Dc) for these same sectors was 1.28 ± 0.07, 1.45 ± 0.16 and 1.65 ± 0.19. For W, SE and E air masses, the number fraction of BCsf ranged from 6 ± 2% to 11 ± 5% to 18 ± 10%, respectively, but importantly the larger BC core sizes lead to an increased fraction of BCsf in terms of mass than number (for W, SE and E air masses, the BCsf mass fractions ranged from 16 ± 6%, 24 ± 10% and 39 ± 14%, respectively). An increased fraction of non-BC particles (particles that did not contain a BC core) was also observed when SF sources were more significant. The BC mass attribution by the SP2 method agreed well with the BC–AMS–PMF multiple linear regression method (BC–AMS–PMF : SP2 ratio = 1.05, r2 = 0.80) over the entire experimental period. Good agreement was found between BCsf attributed with the Aethalometer model and the SP2. However, the assumed absorption Ångström exponent (αwb) had to be changed according to the different air mass sectors to yield the best comparison with the SP2. This could be due to influences of fuel type or burn phase.

Posted on 22 September 2014 | 12:00 am


Experimental determination of the temperature dependence of water activities for a selection of aqueous organic solutions

Experimental determination of the temperature dependence of water activities for a selection of aqueous organic solutions

Atmospheric Chemistry and Physics, 14, 9993-10012, 2014

Author(s): G. Ganbavale, C. Marcolli, U. K. Krieger, A. Zuend, G. Stratmann, and T. Peter

This work presents experimental data of the temperature dependence of water activity in aqueous organic solutions relevant for tropospheric conditions (200–273 K). Water activity (aw) at low temperatures (T) is a crucial parameter for predicting homogeneous ice nucleation. We investigated temperature-dependent water activities, ice freezing and melting temperatures of solutions, and vapour pressures of a selection of atmospherically relevant aqueous organic systems. To measure aw over a wide composition range and with a focus on low temperatures, we use various aw measurement techniques and instruments: a dew point water activity meter, an electrodynamic balance (EDB), differential scanning calorimetry (DSC), and a setup to measure the total gas phase pressure at equilibrium over aqueous solutions. Water activity measurements were performed for aqueous multicomponent and multifunctional organic mixtures containing the functional groups typically found in atmospheric organic aerosols, such as hydroxyl, carboxyl, ketone, ether, ester, and aromatic groups. The aqueous organic systems studied at several fixed compositions over a considerable temperature range differ significantly in their temperature dependence. Aqueous organic systems of 1,4-butanediol and methoxyacetic acid show a moderate decrease in aw with decreasing temperature. The aqueous M5 system (a multicomponent system containing five different dicarboxylic acids) and aqueous 2-(2-ethoxyethoxy)ethanol solutions both show a strong increase of water activity with decreasing temperature at high solute concentrations for T < 270 K and T < 260 K, respectively. These measurements show that the temperature trend of aw can be reversed at low temperatures and that linear extrapolations of high-temperature data may lead to erroneous predictions. To avoid this, experimentally determined aw at low temperature are needed to improve thermodynamic models towards lower temperatures and for improved predictions of the ice nucleation ability of organic–water systems.

Posted on 19 September 2014 | 12:00 am


Source apportionment and seasonal variation of PM2.5 in a Sub-Saharan African city: Nairobi, Kenya

Source apportionment and seasonal variation of PM2.5 in a Sub-Saharan African city: Nairobi, Kenya

Atmospheric Chemistry and Physics, 14, 9977-9991, 2014

Author(s): S. M. Gaita, J. Boman, M. J. Gatari, J. B. C. Pettersson, and S. Janhäll

Sources of airborne particulate matter and their seasonal variation in urban areas in Sub-Saharan Africa are poorly understood due to lack of long-term measurement data. In view of this, filter samples of airborne particulate matter (particle diameter ≤2.5 ?m, PM2.5) were collected between May 2008 and April 2010 at two sites (urban background site and suburban site) within the Nairobi metropolitan area. A total of 780 samples were collected and analyzed for particulate mass, black carbon (BC) and 13 trace elements. The average PM2.5 concentration at the urban background site was 21±9.5 ?g m?3, whereas the concentration at the suburban site was 13±7.3 ?g m?3. The daily PM2.5 concentrations exceeded 25 ?g m?3 (the World Health Organization 24 h guideline value) on 29% of the days at the urban background site and 7% of the days at the suburban site. At both sites, BC, Fe, S and Cl accounted for approximately 80% of all detected elements. Positive matrix factorization analysis identified five source factors that contribute to PM2.5 in Nairobi, namely traffic, mineral dust, industry, combustion and a mixed factor (composed of biomass burning, secondary aerosol and aged sea salt). Mineral dust and traffic factors were related to approximately 74% of PM2.5. The identified source factors exhibited seasonal variation, apart from the traffic factor, which was prominently consistent throughout the sampling period. Weekly variations were observed in all factors, with weekdays having higher concentrations than weekends. The results provide information that can be exploited for policy formulation and mitigation strategies to control air pollution in Sub-Saharan African cities.

Posted on 19 September 2014 | 12:00 am


Large mixing ratios of atmospheric nitrous acid (HONO) at Concordia (East Antarctic Plateau) in summer: a strong source from surface snow?

Large mixing ratios of atmospheric nitrous acid (HONO) at Concordia (East Antarctic Plateau) in summer: a strong source from surface snow?

Atmospheric Chemistry and Physics, 14, 9963-9976, 2014

Author(s): M. Legrand, S. Preunkert, M. Frey, Th. Bartels-Rausch, A. Kukui, M. D. King, J. Savarino, M. Kerbrat, and B. Jourdain

During the austral summer 2011/2012 atmospheric nitrous acid (HONO) was investigated for the second time at the Concordia site (75°06' S, 123°33' E), located on the East Antarctic Plateau, by deploying a long-path absorption photometer (LOPAP). Hourly mixing ratios of HONO measured in December 2011/January 2012 (35 ± 5.0 pptv) were similar to those measured in December 2010/January 2011 (30.4 ± 3.5 pptv). The large value of the HONO mixing ratio at the remote Concordia site suggests a local source of HONO in addition to weak production from oxidation of NO by the OH radical. Laboratory experiments demonstrate that surface snow removed from Concordia can produce gas-phase HONO at mixing ratios half that of the NOx mixing ratio produced in the same experiment at typical temperatures encountered at Concordia in summer. Using these lab data and the emission flux of NOx from snow estimated from the vertical gradient of atmospheric concentrations measured during the campaign, a mean diurnal HONO snow emission ranging between 0.5 and 0.8 × 109 molecules cm?2 s?1 is calculated. Model calculations indicate that, in addition to around 1.2 pptv of HONO produced by the NO oxidation, these HONO snow emissions can only explain 6.5 to 10.5 pptv of HONO in the atmosphere at Concordia. To explain the difference between observed and simulated HONO mixing ratios, tests were done both in the field and at lab to explore the possibility that the presence of HNO4 had biased the measurements of HONO.

Posted on 19 September 2014 | 12:00 am


How stratospheric are deep stratospheric intrusions?

How stratospheric are deep stratospheric intrusions?

Atmospheric Chemistry and Physics, 14, 9941-9961, 2014

Author(s): T. Trickl, H. Vogelmann, H. Giehl, H.-E. Scheel, M. Sprenger, and A. Stohl

Preliminary attempts of quantifying the stratospheric ozone contribution in the observations at the Zugspitze summit (2962 m a.s.l.) next to Garmisch-Partenkirchen in the German Alps had yielded an approximate doubling of the stratospheric fraction of the Zugspitze ozone during the time period 1978 to 2004. These investigations had been based on data filtering by using low relative humidity (RH) and elevated 7Be as the criteria for selecting half-hour intervals of ozone data representative of stratospheric intrusion air. To quantify the residual stratospheric component in stratospherically influenced air masses, however, the mixing of tropospheric air into the stratospheric intrusion layers must be taken into account. In fact, the dewpoint mirror instrument at the Zugspitze summit station rarely registers RH values lower than 10% in stratospheric air intrusions. Since 2007 a programme of routine lidar sounding of ozone, water vapour and aerosol has been conducted in the Garmisch-Partenkirchen area. The lidar results demonstrate that the intrusion layers are drier by roughly one order of magnitude than indicated in the in situ measurements. Even in thin layers RH values clearly below 1% have frequently been observed. These thin, undiluted layers present an important challenge for atmospheric modelling. Although the ozone values never reach values typical of the lower-stratosphere it becomes, thus, obvious that, without strong wind shear or convective processes, mixing of stratospheric and tropospheric air must be very slow in most of the free troposphere. As a consequence, the analysis the Zugspitze data can be assumed to be more reliable than anticipated. Finally, the concentrations of Zugspitze carbon monoxide rarely drop inside intrusion layers and normally stay clearly above full stratospheric values. This indicates that most of the CO, and thus the intrusion air mass, originates in the shallow "mixing layer" around the thermal tropopause. The CO mixing ratio in these descending layers between 1990 and 2004 exhibits a slightly positive trend indicating some Asian influence on the lowermost stratosphere in the high-latitude source region of most intrusions reaching the station.

Posted on 19 September 2014 | 12:00 am


Aviation 2006 NOx-induced effects on atmospheric ozone and HOx in Community Earth System Model (CESM)

Aviation 2006 NOx-induced effects on atmospheric ozone and HOx in Community Earth System Model (CESM)

Atmospheric Chemistry and Physics, 14, 9925-9939, 2014

Author(s): A. Khodayari, S. Tilmes, S. C. Olsen, D. B. Phoenix, D. J. Wuebbles, J.-F. Lamarque, and C.-C. Chen

The interaction between atmospheric chemistry and ozone (O3) in the upper troposphere–lower stratosphere (UTLS) presents a major uncertainty in understanding the effects of aviation on climate. In this study, two configurations of the atmospheric model from the Community Earth System Model (CESM), Community Atmosphere Model with Chemistry, Version 4 (CAM4) and Version 5 (CAM5), are used to evaluate the effects of aircraft nitrogen oxide (NOx = NO + NO2) emissions on ozone and the background chemistry in the UTLS. CAM4 and CAM5 simulations were both performed with extensive tropospheric and stratospheric chemistry including 133 species and 330 photochemical reactions. CAM5 includes direct and indirect aerosol effects on clouds using a modal aerosol module (MAM), whereby CAM4 uses a bulk aerosol module, which can only simulate the direct effect. To examine the accuracy of the aviation NOx-induced ozone distribution in the two models, results from the CAM5 and CAM4 simulations are compared to ozonesonde data. Aviation NOx emissions for 2006 were obtained from the AEDT (Aviation Environmental Design Tool) global commercial aircraft emissions inventory. Differences between simulated O3 concentrations and ozonesonde measurements averaged at representative levels in the troposphere and different regions are 13% in CAM5 and 18% in CAM4. Results show a localized increase in aviation-induced O3 concentrations at aviation cruise altitudes that stretches from 40° N to the North Pole. The results indicate a greater and more disperse production of aviation NOx-induced ozone in CAM5, with the annual tropospheric mean O3 perturbation of 1.2 ppb (2.4%) for CAM5 and 1.0 ppb (1.9%) for CAM4. The annual mean O3 perturbation peaks at about 8.2 ppb (6.4%) and 8.8 ppb (5.2%) in CAM5 and CAM4, respectively. Aviation emissions also result in increased hydroxyl radical (OH) concentrations and methane (CH4) loss rates, reducing the tropospheric methane lifetime in CAM5 and CAM4 by 1.69 and 1.40%, respectively. Aviation NOx emissions are associated with an instantaneous change in global mean short-term O3 radiative forcing (RF) of 40.3 and 36.5 mWm?2 in CAM5 and CAM4, respectively.

Posted on 19 September 2014 | 12:00 am


Corrigendum to "Advances in understanding and parameterization of small-scale physical processes in the marine Arctic climate system: a review" published in Atmos. Chem. Phys., 14, 9403–9450, 2014

Corrigendum to "Advances in understanding and parameterization of small-scale physical processes in the marine Arctic climate system: a review" published in Atmos. Chem. Phys., 14, 9403–9450, 2014

Atmospheric Chemistry and Physics, 14, 9923-9923, 2014

Author(s): T. Vihma, R. Pirazzini, I. Fer, I. A. Renfrew, J. Sedlar, M. Tjernström, C. Lüpkes, T. Nygård, D. Notz, J. Weiss, D. Marsan, B. Cheng, G. Birnbaum, S. Gerland, D. Chechin, and J. C. Gascard

No abstract available.

Posted on 19 September 2014 | 12:00 am


Technical Note: The horizontal scale dependence of the cloud overlap parameter ?

Technical Note: The horizontal scale dependence of the cloud overlap parameter ?

Atmospheric Chemistry and Physics, 14, 9917-9922, 2014

Author(s): I. Astin and L. Di Girolamo

The cloud overlap parameter ? relates the combined cloud fraction between two altitude levels in a grid box to the cloud fraction as derived under the maximum and random overlap assumptions. In a number of published studies in this and other journals, it is found that ? tends to increase with an increasing scale. In this Technical Note, we investigate this analytically by considering what happens to ? when two grid boxes are merged to give a grid box with twice the area. Assuming that ? depends only on scale, then between any two fixed altitudes, there will be a linear relationship between the values of ? on the two scales. We illustrate this by finding the relationship when cloud cover fractions are assumed to be uniformly distributed, but with varying degrees of horizontal and vertical correlation. Based on this, we conclude that ? increases with scale if its value is less than the vertical correlation coefficient in cloud fraction between the two altitude levels. This occurs when the clouds are deeper than would be expected at random (i.e. for exponentially distributed cloud depths).

Posted on 19 September 2014 | 12:00 am


Lightning NOx, a key chemistry–climate interaction: impacts of future climate change and consequences for tropospheric oxidising capacity

Lightning NOx, a key chemistry–climate interaction: impacts of future climate change and consequences for tropospheric oxidising capacity

Atmospheric Chemistry and Physics, 14, 9871-9881, 2014

Author(s): A. Banerjee, A. T. Archibald, A. C. Maycock, P. Telford, N. L. Abraham, X. Yang, P. Braesicke, and J. A. Pyle

Lightning is one of the major natural sources of NOx in the atmosphere. A suite of time slice experiments using a stratosphere-resolving configuration of the Unified Model (UM), containing the United Kingdom Chemistry and Aerosols sub-model (UKCA), has been performed to investigate the impact of climate change on emissions of NOx from lightning (LNOx) and to highlight its critical impacts on photochemical ozone production and the oxidising capacity of the troposphere. Two Representative Concentration Pathway (RCP) scenarios (RCP4.5 and RCP8.5) are explored. LNOx is simulated to increase in a year-2100 climate by 33% (RCP4.5) and 78% (RCP8.5), primarily as a result of increases in the depth of convection. The total tropospheric chemical odd oxygen production (P(Ox)) increases linearly with increases in total LNOx and consequently, tropospheric ozone burdens of 29 ± 4 Tg(O3) (RCP4.5) and 46 ± 4 Tg(O3) (RCP8.5) are calculated here. By prescribing a uniform surface boundary concentration for methane in these simulations, methane-driven feedbacks are essentially neglected. A simple estimate of the contribution of the feedback reduces the increase in ozone burden to 24 and 33 Tg(O3), respectively. We thus show that, through changes in LNOx, the effects of climate change counteract the simulated mitigation of the ozone burden, which results from reductions in ozone precursor emissions as part of air quality controls projected in the RCP scenarios. Without the driver of increased LNOx, our simulations suggest that the net effect of climate change would be to lower free tropospheric ozone.

In addition, we identify large climate-change-induced enhancements in the concentration of the hydroxyl radical (OH) in the tropical upper troposphere (UT), particularly over the Maritime Continent, primarily as a consequence of greater LNOx. The OH enhancement in the tropics increases oxidation of both methane (with feedbacks onto chemistry and climate) and very short-lived substances (VSLS) (with implications for stratospheric ozone depletion). We emphasise that it is important to improve our understanding of LNOx in order to gain confidence in model projections of composition change under future climate.

Posted on 18 September 2014 | 12:00 am


Characterising terrestrial influences on Antarctic air masses using Radon-222 measurements at King George Island

Characterising terrestrial influences on Antarctic air masses using Radon-222 measurements at King George Island

Atmospheric Chemistry and Physics, 14, 9903-9916, 2014

Author(s): S. D. Chambers, S.-B. Hong, A. G. Williams, J. Crawford, A. D. Griffiths, and S.-J. Park

We report on one year of high-precision direct hourly radon observations at King Sejong Station (King George Island) beginning in February 2013. Findings are compared with historic and ongoing radon measurements from other Antarctic sites. Monthly median concentrations reduced from 72 mBq m?3 in late-summer to 44 mBq m?3 in late winter and early spring. Monthly 10th percentiles, ranging from 29 to 49 mBq m?3, were typical of oceanic baseline values. Diurnal cycles were rarely evident and local influences were minor, consistent with regional radon flux estimates one tenth of the global average for ice-free land. The predominant fetch region for terrestrially influenced air masses was South America (47–53° S), with minor influences also attributed to aged Australian air masses and local sources. Plume dilution factors of 2.8–4.0 were estimated for the most terrestrially influenced (South American) air masses, and a seasonal cycle in terrestrial influence on tropospheric air descending at the pole was identified and characterised.

Posted on 18 September 2014 | 12:00 am


Drivers of column-average CO2 variability at Southern Hemispheric Total Carbon Column Observing Network sites

Drivers of column-average CO2 variability at Southern Hemispheric Total Carbon Column Observing Network sites

Atmospheric Chemistry and Physics, 14, 9883-9901, 2014

Author(s): N. M. Deutscher, V. Sherlock, S. E. Mikaloff Fletcher, D. W. T. Griffith, J. Notholt, R. Macatangay, B. J. Connor, J. Robinson, H. Shiona, V. A. Velazco, Y. Wang, P. O. Wennberg, and D. Wunch

We investigate factors that drive the variability in total column CO2 at the Total Carbon Column Observing Network sites in the Southern Hemisphere using fluxes tagged by process and by source region from the CarbonTracker analysed product as well as the Simple Biosphere model. We show that the terrestrial biosphere is the largest driver of variability in the Southern Hemisphere column CO2. However, it does not dominate in the same fashion as in the Northern Hemisphere. Local- and hemispheric-scale biomass burning can also play an important role, particularly at the tropical site, Darwin. The magnitude of seasonal variability in the column-average dry-air mole fraction of CO2, XCO2, is also much smaller in the Southern Hemisphere and comparable in magnitude to the annual increase. Comparison of measurements to the model simulations highlights that there is some discrepancy between the two time series, especially in the early part of the Darwin data record. We show that this mismatch is most likely due to erroneously estimated local fluxes in the Australian tropical region, which are associated with enhanced photosynthesis caused by early rainfall during the tropical monsoon season.

Posted on 18 September 2014 | 12:00 am


Tropospheric ozone increases over the southern Africa region: bellwether for rapid growth in Southern Hemisphere pollution?

Tropospheric ozone increases over the southern Africa region: bellwether for rapid growth in Southern Hemisphere pollution?

Atmospheric Chemistry and Physics, 14, 9855-9869, 2014

Author(s): A. M. Thompson, N. V. Balashov, J. C. Witte, J. G. R. Coetzee, V. Thouret, and F. Posny

Increases in free-tropospheric (FT) ozone based on ozonesonde records from the early 1990s through 2008 over two subtropical stations, Irene (near Pretoria, South Africa) and Réunion (21° S, 55° E; ~2800 km NE of Irene in the Indian Ocean), have been reported. Over Irene a large increase in the urban-influenced boundary layer (BL, 1.5–4 km) was also observed during the 18-year period, equivalent to 30% decade?1. Here we show that the Irene BL trend is at least partly due to a gradual change in the sonde launch times from early morning to the midday period. The FT ozone profiles over Irene in 1990–2007 are re-examined, filling in a 1995–1999 gap with ozone profiles taken during the Measurements of Ozone by Airbus In-service Aircraft (MOZAIC) project over nearby Johannesburg. A multivariate regression model that accounts for the annual ozone cycle, El Niño–Southern Oscillation (ENSO) and possible tropopause changes was applied to monthly averaged Irene data from 4 to 11 km and to 1992–2011 Réunion sonde data from 4 to 15 km. Statistically significant trends appear predominantly in the middle and upper troposphere (UT; 4–11 km over Irene, 4–15 km over Réunion) in winter (June–August), with increases ~1 ppbv yr?1 over Irene and ~2 ppbv yr?1 over Réunion. These changes are equivalent to ~25 and 35–45% decade?1, respectively. Both stations also display smaller positive trends in summer, with a 45% decade?1 ozone increase near the tropopause over Réunion in December. To explain the ozone increases, we investigated a time series of dynamical markers, e.g., potential vorticity (PV) at 330–350 K. PV affects UT ozone over Irene in November–December but displays little relationship with ozone over Réunion. A more likely reason for wintertime FT ozone increases over Irene and Réunion appears to be long-range transport of growing pollution in the Southern Hemisphere. The ozone increases are consistent with trajectory origins of air parcels sampled by the sondes and with recent NOx emissions trends estimated for Africa, South America and Madagascar. For Réunion trajectories also point to sources from the eastern Indian Ocean and Asia.

Posted on 17 September 2014 | 12:00 am


Cloud droplet activity changes of soot aerosol upon smog chamber ageing

Cloud droplet activity changes of soot aerosol upon smog chamber ageing

Atmospheric Chemistry and Physics, 14, 9831-9854, 2014

Author(s): C. Wittbom, A. C. Eriksson, J. Rissler, J. E. Carlsson, P. Roldin, E. Z. Nordin, P. T. Nilsson, E. Swietlicki, J. H. Pagels, and B. Svenningsson

Particles containing soot, or black carbon, are generally considered to contribute to global warming. However, large uncertainties remain in the net climate forcing resulting from anthropogenic emissions of black carbon (BC), to a large extent due to the fact that BC is co-emitted with gases and primary particles, both organic and inorganic, and subject to atmospheric ageing processes. In this study, diesel exhaust particles and particles from a flame soot generator spiked with light aromatic secondary organic aerosol (SOA) precursors were processed by UV radiation in a 6 m3 Teflon chamber in the presence of NOx. The time-dependent changes of the soot nanoparticle properties were characterised using a Cloud Condensation Nuclei Counter, an Aerosol Particle Mass Analyzer and a Soot Particle Aerosol Mass Spectrometer. The results show that freshly emitted soot particles do not activate into cloud droplets at supersaturations ≤2%, i.e. the BC core coated with primary organic aerosol (POA) from the exhaust is limited in hygroscopicity. Before the onset of UV radiation it is unlikely that any substantial SOA formation is taking place. An immediate change in cloud-activation properties occurs at the onset of UV exposure. This change in hygroscopicity is likely attributed to SOA formed from intermediate volatility organic compounds (IVOCs) in the diesel engine exhaust. The change of cloud condensation nuclei (CCN) properties at the onset of UV radiation implies that the lifetime of soot particles in the atmosphere is affected by the access to sunlight, which differs between latitudes. The ageing of soot particles progressively enhances their ability to act as cloud condensation nuclei, due to changes in: (I) organic fraction of the particle, (II) chemical properties of this fraction (e.g. primary or secondary organic aerosol), (III) particle size, and (IV) particle morphology. Applying κ-Köhler theory, using a κSOA value of 0.13 (derived from independent input parameters describing the organic material), showed good agreement with cloud droplet activation measurements for particles with a SOA mass fraction ≥0.12 (slightly aged particles). The activation properties are enhanced with only a slight increase in organic material coating the soot particles (SOA mass fraction < 0.12), however not as much as predicted by Köhler theory. The discrepancy between theory and experiments during the early stages of ageing might be due to solubility limitations, unevenly distributed organic material or hindering particle morphology.

The change in properties of soot nanoparticles upon photochemical processing clearly increases their hygroscopicity, which affects their behaviour both in the atmosphere and in the human respiratory system.

Posted on 17 September 2014 | 12:00 am


To what extent could water isotopic measurements help us understand model biases in the water cycle over Western Siberia

To what extent could water isotopic measurements help us understand model biases in the water cycle over Western Siberia

Atmospheric Chemistry and Physics, 14, 9807-9830, 2014

Author(s): V. Gryazin, C. Risi, J. Jouzel, N. Kurita, J. Worden, C. Frankenberg, V. Bastrikov, K. Gribanov, and O. Stukova

We evaluate the isotopic composition of water vapor and precipitation simulated by the LMDZ (Laboratoire de Météorologie Dynamique-Zoom) GCM (General Circulation Model) over Siberia using several data sets: TES (Tropospheric Emission Spectrometer) and GOSAT (Greenhouse gases Observing SATellite) satellite observations of tropospheric water vapor, GNIP (Global Network for Isotopes in Precipitation) and SNIP (Siberian Network for Isotopes in Precipitation) precipitation networks, and daily, in situ measurements of water vapor and precipitation at the Kourovka site in Western Siberia. LMDZ captures the spatial, seasonal and daily variations reasonably well, but it underestimates humidity (q) in summer and overestimates ?D in the vapor and precipitation in all seasons. The performance of LMDZ is put in the context of other isotopic models from the SWING2 (Stable Water Intercomparison Group phase 2) models. There is significant spread among models in the simulation of ?D, and of the ?D-q relationship. This confirms that ?D brings additional information compared to q only. We specifically investigate the added value of water isotopic measurements to interpret the warm and dry bias featured by most GCMs over mid and high latitude continents in summer. The analysis of the slopes in ?D-q diagrams and of processes controlling ?D and q variations suggests that the cause of the dry bias could be either a problem in the large-scale advection transporting too much dry and warm air from the south, or too strong boundary-layer mixing. However, ?D-q diagrams using the available data do not tell the full story. Additional measurements would be needed, or a more sophisticated theoretical framework would need to be developed.

Posted on 17 September 2014 | 12:00 am


High-resolution mapping of vehicle emissions in China in 2008

High-resolution mapping of vehicle emissions in China in 2008

Atmospheric Chemistry and Physics, 14, 9787-9805, 2014

Author(s): B. Zheng, H. Huo, Q. Zhang, Z. L. Yao, X. T. Wang, X. F. Yang, H. Liu, and K. B. He

This study is the first in a series of papers that aim to develop high-resolution emission databases for different anthropogenic sources in China. Here we focus on on-road transportation. Because of the increasing impact of on-road transportation on regional air quality, developing an accurate and high-resolution vehicle emission inventory is important for both the research community and air quality management. This work proposes a new inventory methodology to improve the spatial and temporal accuracy and resolution of vehicle emissions in China. We calculate, for the first time, the monthly vehicle emissions for 2008 in 2364 counties (an administrative unit one level lower than city) by developing a set of approaches to estimate vehicle stock and monthly emission factors at county-level, and technology distribution at provincial level. We then introduce allocation weights for the vehicle kilometers traveled to assign the county-level emissions onto 0.05° × 0.05° grids based on the China Digital Road-network Map (CDRM). The new methodology overcomes the common shortcomings of previous inventory methods, including neglecting the geographical differences between key parameters and using surrogates that are weakly related to vehicle activities to allocate vehicle emissions. The new method has great advantages over previous methods in depicting the spatial distribution characteristics of vehicle activities and emissions. This work provides a better understanding of the spatial representation of vehicle emissions in China and can benefit both air quality modeling and management with improved spatial accuracy.

Posted on 17 September 2014 | 12:00 am


Analysis of the diurnal development of a lake-valley circulation in the Alps based on airborne and surface measurements

Analysis of the diurnal development of a lake-valley circulation in the Alps based on airborne and surface measurements

Atmospheric Chemistry and Physics, 14, 9771-9786, 2014

Author(s): L. Laiti, D. Zardi, M. de Franceschi, G. Rampanelli, and L. Giovannini

This study investigates the thermal structures of the atmospheric boundary layer (ABL) and the near-surface wind field associated with a lake-valley circulation in the south-eastern Italian Alps – the so-called Ora del Garda. Two flights of an equipped motorglider allowed for the exploration of the diurnal evolution of this circulation, from the onset, on Lake Garda's shoreline, throughout its development along the Sarca Valley and Lakes Valley (Valle dei Laghi), to the outflow into the Adige Valley. At the same time, surface observations, both from a targeted field campaign and from routinely operated weather stations, supported the analysis of the development of the Ora del Garda at the valley floor.

In particular, in the valleys typical ABL vertical structures, characterized by rather shallow convective mixed layers (~ 500 m) and (deeper) weakly stable layers above, up to the lateral crest height, are identified in the late morning. In contrast, close to the lake the ABL is stably stratified down to very low heights, as a consequence of the intense advection of colder air associated with the Ora del Garda flow (up to 6 m s–1). The combined analysis of surface and airborne observations (remapped over 3-D high-resolution grids) suggests that the lake-breeze front propagating up-valley from the shoreline in the late morning penetrates slightly later at the eastern end of the valley inlet (delay: ~ 1 h), probably due to the asymmetric radiative forcing caused by the N–S valley orientation. On the other hand, in the early afternoon the Ora del Garda overflows through an elevated gap, producing an anomalous, strong cross-valley wind (5 m s–1) at the Adige Valley floor north of Trento, which overwhelms the local up-valley wind. This feature is associated with a strong deepening of the local mixed layer (from 400 to 1300 m). The potential temperature 3-D field suggests that the intense turbulent mixing may be attributed to the development of a downslope wind across the gap, followed by a hydraulic jump downstream.

Posted on 17 September 2014 | 12:00 am


Simulation of the interannual variations of aerosols in China: role of variations in meteorological parameters

Simulation of the interannual variations of aerosols in China: role of variations in meteorological parameters

Atmospheric Chemistry and Physics, 14, 9597-9612, 2014

Author(s): Q. Mu and H. Liao

We used the nested grid version of the global three-dimensional Goddard Earth Observing System chemical transport model (GEOS-Chem) to examine the interannual variations (IAVs) of aerosols over heavily polluted regions in China for years 2004–2012. The role of variations in meteorological parameters was quantified by a simulation with fixed anthropogenic emissions at year 2006 levels and changes in meteorological parameters over 2004–2012. Simulated PM2.5 (particles with a diameter of 2.5 ?m or less) aerosol concentrations exhibited large IAVs in North China (NC; 32–42° N, 110–120° E), with regionally averaged absolute percent departure from the mean (APDM) values of 17, 14, 14, and 11% in December-January-February (DJF), March-April-May (MAM), June-July-August (JJA), and September-October-November (SON), respectively. Over South China (SC; 22–32° N, 110–120° E), the IAVs in PM2.5 were found to be the largest in JJA, with the regional mean APDM values of 14% in JJA and of about 9% in other seasons. The concentrations of PM2.5 over the Sichuan Basin (SCB; 27–33° N, 102–110° E) were simulated to have the smallest IAVs among the polluted regions examined in this work, with APDM values of 8–9% in all seasons. All aerosol species (sulfate, nitrate, ammonium, black carbon, and organic carbon) were simulated to have the largest IAVs over NC in DJF, corresponding to the large variations in meteorological parameters over NC in this season. Process analyses were performed to identify the key meteorological parameters that determined the IAVs of different aerosol species in different regions. While the variations in temperature and specific humidity, which influenced the gas-phase formation of sulfate, jointly determined the IAVs of sulfate over NC in both DJF and JJA, wind (or convergence of wind) in DJF and precipitation in JJA were the dominant meteorological factors to influence IAVs of sulfate over SC and the SCB. The IAVs in temperature and specific humidity influenced gas-to-aerosol partitioning, which were the major factors that led to the IAVs of nitrate aerosol in China. The IAVs in wind and precipitation were found to drive the IAVs of organic carbon aerosol. We also compared the IAVs of aerosols simulated with variations in meteorological parameters alone with those simulated with variations in anthropogenic emissions alone; the variations in meteorological fields were found to dominate the IAVs of aerosols in northern and southern China over 2004–2012. Considering that the IAVs in meteorological fields are mainly associated with natural variability in the climate system, the IAVs in aerosol concentrations driven by meteorological parameters have important implications for the effectiveness of short-term air quality control strategies in China.

Posted on 16 September 2014 | 12:00 am


Analysing time-varying trends in stratospheric ozone time series using the state space approach

Analysing time-varying trends in stratospheric ozone time series using the state space approach

Atmospheric Chemistry and Physics, 14, 9707-9725, 2014

Author(s): M. Laine, N. Latva-Pukkila, and E. Kyrölä

We describe a hierarchical statistical state space model for ozone profile time series. The time series are from satellite measurements by the Stratospheric Aerosol and Gas Experiment (SAGE) II and the Global Ozone Monitoring by Occultation of Stars (GOMOS) instruments spanning the years 1984–2011. Vertical ozone profiles were linearly interpolated on an altitude grid with 1 km resolution covering 20–60 km. Monthly averages were calculated for each altitude level and 10° wide latitude bins between 60° S and 60° N. In the analysis, mean densities are studied separately for the 25–35, 35–45, and 45–55 km layers. Model variables include the ozone mean level, local trend, seasonal oscillations, and proxy variables for solar activity, the Quasi-Biennial Oscillation (QBO), and the El Niño–Southern Oscillation (ENSO).

This is a companion paper to Kyrölä et al. (2013), where a piecewise linear model was used together with the same proxies as in this work (excluding ENSO). The piecewise linear trend was allowed to change at the beginning of 1997 in all latitudes and altitudes. In the modelling of the present paper such an assumption is not needed as the linear trend is allowed to change continuously at each time step. This freedom is also allowed for the seasonal oscillations whereas other regression coefficients are taken independent of time. According to our analyses, the slowly varying ozone background shows roughly three general development patterns. A continuous decay for the whole period 1984–2011 is evident in the southernmost latitude belt 50–60° S in all altitude regions and in 50–60° N in the lowest altitude region 25–35 km. A second pattern, where a recovery after an initial decay is followed by a further decay, is found at northern latitudes from the equator to 50° N in the lowest altitude region (25–35 km) and between 40° N and 60° N in the 35–45 km altitude region. Further ozone loss occurred after 2007 in these regions. Everywhere else a decay is followed by a recovery. This pattern is shown at all altitudes and latitudes in the Southern Hemisphere (10–50° S) and in the 45–55 km layer in the Northern Hemisphere (from the equator to 40° N). In the 45–55 km range the trend, measured as an average change in 10 years, has mostly turned from negative to positive before the year 2000. In those regions where the "V" type of change of the trend is appropriate, the turning point is around the years 1997–2001. To compare results for the trend changes with the companion paper, we studied the difference in trends between the years from 1984 to 1997 and from 1997 to 2011. Overall, the two methods produce very similar ozone recovery patterns with the maximum trend change of 10% in 35–45 km. The state space method (used in this paper) shows a somewhat faster recovery than the piecewise linear model. For the percent change of the ozone density per decade the difference between the results is below three percentage units.

Posted on 16 September 2014 | 12:00 am


Corrigendum to "Controls on the movement and composition of firn air at the West Antarctic Ice Sheet Divide"

Corrigendum to "Controls on the movement and composition of firn air at the West Antarctic Ice Sheet Divide"

Atmospheric Chemistry and Physics, 14, 9511-9511, 2014

Author(s): M. O. Battle, J. P. Severinghaus, E. D. Sofen, D. Plotkin, A. J. Orsi, M. Aydin, S. A. Montzka, T. Sowers, and P. P. Tans

No abstract available.

Posted on 16 September 2014 | 12:00 am


Aqueous-phase photooxidation of levoglucosan – a mechanistic study using aerosol time-of-flight chemical ionization mass spectrometry (Aerosol ToF-CIMS)

Aqueous-phase photooxidation of levoglucosan – a mechanistic study using aerosol time-of-flight chemical ionization mass spectrometry (Aerosol ToF-CIMS)

Atmospheric Chemistry and Physics, 14, 9695-9706, 2014

Author(s): R. Zhao, E. L. Mungall, A. K. Y. Lee, D. Aljawhary, and J. P. D. Abbatt

Levoglucosan (LG) is a widely employed tracer for biomass burning (BB). Recent studies have shown that LG can react rapidly with hydroxyl (OH) radicals in the aqueous phase despite many mass balance receptor models assuming it to be inert during atmospheric transport. In the current study, aqueous-phase photooxidation of LG by OH radicals was performed in the laboratory. The reaction kinetics and products were monitored by aerosol time-of-flight chemical ionization mass spectrometry (Aerosol ToF-CIMS). Approximately 50 reaction products were detected by the Aerosol ToF-CIMS during the photooxidation experiments, representing one of the most detailed product studies yet performed. By following the evolution of mass defects of product peaks, unique trends of adding oxygen (+O) and removing hydrogen (?2H) were observed among the products detected, providing useful information for determining potential reaction mechanisms and sequences. Additionally, bond-scission reactions take place, leading to reaction intermediates with lower carbon numbers. We introduce a data analysis framework where the average oxidation state (OSc) is plotted against a novel molecular property: double-bond-equivalence-to-carbon ratio (DBE/#C). The trajectory of LG photooxidation on this plot suggests formation of polycarbonyl intermediates and their subsequent conversion to carboxylic acids as a general reaction trend. We also determined the rate constant of LG with OH radicals at room temperature to be 1.08 ± 0.16 × 109 M?1 s?1. By coupling an aerosol mass spectrometer (AMS) to the system, we observed a rapid decay of the mass fraction of organic signals at mass-to-charge ratio 60 (f60), corresponding closely to the LG decay monitored by the Aerosol ToF-CIMS. The trajectory of LG photooxidation on a f44–f60 correlation plot matched closely to literature field measurement data. This implies that aqueous-phase photooxidation might be partially contributing to aging of BB particles in the ambient atmosphere.

Posted on 16 September 2014 | 12:00 am


Estimates of European emissions of methyl chloroform using a Bayesian inversion method

Estimates of European emissions of methyl chloroform using a Bayesian inversion method

Atmospheric Chemistry and Physics, 14, 9755-9770, 2014

Author(s): M. Maione, F. Graziosi, J. Arduini, F. Furlani, U. Giostra, D. R. Blake, P. Bonasoni, X. Fang, S. A. Montzka, S. J. O'Doherty, S. Reimann, A. Stohl, and M. K. Vollmer

Methyl chloroform (MCF) is a man-made chlorinated solvent contributing to the destruction of stratospheric ozone and is controlled under the "Montreal Protocol on Substances that Deplete the Ozone Layer" and its amendments, which called for its phase-out in 1996 in developed countries and 2015 in developing countries. Long-term, high-frequency observations of MCF carried out at three European sites show a constant decline in the background mixing ratios of MCF. However, we observe persistent non-negligible mixing ratio enhancements of MCF in pollution episodes, suggesting unexpectedly high ongoing emissions in Europe. In order to identify the source regions and to give an estimate of the magnitude of such emissions, we have used a Bayesian inversion method and a point source analysis, based on high-frequency long-term observations at the three European sites.

The inversion identified southeastern France (SEF) as a region with enhanced MCF emissions. This estimate was confirmed by the point source analysis. We performed this analysis using an 11-year data set, from January 2002 to December 2012. Overall, emissions estimated for the European study domain decreased nearly exponentially from 1.1 Gg yr?1 in 2002 to 0.32 Gg yr?1 in 2012, of which the estimated emissions from the SEF region accounted for 0.49 Gg yr?1 in 2002 and 0.20 Gg yr?1 in 2012. The European estimates are a significant fraction of the total semi-hemisphere (30–90° N) emissions, contributing a minimum of 9.8% in 2004 and a maximum of 33.7% in 2011, of which on average 50% are from the SEF region. On the global scale, the SEF region is thus responsible for a minimum of 2.6% (in 2003) and a maximum of 10.3% (in 2009) of the global MCF emissions.

Posted on 16 September 2014 | 12:00 am


Links between satellite-retrieved aerosol and precipitation

Links between satellite-retrieved aerosol and precipitation

Atmospheric Chemistry and Physics, 14, 9677-9694, 2014

Author(s): E. Gryspeerdt, P. Stier, and D. G. Partridge

Many theories have been proposed detailing how aerosols might impact precipitation, predicting both increases and decreases depending on the prevailing meteorological conditions and aerosol type. In convective clouds, increased aerosol concentrations have been speculated to invigorate convective activity. Previous studies have shown large increases in precipitation with increasing aerosol optical depth, concluding an aerosol effect on precipitation. Our analysis reveals that these studies may have been influenced by cloud effects on the retrieved aerosol, as well as by meteorological covariations.

We use a regime-based approach to separate out different cloud regimes, allowing for the study of aerosol–cloud interactions in individual cloud regimes. We account for the influence of cloud properties on the aerosol retrieval and make use of the diurnal sampling of the TRMM satellite and the TRMM merged precipitation product to investigate the precipitation development.

We find that whilst there is little effect on precipitation at the time of the aerosol retrieval, in the 6 h after the aerosol retrieval, there is an increase in precipitation from cloud in high-aerosol environments, consistent with the invigoration hypothesis. Increases in lightning flash count with increased aerosol are also observed in this period. The invigoration effect appears to be dependent on the cloud-top temperature, with clouds with tops colder than 0 °C showing increases in precipitation at times after the retrieval, as well as increases in wet scavenging. Warm clouds show little change in precipitation development with increasing aerosol, suggesting ice processes are important for the invigoration of precipitation.

Posted on 16 September 2014 | 12:00 am


Impact of black carbon aerosol over Italian basin valleys: high-resolution measurements along vertical profiles, radiative forcing and heating rate

Impact of black carbon aerosol over Italian basin valleys: high-resolution measurements along vertical profiles, radiative forcing and heating rate

Atmospheric Chemistry and Physics, 14, 9641-9664, 2014

Author(s): L. Ferrero, M. Castelli, B. S. Ferrini, M. Moscatelli, M. G. Perrone, G. Sangiorgi, L. D'Angelo, G. Rovelli, B. Moroni, F. Scardazza, G. Mo?nik, E. Bolzacchini, M. Petitta, and D. Cappelletti

A systematic study of black carbon (BC) vertical profiles measured at high-resolution over three Italian basin valleys (Terni Valley, Po Valley and Passiria Valley) is presented. BC vertical profiles are scarcely available in literature. The campaign lasted 45 days and resulted in 120 measured vertical profiles. Besides the BC mass concentration, measurements along the vertical profiles also included aerosol size distributions in the optical particle counter range, chemical analysis of filter samples and a full set of meteorological parameters. Using the collected experimental data, we performed calculations of aerosol optical properties along the vertical profiles. The results, validated with AERONET data, were used as inputs to a radiative transfer model (libRadtran). The latter allowed an estimation of vertical profiles of the aerosol direct radiative effect, the atmospheric absorption and the heating rate in the lower troposphere.

The present measurements revealed some common behaviors over the studied basin valleys. Specifically, at the mixing height, marked concentration drops of both BC (range: from −48.4 ± 5.3 to −69.1 ± 5.5%) and aerosols (range: from −23.9 ± 4.3 to −46.5 ± 7.3%) were found. The measured percentage decrease of BC was higher than that of aerosols: therefore, the BC aerosol fraction decreased upwards. Correspondingly, both the absorption and scattering coefficients decreased strongly across the mixing layer (range: from −47.6 ± 2.5 to −71.3 ± 3.0% and from −23.5 ± 0.8 to −61.2 ± 3.1%, respectively) resulting in a single-scattering albedo increase along height (range: from +4.9 ± 2.2 to +7.4 ± 1.0%).

This behavior influenced the vertical distribution of the aerosol direct radiative effect and of the heating rate. In this respect, the highest atmospheric absorption of radiation was predicted below the mixing height (~ 2–3 times larger than above it) resulting in a heating rate characterized by a vertical negative gradient (range: from −2.6 ± 0.2 to −8.3 ± 1.2 K day?1 km?1). In conclusion, the present results suggest that the BC below the mixing height has the potential to promote a negative feedback on the atmospheric stability over basin valleys, weakening the ground-based thermal inversions and increasing the dispersal conditions.

Posted on 16 September 2014 | 12:00 am


Estimation of the direct and indirect impacts of fireworks on the physicochemical characteristics of atmospheric PM10 and PM2.5

Estimation of the direct and indirect impacts of fireworks on the physicochemical characteristics of atmospheric PM10 and PM2.5

Atmospheric Chemistry and Physics, 14, 9469-9479, 2014

Author(s): Y. Z. Tian, J. Wang, X. Peng, G. L. Shi, and Y. C. Feng

To quantify the total, direct and indirect impacts of fireworks individually, size-resolved PM samples were collected before, during and after a Chinese folk festival (Chinese New Year) in a megacity in China. Through chemical analysis and morphological characterisation, a strong influence of fireworks on the physicochemical characteristics of PM10 and PM2.5 was observed. The concentrations of many species exhibited an increasing trend during the heavy-firework period, especially for K+, Mg2+ and Cr; the results of the non-sea-salt ions demonstrated an anthropogenic influence on K+ and Mg2+. Then, source apportionment was conducted by receptor models and peak analysis (PA). The total influence of the fireworks was quantified by positive matrix factorisation (PMF), showing that the fireworks contributed higher fractions (23.40% for PM10 and 29.66% for PM2.5) during the heavy-firework period than during the light-firework period (4.28% for PM10 and 7.18% for PM2.5). The profiles of the total fireworks obtained by two independent methods (PMF and peak analysis) were consistent, with higher abundances of K+, Al, Si, Ca and organic carbon (OC). Finally, the individual contributions of the direct and indirect impacts of fireworks were quantified by chemical mass balance (CMB). The percentage contributions of resuspended dust, biomass combustion and direct fireworks were 36.8 ± 8.37%, 14.1 ± 2.82% and 44.4 ± 8.26%, respectively, for PM10 and 34.9 ± 4.19%, 16.6 ± 3.05% and 52.5 ± 9.69%, respectively, for PM2.5, in terms of the total fireworks. The quantification of the total, direct and indirect impacts of fireworks in the ambient PM gives a original contribution for understanding the physicochemical characteristics and mechanisms of such high-intensity anthropogenic activities.

Posted on 16 September 2014 | 12:00 am


Seasonality of halogen deposition in polar snow and ice

Seasonality of halogen deposition in polar snow and ice

Atmospheric Chemistry and Physics, 14, 9613-9622, 2014

Author(s): A. Spolaor, P. Vallelonga, J. Gabrieli, T. Martma, M. P. Björkman, E. Isaksson, G. Cozzi, C. Turetta, H. A. Kjær, M. A. J. Curran, A. D. Moy, A. Schönhardt, A.-M. Blechschmidt, J. P. Burrows, J. M. C. Plane, and C. Barbante

The atmospheric chemistry of iodine and bromine in Polar regions is of interest due to the key role of halogens in many atmospheric processes, particularly tropospheric ozone destruction. Bromine is emitted from the open ocean but is enriched above first-year sea ice during springtime bromine explosion events, whereas iodine emission is attributed to biological communities in the open ocean and hosted by sea ice. It has been previously demonstrated that bromine and iodine are present in Antarctic ice over glacial–interglacial cycles. Here we investigate seasonal variability of bromine and iodine in polar snow and ice, to evaluate their emission, transport and deposition in Antarctica and the Arctic and better understand potential links to sea ice. We find that bromine and iodine concentrations and Br enrichment (relative to sea salt content) in polar ice do vary seasonally in Arctic snow and Antarctic ice. Although seasonal variability in halogen emission sources is recorded by satellite-based observations of tropospheric halogen concentrations, seasonal patterns observed in snowpack are likely also influenced by photolysis-driven processes. Peaks of bromine concentration and Br enrichment in Arctic snow and Antarctic ice occur in spring and summer, when sunlight is present. A secondary bromine peak, observed at the end of summer, is attributed to bromine deposition at the end of the polar day. Iodine concentrations are largest in winter Antarctic ice strata, contrary to contemporary observations of summer maxima in iodine emissions. These findings support previous observations of iodine peaks in winter snow strata attributed to the absence of sunlight-driven photolytic re-mobilisation of iodine from surface snow. Further investigation is required to confirm these proposed mechanisms explaining observations of halogens in polar snow and ice, and to evaluate the extent to which halogens may be applied as sea ice proxies.

Posted on 16 September 2014 | 12:00 am


Factors controlling temporal variability of near-ground atmospheric 222Rn concentration over central Europe

Factors controlling temporal variability of near-ground atmospheric 222Rn concentration over central Europe

Atmospheric Chemistry and Physics, 14, 9567-9581, 2014

Author(s): M. Zimnoch, P. Wach, L. Chmura, Z. Gorczyca, K. Rozanski, J. Godlowska, J. Mazur, K. Kozak, and A. Jeri?evi?

Concentration of radon (222Rn) in the near-ground atmosphere has been measured quasi-continuously from January 2005 to December 2009 at two continental sites in Europe: Heidelberg (south-west Germany) and Krakow (southern Poland). The atmosphere was sampled at ca. 30 and 20 m above the local ground. Both stations were equipped with identical instruments. Regular observations of 222Rn were supplemented by measurements of surface fluxes of this gas in the Krakow urban area, using two different approaches. The measured concentrations of 222Rn varied at both sites in a wide range, from less than 2.0 Bq m?3 to approximately 40 Bq m?3 in Krakow and 35 Bq m?3 in Heidelberg. The mean 222Rn content in Krakow, when averaged over the entire observation period, was 30% higher than in Heidelberg (5.86 ± 0.09 and 4.50 ± 0.07 Bq m?3, respectively). Distinct seasonality of 222Rn signal is visible in the obtained time series of 222Rn concentration, with higher values recorded generally during late summer and autumn. The surface 222Rn fluxes measured in Krakow also revealed a distinct seasonality, with broad maximum observed during summer and early autumn and minimum during the winter. When averaged over a 5-year observation period, the night-time surface 222Rn flux was equal to 46.8 ± 2.4 Bq m?2 h?1. Although the atmospheric 222Rn levels at Heidelberg and Krakow appeared to be controlled primarily by local factors, it was possible to evaluate the "continental effect" in atmospheric 222Rn content between both sites, related to gradual build-up of 222Rn concentration in the air masses travelling between Heidelberg and Krakow. The mean value of this build-up was equal to 0.78 ± 0.12 Bq m?3. The measured minimum 222Rn concentrations at both sites and the difference between them was interpreted in the framework of a simple box model coupled with HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory) analysis of air mass trajectories. The best fit of experimental data was obtained for the mean 222Rn flux over the European continent equal to 52 Bq m?2 h?1, the mean transport velocity of the air masses within the convective mixed layer of the planetary boundary layer (PBL) on their route from the Atlantic coast to Heidelberg and Krakow equal to 3.5 m s?1, the mean rate constant of 222Rn removal across the top of the PBL equal to the 222Rn decay constant and the mean height of the convective mixed layer equal to 1600 m.

Posted on 16 September 2014 | 12:00 am


Comparison of IASI water vapor retrieval with H2O-Raman lidar in the framework of the Mediterranean HyMeX and ChArMEx programs

Comparison of IASI water vapor retrieval with H2O-Raman lidar in the framework of the Mediterranean HyMeX and ChArMEx programs

Atmospheric Chemistry and Physics, 14, 9583-9596, 2014

Author(s): P. Chazette, F. Marnas, J. Totems, and X. Shang

The Infrared Atmospheric Sounding Interferometer (IASI) is a new generation spaceborne passive sensor mainly dedicated to meteorological applications. Operational Level-2 products have been available via the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) for several years. In particular, vertical profiles of water vapor measurements are retrieved from infrared radiances at the global scale. Nevertheless, the robustness of such products has to be checked because only a few validations have been reported. For this purpose, the field experiments that were held during the HyMeX and ChArMEx international programs are a very good opportunity. A H2O-Raman lidar was deployed on the Balearic island of Menorca and operated continuously for ~ 6 and ~ 3 weeks during fall 2012 (Hydrological cycle in the Mediterranean eXperiment – HyMeX) and summer 2013 (Chemistry–Aerosol Mediterranean Experiment – ChArMEx), respectively. It measured simultaneously the water vapor mixing ratio and aerosol optical properties. This article does not aim to describe the IASI operational H2O inversion algorithm, but to compare the vertical profiles derived from IASI onboard (meteorological operational) MetOp-A and the ground-based lidar measurements to assess the reliability of the IASI operational product for the water vapor retrieval in both the lower and middle troposphere. The links between water vapor contents and both the aerosol vertical profiles and the air mass origins are also studied. About 30 simultaneous observations, performed during nighttime in cloud free conditions, have been considered. For altitudes ranging from 2 to 7 km, root mean square errors (correlation) of ~ 0.5 g kg?1 (~ 0.77) and ~ 1.1 g kg?1 (~ 0.72) are derived between the operational IASI product and the available lidar profiles during HyMeX and ChArMEx, respectively. The values of both root mean square error and correlation are meaningful and show that the operational Level-2 product of the IASI-derived vertical water vapor mixing ratio can be considered for meteorological and climatic applications, at least in the framework of field campaigns.

Posted on 16 September 2014 | 12:00 am


Observations of the scale-dependent turbulence and evaluation of the flux–gradient relationship for sensible heat for a closed Douglas-fir canopy in very weak wind conditions

Observations of the scale-dependent turbulence and evaluation of the flux–gradient relationship for sensible heat for a closed Douglas-fir canopy in very weak wind conditions

Atmospheric Chemistry and Physics, 14, 9665-9676, 2014

Author(s): D. Vickers and C. K. Thomas

Observations of the scale-dependent turbulent fluxes, variances, and the bulk transfer parameterization for sensible heat above, within, and beneath a tall closed Douglas-fir canopy in very weak winds are examined. The daytime sub-canopy vertical velocity spectra exhibit a double-peak structure with peaks at timescales of 0.8 s and 51.2 s. A double-peak structure is also observed in the daytime sub-canopy heat flux co-spectra. The daytime momentum flux co-spectra in the upper bole space and in the sub-canopy are characterized by a relatively large cross-wind component, likely due to the extremely light and variable winds, such that the definition of a mean wind direction, and subsequent partitioning of the momentum flux into along- and cross-wind components, has little physical meaning. Positive values of both momentum flux components in the sub-canopy contribute to upward transfer of momentum, consistent with the observed sub-canopy secondary wind speed maximum. For the smallest resolved scales in the canopy at nighttime, we find increasing vertical velocity variance with decreasing timescale, consistent with very small eddies possibly generated by wake shedding from the canopy elements that transport momentum, but not heat. Unusually large values of the velocity aspect ratio within the canopy were observed, consistent with enhanced suppression of the horizontal wind components compared to the vertical by the very dense canopy.

The flux–gradient approach for sensible heat flux is found to be valid for the sub-canopy and above-canopy layers when considered separately in spite of the very small fluxes on the order of a few W m−2 in the sub-canopy. However, single-source approaches that ignore the canopy fail because they make the heat flux appear to be counter-gradient when in fact it is aligned with the local temperature gradient in both the sub-canopy and above-canopy layers. While sub-canopy Stanton numbers agreed well with values typically reported in the literature, our estimates for the above-canopy Stanton number were much larger, which likely leads to underestimated modeled sensible heat fluxes above dark warm closed canopies.

Posted on 16 September 2014 | 12:00 am


Constraining the N2O5 UV absorption cross section from spectroscopic trace gas measurements in the tropical mid-stratosphere

Constraining the N2O5 UV absorption cross section from spectroscopic trace gas measurements in the tropical mid-stratosphere

Atmospheric Chemistry and Physics, 14, 9555-9566, 2014

Author(s): L. Kritten, A. Butz, M. P. Chipperfield, M. Dorf, S. Dhomse, R. Hossaini, H. Oelhaf, C. Prados-Roman, G. Wetzel, and K. Pfeilsticker

The absorption cross section of N2O5, σN2O5(λ, T), which is known from laboratory measurements with the uncertainty of a factor of 2 (Table 4-2 in (Jet Propulsion Laboratory) JPL-2011; the spread in laboratory data, however, points to an uncertainty in the range of 25 to 30%, Sander et al., 2011), was investigated by balloon-borne observations of the relevant trace gases in the tropical mid-stratosphere. The method relies on the observation of the diurnal variation of NO2 by limb scanning DOAS (differential optical absorption spectroscopy) measurements (Weidner et al., 2005; Kritten et al., 2010), supported by detailed photochemical modelling of NOy (NOx(= NO + NO2) + NO3 + 2N2O5 + ClONO2 + HO2NO2 + BrONO2 + HNO3) photochemistry and a non-linear least square fitting of the model result to the NO2 observations. Simulations are initialised with O3 measured by direct sun observations, the NOy partitioning from MIPAS-B (Michelson Interferometer for Passive Atmospheric Sounding – Balloon-borne version) observations in similar air masses at night-time, and all other relevant species from simulations of the SLIMCAT (Single Layer Isentropic Model of Chemistry And Transport) chemical transport model (CTM). Best agreement between the simulated and observed diurnal increase of NO2 is found if the σN2O5(λ, T) is scaled by a factor of 1.6 ± 0.8 in the UV-C (200–260 nm) and by a factor of 0.9 ± 0.26 in the UV-B/A (260–350 nm), compared to current recommendations. As a consequence, at 30 km altitude, the N2O5 lifetime against photolysis becomes a factor of 0.77 shorter at solar zenith angle (SZA) of 30° than using the recommended σN2O5(λ, T), and stays more or less constant at SZAs of 60°. Our scaled N2O5 photolysis frequency slightly reduces the lifetime (0.2–0.6%) of ozone in the tropical mid- and upper stratosphere, but not to an extent to be important for global ozone.

Posted on 16 September 2014 | 12:00 am


Hygroscopic properties and mixing state of aerosol measured at the high-altitude site Puy de Dôme (1465 m a.s.l.), France

Hygroscopic properties and mixing state of aerosol measured at the high-altitude site Puy de Dôme (1465 m a.s.l.), France

Atmospheric Chemistry and Physics, 14, 9537-9554, 2014

Author(s): H. Holmgren, K. Sellegri, M. Hervo, C. Rose, E. Freney, P. Villani, and P. Laj

A Hygroscopicity Tandem Differential Mobility Analyser (HTDMA) was used to evaluate the hygroscopic properties of aerosol particles measured at the Puy de Dôme research station in central France, periodically from September 2008 to January 2010, and almost continuously from October 2010 to December 2012. This high-altitude site is ideally situated to allow for both the upper part of the planetary boundary layer and the lower free troposphere to be sampled. The aim of the study is to investigate both the influence of year-to-year, seasonal and diurnal cycles, as well as the influence of air mass type on particle hygroscopicity and mixing state.

Results show that particle hygroscopicity increases with particle size and depends both on air mass type and on season. Average growth factor values, GFs, are lowest in winter (1.21 ± 0.13, 1.23 ± 0.18 and 1.38 ± 0.25 for 25, 50 and 165 nm particles, respectively) and highest in autumn (1.27 ± 0.11, 1.32 ± 0.12 and 1.49 ± 0.15 for 25, 50 and 165 nm particles, respectively). Particles are generally more hygroscopic at night than during the day. The seasonal and diurnal variations are likely to be strongly influenced by boundary layer dynamics. Furthermore, particles originating from oceanic and continental regions tend to be more hygroscopic than those measured in African and local air masses. The high hygroscopicity of oceanic aerosol can be explained by large proportions of inorganic aerosol and sea salts.

Aerosols measured at the Puy de Dôme display a high degree of external mixing, and hygroscopic growth spectra can be divided into three different hygroscopic modes: a less-hygroscopic mode (GF < 1.3), a hygroscopic mode (GF~1.3–1.7) and a more-hygroscopic mode (GF > 1.7). The majority of particles measured can be classified as being in either the less-hygroscopic mode or the hygroscopic mode, and only few of them have more-hygroscopic properties. The degree of external mixing, evaluated as the fraction of time when the aerosol is found with two or more aerosol populations with different hygroscopic properties, increases with particle size (average yearly values are 20, 28 and 45 {%} for 25, 50, and 165 nm particles, respectively). The degree of external mixing is more sensitive to season than to air mass type, and it is higher in the cold seasons than in the warm seasons.

With more than two years of nearly continuous measurements, this study gathers the results from one of the longest data sets of hygroscopic growth factor measurements to date, allowing a statistically relevant hygroscopic growth parameterization to be determined as a function of both air mass type and season.

Posted on 16 September 2014 | 12:00 am


Volatility basis-set approach simulation of organic aerosol formation in East Asia: implications for anthropogenic–biogenic interaction and controllable amounts

Volatility basis-set approach simulation of organic aerosol formation in East Asia: implications for anthropogenic–biogenic interaction and controllable amounts

Atmospheric Chemistry and Physics, 14, 9513-9535, 2014

Author(s): H. Matsui, M. Koike, Y. Kondo, A. Takami, J. D. Fast, Y. Kanaya, and M. Takigawa

Organic aerosol (OA) simulations using the volatility basis-set approach were made for East Asia and its outflow region. Model simulations were evaluated through comparisons with OA measured by aerosol mass spectrometers in and around Tokyo (at Komaba and Kisai in summer 2003 and 2004) and over the outflow region in East Asia (at Fukue and Hedo in spring 2009). The simulations with aging processes of organic vapors reproduced the mass concentrations, temporal variations, and formation efficiencies of observed OA at all of the sites reasonably well. As OA mass was severely underestimated in the simulations without the aging processes, the oxidations of organic vapors are essential for reasonable OA simulations over East Asia. By considering the aging processes, simulated OA concentrations increased from 0.24 to 1.28 ?g m?3 in the boundary layer over the whole of East Asia. OA formed from the interaction of anthropogenic and biogenic sources was also enhanced by the aging processes. The fraction of controllable OA was estimated to be 87% of total OA over the whole of East Asia, which indicated that most of the OA in our simulations were formed anthropogenically (from controllable combustion sources). A large portion of biogenic secondary OA (78% of biogenic secondary OA) was formed through the influence of anthropogenic sources. These fractions were higher than the fraction of anthropogenic emissions. An important reason for these higher controllable fractions was higher oxidant concentrations and the resulting faster oxidation rates of OA precursors by considering anthropogenic sources. Both the amounts (from 0.18 to 1.12 ?g m−3) and the fraction (from 75 to 87%) of controllable OA were increased by aging processes of organic vapors over East Asia.

Posted on 16 September 2014 | 12:00 am


Sensitivity of high-temperature weather to initial soil moisture: a case study using the WRF model

Sensitivity of high-temperature weather to initial soil moisture: a case study using the WRF model

Atmospheric Chemistry and Physics, 14, 9623-9639, 2014

Author(s): X.-M. Zeng, B. Wang, Y. Zhang, S. Song, X. Huang, Y. Zheng, C. Chen, and G. Wang

Using a succession of 24 h Weather Research and Forecasting model (WRF) simulations, we investigate the sensitivity to initial soil moisture of a short-range high-temperature weather event that occurred in late July 2003 in East China. The initial soil moisture (SMOIS) in the Noah land surface scheme is adjusted (relative to the control run, CTL) for four groups of simulations: DRY25 (?25%), DRY50 (?50%), WET25 (+25%) and WET50 (+50%). Ten 24 h integrations are performed in each group.

We focus on 2 m surface air temperature (SAT) greater than 35 °C (the threshold of "high-temperature" events in China) at 06:00 UTC (roughly 14:00 LT in the study domain) to analyse the occurrence of the high-temperature event. The 10-day mean results show that the 06:00 UTC SAT (SAT06) is sensitive to the SMOIS change; specifically, SAT06 exhibits an apparent increase with the SMOIS decrease (e.g. compared with CTL, DRY25 generally results in a 1 °C SAT06 increase over the land surface of East China), areas with 35 °C or higher SAT06 are the most affected, and the simulations are more sensitive to the SMOIS decrease than to the SMOIS increase, which suggests that hot weather can be amplified under low soil moisture conditions. Regarding the mechanism underlying the extremely high SAT06, sensible heat flux has been shown to directly heat the lower atmosphere, and latent heat flux has been found to be more sensitive to the SMOIS change, resulting in an overall increase in surface net radiation due to the increased greenhouse effect (e.g. with the SMOIS increase from DRY25 to CTL, the 10-day mean net radiation increases by 5 W m−2). Additionally, due to the unique and dynamic nature of the western Pacific subtropical high, negative feedback occurs between the regional atmospheric circulation and the air temperature in the lower atmosphere while positive feedback occurs in the mid-troposphere.

Using a method based on an analogous temperature relationship, a detailed analysis of the physical processes shows that for the SAT change, the SMOIS change affects diabatic processes (e.g. surface fluxes) more strongly than the adiabatic process of subsidence in the western Pacific subtropical high in the five groups of simulations. Interestingly, although diabatic processes dominate subsidence during the daytime and night-time separately, they do not necessarily dominate during the 24 h periods (e.g. they are dominant in the WET and CTL simulations only). Further, as the SMOIS decreases, the SAT06 increases, which is largely due to the reduced cooling effect of the diabatic processes, rather than the warming effect of subsidence.

Unlike previous studies on heatwave events at climate timescales, this paper presents the sensitivity of simulated short-term hot weather to initial soil moisture and emphasises the importance of appropriate soil moisture initialization when simulating hot weather.

Posted on 16 September 2014 | 12:00 am


Downslope föhn winds over the Antarctic Peninsula and their effect on the Larsen ice shelves

Downslope föhn winds over the Antarctic Peninsula and their effect on the Larsen ice shelves

Atmospheric Chemistry and Physics, 14, 9481-9509, 2014

Author(s): D. P. Grosvenor, J. C. King, T. W. Choularton, and T. Lachlan-Cope

Mesoscale model simulations are presented of a westerly föhn event over the Antarctic Peninsula mountain ridge and onto the Larsen C ice shelf, just south of the recently collapsed Larsen B ice shelf. Aircraft observations showed the presence of föhn jets descending near the ice shelf surface with maximum wind speeds at 250–350 m in height. Surface flux measurements suggested that melting was occurring. Simulated profiles of wind speed, temperature and wind direction were very similar to the observations. However, the good match only occurred at a model time corresponding to ~9 h before the aircraft observations were made since the model föhn jets died down after this. This was despite the fact that the model was nudged towards analysis for heights greater than ~1.15 km above the surface.

Timing issues aside, the otherwise good comparison between the model and observations gave confidence that the model flow structure was similar to that in reality. Details of the model jet structure are explored and discussed and are found to have ramifications for the placement of automatic weather station (AWS) stations on the ice shelf in order to detect föhn flow. Cross sections of the flow are also examined and were found to compare well to the aircraft measurements. Gravity wave breaking above the mountain crest likely created a~situation similar to hydraulic flow and allowed föhn flow and ice shelf surface warming to occur despite strong upwind blocking, which in previous studies of this region has generally not been considered. Our results therefore suggest that reduced upwind blocking, due to wind speed increases or stability decreases, might not result in an increased likelihood of föhn events over the Antarctic Peninsula, as previously suggested.

The surface energy budget of the model during the melting periods showed that the net downwelling short-wave surface flux was the largest contributor to the melting energy, indicating that the cloud clearing effect of föhn events is likely to be the most important factor for increased melting relative to non-föhn days. The results also indicate that the warmth of the föhn jets through sensible heat flux ("SH") may not be critical in causing melting beyond boundary layer stabilisation effects (which may help to prevent cloud cover and suppress loss of heat by convection) and are actually cancelled by latent heat flux ("LH") effects (snow ablation). It was found that ground heat flux ("GRD") was likely to be an important factor when considering the changing surface energy budget for the southern regions of the ice shelf as the climate warms.

Posted on 16 September 2014 | 12:00 am


Trace gas emissions from combustion of peat, crop residue, domestic biofuels, grasses, and other fuels: configuration and Fourier transform infrared (FTIR) component of the fourth Fire Lab at Missoula Experiment (FLAME-4)

Trace gas emissions from combustion of peat, crop residue, domestic biofuels, grasses, and other fuels: configuration and Fourier transform infrared (FTIR) component of the fourth Fire Lab at Missoula Experiment (FLAME-4)

Atmospheric Chemistry and Physics, 14, 9727-9754, 2014

Author(s): C. E. Stockwell, R. J. Yokelson, S. M. Kreidenweis, A. L. Robinson, P. J. DeMott, R. C. Sullivan, J. Reardon, K. C. Ryan, D. W. T. Griffith, and L. Stevens

During the fourth Fire Lab at Missoula Experiment (FLAME-4, October–November 2012) a large variety of regionally and globally significant biomass fuels was burned at the US Forest Service Fire Sciences Laboratory in Missoula, Montana. The particle emissions were characterized by an extensive suite of instrumentation that measured aerosol chemistry, size distribution, optical properties, and cloud-nucleating properties. The trace gas measurements included high-resolution mass spectrometry, one- and two-dimensional gas chromatography, and open-path Fourier transform infrared (OP-FTIR) spectroscopy. This paper summarizes the overall experimental design for FLAME-4 – including the fuel properties, the nature of the burn simulations, and the instrumentation employed – and then focuses on the OP-FTIR results. The OP-FTIR was used to measure the initial emissions of 20 trace gases: CO2, CO, CH4, C2H2, C2H4, C3H6, HCHO, HCOOH, CH3OH, CH3COOH, glycolaldehyde, furan, H2O, NO, NO2, HONO, NH3, HCN, HCl, and SO2. These species include most of the major trace gases emitted by biomass burning, and for several of these compounds, this is the first time their emissions are reported for important fuel types. The main fire types included African grasses, Asian rice straw, cooking fires (open (three-stone), rocket, and gasifier stoves), Indonesian and extratropical peat, temperate and boreal coniferous canopy fuels, US crop residue, shredded tires, and trash. Comparisons of the OP-FTIR emission factors (EFs) and emission ratios (ERs) to field measurements of biomass burning verify that the large body of FLAME-4 results can be used to enhance the understanding of global biomass burning and its representation in atmospheric chemistry models.

Crop residue fires are widespread globally and account for the most burned area in the US, but their emissions were previously poorly characterized. Extensive results are presented for burning rice and wheat straw: two major global crop residues. Burning alfalfa produced the highest average NH3 EF observed in the study (6.63 ± 2.47 g kg?1), while sugar cane fires produced the highest EF for glycolaldehyde (6.92 g kg?1) and other reactive oxygenated organic gases such as HCHO, HCOOH, and CH3COOH. Due to the high sulfur and nitrogen content of tires, they produced the highest average SO2 emissions (26.2 ± 2.2 g kg?1) and high NOx and HONO emissions. High variability was observed for peat fire emissions, but they were consistently characterized by large EFs for NH3 (1.82 ± 0.60 g kg?1) and CH4 (10.8 ± 5.6 g kg?1). The variability observed in peat fire emissions, the fact that only one peat fire had previously been subject to detailed emissions characterization, and the abundant emissions from tropical peatlands all impart high value to our detailed measurements of the emissions from burning three Indonesian peat samples. This study also provides the first EFs for HONO and NO2 for Indonesian peat fires. Open cooking fire emissions of HONO and HCN are reported for the first time, and the first emissions data for HCN, NO, NO2, HONO, glycolaldehyde, furan, and SO2 are reported for "rocket" stoves: a common type of improved cookstove. The HCN / CO emission ratios for cooking fires (1.72 × 10?3 ± 4.08 × 10−4) and peat fires (1.45 × 10?2 ± 5.47 × 10?3) are well below and above the typical values for other types of biomass burning, respectively. This would affect the use of HCN / CO observations for source apportionment in some regions. Biomass burning EFs for HCl are rare and are reported for the first time for burning African savanna grasses. High emissions of HCl were also produced by burning many crop residues and two grasses from coastal ecosystems. HCl could be the main chlorine-containing gas in very fresh smoke, but rapid partitioning to aerosol followed by slower outgassing probably occurs.

Posted on 16 September 2014 | 12:00 am


Advances in understanding and parameterization of small-scale physical processes in the marine Arctic climate system: a review

Advances in understanding and parameterization of small-scale physical processes in the marine Arctic climate system: a review

Atmospheric Chemistry and Physics, 14, 9403-9450, 2014

Author(s): T. Vihma, R. Pirazzini, I. Fer, I. A. Renfrew, J. Sedlar, M. Tjernström, C. Lüpkes, T. Nygård, D. Notz, J. Weiss, D. Marsan, B. Cheng, G. Birnbaum, S. Gerland, D. Chechin, and J. C. Gascard

The Arctic climate system includes numerous highly interactive small-scale physical processes in the atmosphere, sea ice, and ocean. During and since the International Polar Year 2007–2009, significant advances have been made in understanding these processes. Here, these recent advances are reviewed, synthesized, and discussed. In atmospheric physics, the primary advances have been in cloud physics, radiative transfer, mesoscale cyclones, coastal, and fjordic processes as well as in boundary layer processes and surface fluxes. In sea ice and its snow cover, advances have been made in understanding of the surface albedo and its relationships with snow properties, the internal structure of sea ice, the heat and salt transfer in ice, the formation of superimposed ice and snow ice, and the small-scale dynamics of sea ice. For the ocean, significant advances have been related to exchange processes at the ice–ocean interface, diapycnal mixing, double-diffusive convection, tidal currents and diurnal resonance. Despite this recent progress, some of these small-scale physical processes are still not sufficiently understood: these include wave–turbulence interactions in the atmosphere and ocean, the exchange of heat and salt at the ice–ocean interface, and the mechanical weakening of sea ice. Many other processes are reasonably well understood as stand-alone processes but the challenge is to understand their interactions with and impacts and feedbacks on other processes. Uncertainty in the parameterization of small-scale processes continues to be among the greatest challenges facing climate modelling, particularly in high latitudes. Further improvements in parameterization require new year-round field campaigns on the Arctic sea ice, closely combined with satellite remote sensing studies and numerical model experiments.

Posted on 10 September 2014 | 12:00 am


Will the role of intercontinental transport change in a changing climate?

Will the role of intercontinental transport change in a changing climate?

Atmospheric Chemistry and Physics, 14, 9379-9402, 2014

Author(s): T. Glotfelty, Y. Zhang, P. Karamchandani, and D. G. Streets

Intercontinental transport of atmospheric pollution (ITAP) can offset the impact of local emission control efforts, impact human and ecosystem health, and play a role in climate forcing. This study aims to determine the role of ITAP caused by East Asian anthropogenic emissions (EAAEs) under current and future emission and climate scenarios. The contribution from EAAEs is determined using a "brute force method" in which results from simulations with and without EAAEs are compared. ITAP from East Asia is enhanced in the future due to faster wind speeds aloft and a stronger low pressure center near eastern Russia that facilitate enhanced westerly export in the free troposphere and stronger southerly transport near the surface, increased gaseous precursor emissions, and increased temperatures. As a result, the contribution of ozone (O3) generated by EAAEs to the global average O3 mixing ratio increases by ~0.8 ppb from 1.2 ppb in 2001 to 2.0 ppb in 2050. The contribution of PM2.5 generated by EAAEs to the global PM2.5 level increases by ~0.07 ?g m?3 from 0.32 ?g m?3 in 2001 to 0.39 ?g m?3 in 2050, despite a non-homogenous response in PM2.5 resulting from cloud and radiative feedbacks. EAAEs can increase East Asian biogenic secondary organic aerosol by 10–81%, indicating that it is largely controllable. EAAEs also increase the deposition of nitrogen, black carbon, and mercury both locally and downwind, implying that they may play a role in climate feedbacks and ecosystem health of these regions. These results show that EAAEs have a large impact on global air quality and climate, especially on downwind regions. Such impacts may be enhanced under future climate and emission scenarios, demonstrating a need to synergize global pollution control and climate mitigation efforts.

Posted on 10 September 2014 | 12:00 am


The effects of nitrate on the heterogeneous uptake of sulfur dioxide on hematite

The effects of nitrate on the heterogeneous uptake of sulfur dioxide on hematite

Atmospheric Chemistry and Physics, 14, 9451-9467, 2014

Author(s): L. D. Kong, X. Zhao, Z. Y. Sun, Y. W. Yang, H. B. Fu, S. C. Zhang, T. T. Cheng, X. Yang, L. Wang, and J. M. Chen

Nitrate is often found to be associated with atmospheric particles. Surface nitrate can change the hygroscopicity of these particles, and thus impact their chemical reactivity. However, the influence of nitrate on heterogeneous reactions of atmospheric trace gases is poorly understood. In this work, the effects of nitrate on heterogeneous conversion of SO2 with hematite at 298 K are investigated using an in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and a White cell coupled with Fourier transform infrared spectroscopy (White cell-FTIR). It is found that nitrate participates in heterogeneous reactions of SO2, accelerates the formation rate of sulfate, and leads to the formation of surface-adsorbed HNO3 and gas-phase N2O and HONO. The results indicate that low to moderate amounts of nitrate significantly enhance the reactivity of hematite–nitrate mixtures, the uptake of SO2, and the formation of sulfate on hematite. For mixtures, the sample containing 24% nitrate exhibits the highest sulfate formation rate, and its corresponding uptake coefficient calculated by geometric surface area is about 5.5 times higher than that of hematite alone. The sample containing 48% nitrate presents the highest Brunauer–Emmett–Teller (BET) uptake coefficient, and the value is about 8 times higher than that of pure hematite. No uptake of SO2 and formation of sulfate are observed on pure nitrate. Evidence presented herein implies a significant contribution of the unreleased HNO3 and HONO in the particles for the conversion of SO2 and the enhanced formation of sulfate in the atmosphere. A possible mechanism for the influence of nitrate on the heterogeneous conversion of SO2 on hematite is proposed, and atmospheric implications based on these results are discussed.

Posted on 10 September 2014 | 12:00 am


Greenhouse gas network design using backward Lagrangian particle dispersion modelling − Part 1: Methodology and Australian test case

Greenhouse gas network design using backward Lagrangian particle dispersion modelling − Part 1: Methodology and Australian test case

Atmospheric Chemistry and Physics, 14, 9363-9378, 2014

Author(s): T. Ziehn, A. Nickless, P. J. Rayner, R. M. Law, G. Roff, and P. Fraser

This paper describes the generation of optimal atmospheric measurement networks for determining carbon dioxide fluxes over Australia using inverse methods. A Lagrangian particle dispersion model is used in reverse mode together with a Bayesian inverse modelling framework to calculate the relationship between weekly surface fluxes, comprising contributions from the biosphere and fossil fuel combustion, and hourly concentration observations for the Australian continent. Meteorological driving fields are provided by the regional version of the Australian Community Climate and Earth System Simulator (ACCESS) at 12 km resolution at an hourly timescale. Prior uncertainties are derived on a weekly timescale for biosphere fluxes and fossil fuel emissions from high-resolution model runs using the Community Atmosphere Biosphere Land Exchange (CABLE) model and the Fossil Fuel Data Assimilation System (FFDAS) respectively. The influence from outside the modelled domain is investigated, but proves to be negligible for the network design. Existing ground-based measurement stations in Australia are assessed in terms of their ability to constrain local flux estimates from the land. We find that the six stations that are currently operational are already able to reduce the uncertainties on surface flux estimates by about 30%. A candidate list of 59 stations is generated based on logistic constraints and an incremental optimisation scheme is used to extend the network of existing stations. In order to achieve an uncertainty reduction of about 50%, we need to double the number of measurement stations in Australia. Assuming equal data uncertainties for all sites, new stations would be mainly located in the northern and eastern part of the continent.

Posted on 10 September 2014 | 12:00 am


Evaluation of IASI-derived dust aerosol characteristics over the tropical belt

Evaluation of IASI-derived dust aerosol characteristics over the tropical belt

Atmospheric Chemistry and Physics, 14, 9343-9362, 2014

Author(s): V. Capelle, A. Chédin, M. Siméon, C. Tsamalis, C. Pierangelo, M. Pondrom, C. Crevoisier, L. Crepeau, and N. A. Scott

IASI (Infrared Atmospheric Sounder Interferometer)-derived monthly mean infrared (10 ?m) dust aerosol optical depth (AOD) and altitude are evaluated against ground-based Aerosol RObotic NETwork of sun photometers (AERONET) measurements of the 500 nm coarse-mode AOD and CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) measurements of altitude at 38 AERONET sites (sea and land) within the tropical belt (30° N–30° S). The period covered extends from July 2007 to June 2013. The evaluation goes through the analysis of Taylor diagrams and box-and-whiskers plots, separating situations over oceanic regions and over land. For the AOD, such an evaluation raises the problem of the difference between the two spectral domains used: infrared for IASI and visible for AERONET. Consequently, the two measurements do not share the same metrics. For that reason, AERONET coarse-mode AOD is first "translated" into IASI-equivalent infrared AOD. This is done by the determination, site by site, of an infrared to visible AOD ratio. Because translating visible coarse-mode AOD into infrared AOD requires accurate knowledge of variables, such as the infrared refractive index or the particle size distribution, quantifying the bias between these two sources of AOD is not straightforward. This problem is detailed in this paper, in particular in Appendix A. For the sites over oceanic regions, the overall AOD temporal correlation comes to 0.86 for 786 items (IASI and AERONET monthly mean bins). The overall normalized standard deviation (i.e. ratio of the standard deviation of the test data (IASI) to that of the reference data (AERONET)) is 0.93, close to the desired value of 1. Over land, essentially desert, correlation is 0.74 for 619 items and the normalized standard deviation is 0.86. This slight but significant degradation over land most probably results from the greater complexity of the surface (heterogeneity, elevation) and, to a lesser extent, to the episodic presence of dust within the boundary layer (particularly for sites close to active sources) to which IASI, as any thermal infrared sounder, is poorly sensitive, unlike AERONET. Site by site, disparities appear that are principally due to either the insufficient number of AERONET observations throughout the period considered, to the complexity of the location leading to the mixing of several aerosol types (in the case of the Persian Gulf, for example), to surface heterogeneities (elevation, emissivity, etc.), or to the use of a single aerosol model (called "MITR"). Results using another aerosol model, with a different refractive index, are presented and discussed. Concerning altitude over oceanic regions, correlation is 0.70 for 853 items and the normalized standard deviation is 0.92. A systematic bias of ?0.4 km (IASI–CALIOP) is observed, with a standard deviation of 0.48 km. This result is satisfactory, considering the important differences between the two instruments (space–time coverage, definition of the altitude). Altitude results over land, essentially over deserts, are not satisfactory for a majority of sites. The smaller sensitivity of IASI to altitude compared to its sensitivity to AOD, added to the difficulties met for the determination of the AOD over land (surface heterogeneities), explain this result. Work is in progress to solve this difficulty.

We conclude that the present results demonstrate the usefulness of IASI data, which are planned to cover a long period of time, as an additional constraint to a better knowledge of the impact of aerosols on the climate system.

Posted on 10 September 2014 | 12:00 am


On the wintertime low bias of Northern Hemisphere carbon monoxide found in global model simulations

On the wintertime low bias of Northern Hemisphere carbon monoxide found in global model simulations

Atmospheric Chemistry and Physics, 14, 9295-9316, 2014

Author(s): O. Stein, M. G. Schultz, I. Bouarar, H. Clark, V. Huijnen, A. Gaudel, M. George, and C. Clerbaux

Despite the developments in the global modelling of chemistry and of the parameterization of the physical processes, carbon monoxide (CO) concentrations remain underestimated during Northern Hemisphere (NH) winter by most state-of-the-art chemistry transport models. The consequential model bias can in principle originate from either an underestimation of CO sources or an overestimation of its sinks. We address both the role of surface sources and sinks with a series of MOZART (Model for Ozone And Related Tracers) model sensitivity studies for the year 2008 and compare our results to observational data from ground-based stations, satellite observations, and vertical profiles from measurements on passenger aircraft. In our base case simulation using MACCity (Monitoring Atmospheric Composition and Climate project) anthropogenic emissions, the near-surface CO mixing ratios are underestimated in the Northern Hemisphere by more than 20 ppb from December to April, with the largest bias of up to 75 ppb over Europe in January. An increase in global biomass burning or biogenic emissions of CO or volatile organic compounds (VOCs) is not able to reduce the annual course of the model bias and yields concentrations over the Southern Hemisphere which are too high. Raising global annual anthropogenic emissions with a simple scaling factor results in overestimations of surface mixing ratios in most regions all year round. Instead, our results indicate that anthropogenic CO and, possibly, VOC emissions in the MACCity inventory are too low for the industrialized countries only during winter and spring. Reasonable agreement with observations can only be achieved if the CO emissions are adjusted seasonally with regionally varying scaling factors. A part of the model bias could also be eliminated by exchanging the original resistance-type dry deposition scheme with a parameterization for CO uptake by oxidation from soil bacteria and microbes, which reduces the boreal winter dry deposition fluxes. The best match to surface observations, satellite retrievals, and aircraft observations was achieved when the modified dry deposition scheme was combined with increased wintertime road traffic emissions over Europe and North America (factors up to 4.5 and 2, respectively). One reason for the apparent underestimation of emissions may be an exaggerated downward trend in the Representative Concentration Pathway (RCP) 8.5 scenario in these regions between 2000 and 2010, as this scenario was used to extrapolate the MACCity emissions from their base year 2000. This factor is potentially amplified by a lack of knowledge about the seasonality of emissions. A methane lifetime of 9.7 yr for our basic model and 9.8 yr for the optimized simulation agrees well with current estimates of global OH, but we cannot fully exclude a potential effect from errors in the geographical and seasonal distribution of OH concentrations on the modelled CO.

Posted on 9 September 2014 | 12:00 am


Contributions of vehicular carbonaceous aerosols to PM2.5 in a roadside environment in Hong Kong

Contributions of vehicular carbonaceous aerosols to PM2.5 in a roadside environment in Hong Kong

Atmospheric Chemistry and Physics, 14, 9279-9293, 2014

Author(s): X. H. H. Huang, Q. J. Bian, P. K. K. Louie, and J. Z. Yu

Hourly measurements of elemental carbon (EC) and organic carbon (OC) were made at Mong Kok, a roadside air quality monitoring station in Hong Kong, for a year, from May 2011 to April 2012. The monthly average EC concentrations were 3.8–4.9 ?g C m?3, accounting for 9.2–17.7% of the PM2.5 mass (21.5–49.7 ?g m?3). The EC concentrations showed little seasonal variation and peaked twice daily, coinciding with the traffic rush hours of a day. Strong correlations were found between EC and NOx concentrations, especially during the rush hours in the morning, confirming vehicular emissions as the dominant source of EC at this site. The analysis by means of the minimum OC / EC ratio approach to determine the OC / EC ratio representative of primary vehicular emissions yields a value of 0.5 for (OC / EC)vehicle. By applying the derived (OC / EC)vehicle ratio to the data set, the monthly average vehicle-related OC was estimated to account for 17–64% of the measured OC throughout the year. Vehicle-related OC was also estimated using receptor modeling of a combined data set of hourly NOx, OC, EC and volatile organic compounds characteristic of different types of vehicular emissions. The OCvehicle estimations by the two different approaches were in good agreement. When both EC and vehicle-derived organic matter (OM) (assuming an OM-to-OC ratio of 1.4) are considered, vehicular carbonaceous aerosols contributed ~ 7.3 ?g m?3 to PM2.5, accounting for ~ 20% of PM2.5 mass (38.3 ?g m?3) during winter, when Hong Kong received significant influence of air pollutants transported from outside, and ~ 30% of PM2.5 mass (28.2 ?g m?3) during summertime, when local emission sources were dominant. A reduction of 3.8 ?g m?3 in vehicular carbonaceous aerosols was estimated during 07:00–11:00 (i.e., rush hours on weekdays) on Sundays and public holidays. This could mainly be attributed to less on-road public transportation (e.g., diesel-powered buses) in comparison with non-holidays. These multiple lines of evidence confirm local vehicular emissions as an important source of PM2.5 in an urban roadside environment and suggest the importance of vehicular emission control in reducing exposure to PM2.5 in busy roadside environments.

Posted on 9 September 2014 | 12:00 am


Global data set of biogenic VOC emissions calculated by the MEGAN model over the last 30 years

Global data set of biogenic VOC emissions calculated by the MEGAN model over the last 30 years

Atmospheric Chemistry and Physics, 14, 9317-9341, 2014

Author(s): K. Sindelarova, C. Granier, I. Bouarar, A. Guenther, S. Tilmes, T. Stavrakou, J.-F. Müller, U. Kuhn, P. Stefani, and W. Knorr

The Model of Emissions of Gases and Aerosols from Nature (MEGANv2.1) together with the Modern-Era Retrospective Analysis for Research and Applications (MERRA) meteorological fields were used to create a global emission data set of biogenic volatile organic compounds (BVOC) available on a monthly basis for the time period of 1980–2010. This data set, developed under the Monitoring Atmospheric Composition and Climate project (MACC), is called MEGAN–MACC. The model estimated mean annual total BVOC emission of 760 Tg (C) yr?1 consisting of isoprene (70%), monoterpenes (11%), methanol (6%), acetone (3%), sesquiterpenes (2.5%) and other BVOC species each contributing less than 2%.

Several sensitivity model runs were performed to study the impact of different model input and model settings on isoprene estimates and resulted in differences of up to ±17% of the reference isoprene total. A greater impact was observed for a sensitivity run applying parameterization of soil moisture deficit that led to a 50% reduction of isoprene emissions on a global scale, most significantly in specific regions of Africa, South America and Australia.

MEGAN–MACC estimates are comparable to results of previous studies. More detailed comparison with other isoprene inventories indicated significant spatial and temporal differences between the data sets especially for Australia, Southeast Asia and South America. MEGAN–MACC estimates of isoprene, ?-pinene and group of monoterpenes showed a reasonable agreement with surface flux measurements at sites located in tropical forests in the Amazon and Malaysia. The model was able to capture the seasonal variation of isoprene emissions in the Amazon forest.

Posted on 9 September 2014 | 12:00 am


Photochemical roles of rapid economic growth and potential abatement strategies on tropospheric ozone over South and East Asia in 2030

Photochemical roles of rapid economic growth and potential abatement strategies on tropospheric ozone over South and East Asia in 2030

Atmospheric Chemistry and Physics, 14, 9259-9277, 2014

Author(s): S. Chatani, M. Amann, A. Goel, J. Hao, Z. Klimont, A. Kumar, A. Mishra, S. Sharma, S. X. Wang, Y. X. Wang, and B. Zhao

A regional air quality simulation framework including the Weather Research and Forecasting modeling system (WRF), the Community Multi-scale Air Quality modeling system (CMAQ), and precursor emissions to simulate tropospheric ozone over South and East Asia is introduced. Concentrations of tropospheric ozone and related species simulated by the framework are validated by comparing with observation data of surface monitoring, ozonesondes, and satellites obtained in 2010. The simulation demonstrates acceptable performance on tropospheric ozone over South and East Asia at regional scale. Future energy consumption, carbon dioxide (CO2), nitrogen oxides (NOx), and volatile organic compound (VOC) emissions in 2030 under three future scenarios are estimated. One of the scenarios assumes a business-as-usual (BAU) pathway, and other two scenarios consider implementation of additional energy and environmental strategies to reduce energy consumption, CO2, NOx, and VOC emissions in China and India. Future surface ozone under these three scenarios is predicted by the simulation. The simulation indicates future surface ozone significantly increases around India for a whole year and around northeastern China in summer. NOx is a main driver on significant seasonal increase of surface ozone, whereas VOC as well as increasing background ozone and methane is also an important factor on annual average of surface ozone in East Asia. Warmer weather around India is also preferable for significant increase of surface ozone. Additional energy and environmental strategies assumed in future scenarios are expected to be effective to reduce future surface ozone over South and East Asia.

Posted on 9 September 2014 | 12:00 am


Global emissions of HFC-143a (CH3CF3) and HFC-32 (CH2F2) from in situ and air archive atmospheric observations

Global emissions of HFC-143a (CH3CF3) and HFC-32 (CH2F2) from in situ and air archive atmospheric observations

Atmospheric Chemistry and Physics, 14, 9249-9258, 2014

Author(s): S. O'Doherty, M. Rigby, J. Mühle, D. J. Ivy, B. R. Miller, D. Young, P. G. Simmonds, S. Reimann, M. K. Vollmer, P. B. Krummel, P. J. Fraser, L. P. Steele, B. Dunse, P. K. Salameh, C. M. Harth, T. Arnold, R. F. Weiss, J. Kim, S. Park, S. Li, C. Lunder, O. Hermansen, N. Schmidbauer, L. X. Zhou, B. Yao, R. H. J. Wang, A. J. Manning, and R. G. Prinn

High-frequency, in situ observations from the Advanced Global Atmospheric Gases Experiment (AGAGE), for the period 2003 to 2012, combined with archive flask measurements dating back to 1977, have been used to capture the rapid growth of HFC-143a (CH3CF3) and HFC-32 (CH2F2) mole fractions and emissions into the atmosphere. Here we report the first in situ global measurements of these two gases. HFC-143a and HFC-32 are the third and sixth most abundant hydrofluorocarbons (HFCs) respectively and they currently make an appreciable contribution to the HFCs in terms of atmospheric radiative forcing (1.7 ± 0.04 and 0.7 ± 0.02 mW m−2 in 2012 respectively). In 2012 the global average mole fraction of HFC-143a was 13.4 ± 0.3 ppt (1?) in the lower troposphere and its growth rate was 1.4 ± 0.04 ppt yr?1; HFC-32 had a global mean mole fraction of 6.2 ± 0.2 ppt and a growth rate of 1.1 ± 0.04 ppt yr?1 in 2012. The extensive observations presented in this work have been combined with an atmospheric transport model to simulate global atmospheric abundances and derive global emission estimates. It is estimated that 23 ± 3 Gg yr?1 of HFC-143a and 21 ± 11 Gg yr?1 of HFC-32 were emitted globally in 2012, and the emission rates are estimated to be increasing by 7 ± 5% yr?1 for HFC-143a and 14 ± 11% yr?1 for HFC-32.

Posted on 9 September 2014 | 12:00 am


Estimation of mineral dust long-wave radiative forcing: sensitivity study to particle properties and application to real cases in the region of Barcelona

Estimation of mineral dust long-wave radiative forcing: sensitivity study to particle properties and application to real cases in the region of Barcelona

Atmospheric Chemistry and Physics, 14, 9213-9231, 2014

Author(s): M. Sicard, S. Bertolín, M. Mallet, P. Dubuisson, and A. Comerón

The aerosol radiative effect in the long-wave (LW) spectral range is sometimes not taken into account in atmospheric aerosol forcing studies at local scale because the LW aerosol effect is assumed to be negligible. At regional and global scale this effect is partially taken into account: aerosol absorption is taken into account but scattering is still neglected. However, aerosols with strong absorbing and scattering properties in the LW region, like mineral dust, can have a non-negligible radiative effect in the LW spectral range (both at surface and top of the atmosphere) which can counteract their cooling effect occurring in the short-wave spectral range. The first objective of this research is to perform a sensitivity study of mineral dust LW radiative forcing (RF) as a function of dust microphysical and optical properties using an accurate radiative transfer model which can compute vertically resolved short-wave and long-wave aerosol RF. Radiative forcing simulations in the LW range have shown an important sensitivity to the following parameters: aerosol load, radius of the coarse mode, refractive index, aerosol vertical distribution, surface temperature and surface albedo. The scattering effect has been estimated to contribute to the LW RF up to 18% at the surface and up to 38% at the top of the atmosphere. The second objective is the estimation of the short-wave and long-wave dust RF for 11 dust outbreaks observed in Barcelona. At the surface, the LW RF varies between +2.8 and +10.2 W m?2, which represents between 11 and 26% (with opposite sign) of the SW component, while at the top of the atmosphere the LW RF varies between +0.6 and +5.8 W m?2, which represents between 6 and 26% (with opposite sign) of the SW component.

Posted on 8 September 2014 | 12:00 am


Technical Note: Application of positive matrix factor analysis in heterogeneous kinetics studies utilizing the mixed-phase relative rates technique

Technical Note: Application of positive matrix factor analysis in heterogeneous kinetics studies utilizing the mixed-phase relative rates technique

Atmospheric Chemistry and Physics, 14, 9201-9211, 2014

Author(s): Y. Liu, S.-M. Li, and J. Liggio

The mixed-phase relative rates approach for determining aerosol particle organic heterogeneous reaction kinetics is often performed utilizing mass spectral tracers as a proxy for particle-phase reactant concentration. However, this approach may be influenced by signal contamination from oxidation products during the experiment. In the current study, the mixed-phase relative rates technique has been improved by combining a positive matrix factor (PMF) analysis with electron ionization aerosol mass spectrometry (unit-mass resolution), thereby removing the influence of m / z fragments from reaction products on the reactant signals. To demonstrate the advantages of this approach, the heterogeneous reaction between OH radicals and citric acid (CA) was investigated using a photochemical flow tube coupled to a compact time-of-flight aerosol mass spectrometer (C-ToF-AMS). The measured heterogeneous rate constant (k2) of citric acid toward OH was (3.31 ± 0.29) × 10−12 cm3 molecule?1 s?1 at 298 K and (30 ± 3)% relative humidity (RH) and was several times greater than the results utilizing individual m / z fragments. This phenomenon was further evaluated for particulate-phase organophosphates (triphenyl phosphate (TPhP), tris-1,3-dichloro-2-propyl phosphate (TDCPP) and tris-2-ethylhexyl phosphate (TEHP)), leading to k2 values significantly larger than previously reported. The results suggest that heterogeneous kinetics can be significantly underestimated when the structure of the products is highly similar to the reactant and when a non-molecular tracer is measured with a unit-mass resolution aerosol mass spectrometer. The results also suggest that the heterogeneous lifetime of organic aerosol in models can be overestimated due to underestimated OH uptake coefficients. Finally, a comparison of reported rate constants implies that the heterogeneous oxidation of aerosols will be dependent upon a number of factors related to the reaction system, and that a single rate constant for one system cannot be universally applied under all conditions.

Posted on 8 September 2014 | 12:00 am


Compilation and evaluation of gas phase diffusion coefficients of reactive trace gases in the atmosphere: volume 1. Inorganic compounds

Compilation and evaluation of gas phase diffusion coefficients of reactive trace gases in the atmosphere: volume 1. Inorganic compounds

Atmospheric Chemistry and Physics, 14, 9233-9247, 2014

Author(s): M. J. Tang, R. A. Cox, and M. Kalberer

Diffusion of gas molecules to the surface is the first step for all gas–surface reactions. Gas phase diffusion can influence and sometimes even limit the overall rates of these reactions; however, there is no database of the gas phase diffusion coefficients of atmospheric reactive trace gases. Here we compile and evaluate, for the first time, the diffusivities (pressure-independent diffusion coefficients) of atmospheric inorganic reactive trace gases reported in the literature. The measured diffusivities are then compared with estimated values using a semi-empirical method developed by Fuller et al. (1966). The diffusivities estimated using Fuller's method are typically found to be in good agreement with the measured values within ±30%, and therefore Fuller's method can be used to estimate the diffusivities of trace gases for which experimental data are not available. The two experimental methods used in the atmospheric chemistry community to measure the gas phase diffusion coefficients are also discussed. A different version of this compilation/evaluation, which will be updated when new data become available, is uploaded online (https://sites.google.com/site/mingjintang/home/diffusion).

Posted on 8 September 2014 | 12:00 am


Improvement and further development in CESM/CAM5: gas-phase chemistry and inorganic aerosol treatments

Improvement and further development in CESM/CAM5: gas-phase chemistry and inorganic aerosol treatments

Atmospheric Chemistry and Physics, 14, 9171-9200, 2014

Author(s): J. He and Y. Zhang

Gas-phase chemistry and subsequent gas-to-particle conversion processes such as new particle formation, condensation, and thermodynamic partitioning have large impacts on air quality, climate, and public health through influencing the amounts and distributions of gaseous precursors and secondary aerosols. Their roles in global air quality and climate are examined in this work using the Community Earth System Model version 1.0.5 (CESM1.0.5) with the Community Atmosphere Model version 5.1 (CAM5.1) (referred to as CESM1.0.5/CAM5.1). CAM5.1 includes a simple chemistry that is coupled with a 7-mode prognostic Modal Aerosol Model (MAM7). MAM7 includes classical homogenous nucleation (binary and ternary) and activation nucleation (empirical first-order power law) parameterizations, and a highly simplified inorganic aerosol thermodynamics treatment that only simulates particulate-phase sulfate and ammonium. In this work, a new gas-phase chemistry mechanism based on the 2005 Carbon Bond Mechanism for Global Extension (CB05_GE) and several advanced inorganic aerosol treatments for condensation of volatile species, ion-mediated nucleation (IMN), and explicit inorganic aerosol thermodynamics for sulfate, ammonium, nitrate, sodium, and chloride have been incorporated into CESM/CAM5.1-MAM7. Compared to the simple gas-phase chemistry, CB05_GE can predict many more gaseous species, and thus could improve model performance for PM2.5, PM10, PM components, and some PM gaseous precursors such as SO2 and NH3 in several regions as well as aerosol optical depth (AOD) and cloud properties (e.g., cloud fraction (CF), cloud droplet number concentration (CDNC), and shortwave cloud forcing, SWCF) on the global scale. The modified condensation and aqueous-phase chemistry could further improve the prediction of additional variables such as HNO3, NO2, and O3 in some regions, and new particle formation rate (J) and AOD on the global scale. IMN can improve the prediction of secondary PM2.5 components, PM2.5, and PM10 over Europe as well as AOD and CDNC on the global scale. The explicit inorganic aerosol thermodynamics using the ISORROPIA II model improves the prediction of all major PM2.5 components and their gaseous precursors in some regions as well as downwelling shortwave radiation, SWCF, and cloud condensation nuclei at a supersaturation of 0.5% on the global scale. For simulations of 2001–2005 with all the modified and new treatments, the improved model predicts that on global average, SWCF increases by 2.7 W m?2, reducing the normalized mean bias (NMB) of SWCF from ?5.4 to 1.2%. Uncertainties in emissions can largely explain the inaccurate prediction of precursor gases (e.g., SO2, NH3, and NO) and primary aerosols (e.g., black carbon and primary organic matter). Additional factors leading to the discrepancies between model predictions and observations include assumptions associated with equilibrium partitioning for fine particles assumed in ISORROPIA II, irreversible gas/particle mass transfer treatment for coarse particles, uncertainties in model treatments such as dust emissions, secondary organic aerosol formation, multi-phase chemistry, cloud microphysics, aerosol–cloud interaction, dry and wet deposition, and model parameters (e.g., accommodation coefficients and prefactors of the nucleation power law) as well as uncertainties in model configuration such as the use of a coarse-grid resolution.

Posted on 8 September 2014 | 12:00 am


Comparison of the predictions of two road dust emission models with the measurements of a mobile van

Comparison of the predictions of two road dust emission models with the measurements of a mobile van

Atmospheric Chemistry and Physics, 14, 9155-9169, 2014

Author(s): M. Kauhaniemi, A. Stojiljkovic, L. Pirjola, A. Karppinen, J. Härkönen, K. Kupiainen, L. Kangas, M. A. Aarnio, G. Omstedt, B. R. Denby, and J. Kukkonen

The predictions of two road dust suspension emission models were compared with the on-site mobile measurements of suspension emission factors. Such a quantitative comparison has not previously been reported in the reviewed literature. The models used were the Nordic collaboration model NORTRIP (NOn-exhaust Road TRaffic Induced Particle emissions) and the Swedish–Finnish FORE model (Forecasting Of Road dust Emissions). These models describe particulate matter generated by the wear of road surface due to traction control methods and processes that control the suspension of road dust particles into the air. An experimental measurement campaign was conducted using a mobile laboratory called SNIFFER, along two selected road segments in central Helsinki in 2007 and 2008. The suspended PM10 concentration was measured behind the left rear tyre and the street background PM10 concentration in front of the van. Both models reproduced the measured seasonal variation of suspension emission factors fairly well during both years at both measurement sites. However, both models substantially under-predicted the measured emission values. The article illustrates the challenges in conducting road suspension measurements in densely trafficked urban conditions, and the numerous requirements for input data that are needed for accurately applying road suspension emission models.

Posted on 8 September 2014 | 12:00 am


Comprehensive assessment of meteorological conditions and airflow connectivity during HCCT-2010

Comprehensive assessment of meteorological conditions and airflow connectivity during HCCT-2010

Atmospheric Chemistry and Physics, 14, 9105-9128, 2014

Author(s): A. Tilgner, L. Schöne, P. Bräuer, D. van Pinxteren, E. Hoffmann, G. Spindler, S. A. Styler, S. Mertes, W. Birmili, R. Otto, M. Merkel, K. Weinhold, A. Wiedensohler, H. Deneke, R. Schrödner, R. Wolke, J. Schneider, W. Haunold, A. Engel, A. Wéber, and H. Herrmann

This study presents a comprehensive assessment of the meteorological conditions and atmospheric flow during the Lagrangian-type "Hill Cap Cloud Thuringia 2010" experiment (HCCT-2010), which was performed in September and October 2010 at Mt. Schmücke in the Thuringian Forest, Germany and which used observations at three measurement sites (upwind, in-cloud, and downwind) to study physical and chemical aerosol–cloud interactions. A Lagrangian-type hill cap cloud experiment requires not only suitable cloud conditions but also connected airflow conditions (i.e. representative air masses at the different measurement sites). The primary goal of the present study was to identify time periods during the 6-week duration of the experiment in which these conditions were fulfilled and therefore which are suitable for use in further data examinations. The following topics were studied in detail: (i) the general synoptic weather situations, including the mesoscale flow conditions, (ii) local meteorological conditions and (iii) local flow conditions. The latter were investigated by means of statistical analyses using best-available quasi-inert tracers, SF6 tracer experiments in the experiment area, and regional modelling. This study represents the first application of comprehensive analyses using statistical measures such as the coefficient of divergence (COD) and the cross-correlation in the context of a Lagrangian-type hill cap cloud experiment. This comprehensive examination of local flow connectivity yielded a total of 14 full-cloud events (FCEs), which are defined as periods during which all connected flow and cloud criteria for a suitable Lagrangian-type experiment were fulfilled, and 15 non-cloud events (NCEs), which are defined as periods with connected flow but no cloud at the summit site, and which can be used as reference cases. The overall evaluation of the identified FCEs provides the basis for subsequent investigations of the measured chemical and physical data during HCCT-2010 (see http://www.atmos-chem-phys.net/special_issue287.html).

Results obtained from the statistical flow analyses and regional-scale modelling performed in this study indicate the existence of a strong link between the three measurement sites during the FCEs and NCEs, particularly under conditions of constant southwesterly flow, high wind speeds and slightly stable stratification. COD analyses performed using continuous measurements of ozone and particle (49 nm diameter size bin) concentrations at the three sites revealed, particularly for COD values < 0.1, very consistent time series (i.e. close links between air masses at the different sites). The regional-scale model simulations provided support for the findings of the other flow condition analyses. Cross-correlation analyses revealed typical overflow times of ~15–30 min between the upwind and downwind valley sites under connected flow conditions. The results described here, together with those obtained from the SF6 tracer experiments performed during the experiment, clearly demonstrate that (a) under appropriate meteorological conditions a Lagrangian-type approach is valid and (b) the connected flow validation procedure developed in this work is suitable for identifying such conditions. Overall, it is anticipated that the methods and tools developed and applied in the present study will prove useful in the identification of suitable meteorological and connected airflow conditions during future Lagrangian-type hill cap cloud experiments.

Posted on 5 September 2014 | 12:00 am


Uptake and emission of VOCs near ground level below a mixed forest at Borden, Ontario

Uptake and emission of VOCs near ground level below a mixed forest at Borden, Ontario

Atmospheric Chemistry and Physics, 14, 9087-9097, 2014

Author(s): M. Gordon, A. Vlasenko, R. M. Staebler, C. Stroud, P. A. Makar, J. Liggio, S.-M. Li, and S. Brown

Understanding of the atmosphere/forest canopy exchange of volatile organic compounds (VOCs) requires insight into the deposition, emission, and chemical reactions of VOCs below the canopy. Between 18 July and 9 August 2009, VOCs were measured with proton-transfer-reaction mass spectrometry (PTR-MS) at six heights between 1 and 6 m beneath a 23 m high mixed-forest canopy. Measured VOCs included methanol, isoprene, acetone, methacrolein and methyl vinyl ketone (MACR + MVK), monoterpenes, and sesquiterpenes. There are pronounced differences in the behaviour of isoprene and its by-products and that of the terpenes. Non-terpene mixing ratios increase with height, suggesting predominantly downward fluxes. In contrast, the terpene mixing ratios decrease with height, suggesting upward fluxes. A 1-D canopy model was used to compare results to measurements with and without surface deposition of isoprene and MACR + MVK and emissions of monoterpenes and sesquiterpenes. Results suggest deposition velocities of 2.7 mm s?1 for isoprene and 1.2 mm s?1 for MACR + MVK and daytime surface emission rates of 63 ?g m?2 h?1 for monoterpenes. The modelled isoprene surface deposition is approximately 2% of the canopy-top isoprene emissions and the modelled emissions of monoterpenes comprise approximately 15 to 27% of the canopy-top monoterpene emissions to the atmosphere. These results suggest that surface monoterpene emissions are significant for forest canopy/atmosphere exchange for this mixed-forest location and surface uptake is relatively small for all the species measured in this study.

Posted on 5 September 2014 | 12:00 am


Ozone vegetation damage effects on gross primary productivity in the United States

Ozone vegetation damage effects on gross primary productivity in the United States

Atmospheric Chemistry and Physics, 14, 9137-9153, 2014

Author(s): X. Yue and N. Unger

We apply an off-line process-based vegetation model (the Yale Interactive Terrestrial Biosphere model) to assess the impacts of ozone (O3) vegetation damage on gross primary productivity (GPP) in the United States during the past decade (1998–2007). The model's GPP simulation is evaluated at 40 sites of the North American Carbon Program (NACP) synthesis. The ecosystem-scale model version reproduces interannual variability and seasonality of GPP at most sites, especially in croplands. Inclusion of the O3 damage impact decreases biases of simulated GPP at most of the NACP sites. The simulation with the O3 damage effect reproduces 64% of the observed variance in summer GPP and 42% on the annual average. Based on a regional gridded simulation over the US, summertime average O3-free GPP is 6.1 g C m?2 day?1 (9.5 g C m?2 day?1 in the east of 95° W and 3.9 g C m?2 day?1 in the west). O3 damage decreases GPP by 4–8% on average in the eastern US and leads to significant decreases of 11–17% in east coast hot spots. Sensitivity simulations show that a 25% decrease in surface O3 concentration halves the average GPP damage to only 2–4%, suggesting the substantial co-benefits to ecosystem health that may be achieved via O3 air pollution control.

Posted on 5 September 2014 | 12:00 am


Corrigendum to "Microphysical Process Rates and Global Aerosol-Cloud Interactions" published in Atmos. Chem. Phys., 13, 9855–9867, 2013

Corrigendum to "Microphysical Process Rates and Global Aerosol-Cloud Interactions" published in Atmos. Chem. Phys., 13, 9855–9867, 2013

Atmospheric Chemistry and Physics, 14, 9099-9103, 2014

Author(s): A. Gettelman, H. Morrison, C. R. Terai, and R. Wood

A mistake swapped process rates between autoconversion and accretion in global model solutions. Revised figures are presented. The accretion to autoconversion ratio in the model does increase with Liquid Water Path (LWP) as in the steady state model but biases remain. Simulated autoconversion rates are too high. Adjusting process rates following the steady state model ideas leads to an improvement in process rates. The main conclusion is unaffected.

Posted on 5 September 2014 | 12:00 am


Long-term trends in aerosol optical characteristics in the Po Valley, Italy

Long-term trends in aerosol optical characteristics in the Po Valley, Italy

Atmospheric Chemistry and Physics, 14, 9129-9136, 2014

Author(s): J. P. Putaud, F. Cavalli, S. Martins dos Santos, and A. Dell'Acqua

Aerosol properties have been monitored by ground-based in situ and remote sensing measurements at the station for atmospheric research located in Ispra, on the edge of the Po Valley, for almost one decade. In situ measurements are performed according to Global Atmosphere Watch recommendations, and quality is assured through the participation in regular inter-laboratory comparisons. Sun-photometer data are produced by the Aerosol Robotic Network (AERONET). Data show significant decreasing trends over the 2004–2010 period for a number of variables, including particulate matter (PM) mass concentration, aerosol scattering, backscattering and absorption coefficients, and aerosol optical thickness (AOT). In situ measurement data show no significant trends in the aerosol backscatter ratio, but they do show a significant decreasing trend of about ?0.7 ± 0.3% yr?1 in the aerosol single scattering albedo (SSA) in the visible light range. Similar trends are observed in the SSA retrieved from sun-photometer measurements. Correlations appear between in situ PM mass concentration and aerosol scattering coefficient, on the one hand, and elemental carbon (EC) concentration and aerosol absorption coefficient, on the other hand. However, no increase in the EC / PM ratio was observed, which could have explained the decrease in SSA. The application of a simple approximation to calculate the direct radiative forcing by aerosols suggests a significant diminution in their cooling effect, mainly due to the decrease in AOT. Applying the methodology we present to those sites, where the necessary suite of measurements is available, would provide important information to inform future policies for air-quality enhancement and fast climate change mitigation.

Posted on 5 September 2014 | 12:00 am


Countergradient heat flux observations during the evening transition period

Countergradient heat flux observations during the evening transition period

Atmospheric Chemistry and Physics, 14, 9077-9085, 2014

Author(s): E. Blay-Carreras, E. R. Pardyjak, D. Pino, D. C. Alexander, F. Lohou, and M. Lothon

Gradient-based turbulence models generally assume that the buoyancy flux ceases to introduce heat into the surface layer of the atmospheric boundary layer in temporal consonance with the gradient of the local virtual potential temperature. Here, we hypothesize that during the evening transition a delay exists between the instant when the buoyancy flux goes to zero and the time when the local gradient of the virtual potential temperature indicates a sign change. This phenomenon is studied using a range of data collected over several intensive observational periods (IOPs) during the Boundary Layer Late Afternoon and Sunset Turbulence field campaign conducted in Lannemezan, France. The focus is mainly on the lower part of the surface layer using a tower instrumented with high-speed temperature and velocity sensors.

The results from this work confirm and quantify a flux-gradient delay. Specifically, the observed values of the delay are ~ 30–80 min. The existence of the delay and its duration can be explained by considering the convective timescale and the competition of forces associated with the classical Rayleigh–Bénard problem. This combined theory predicts that the last eddy formed while the sensible heat flux changes sign during the evening transition should produce a delay. It appears that this last eddy is decelerated through the action of turbulent momentum and thermal diffusivities, and that the delay is related to the convective turnover timescale. Observations indicate that as horizontal shear becomes more important, the delay time apparently increases to values greater than the convective turnover timescale.

Posted on 3 September 2014 | 12:00 am


Organic aerosol concentration and composition over Europe: insights from comparison of regional model predictions with aerosol mass spectrometer factor analysis

Organic aerosol concentration and composition over Europe: insights from comparison of regional model predictions with aerosol mass spectrometer factor analysis

Atmospheric Chemistry and Physics, 14, 9061-9076, 2014

Author(s): C. Fountoukis, A. G. Megaritis, K. Skyllakou, P. E. Charalampidis, C. Pilinis, H. A. C. Denier van der Gon, M. Crippa, F. Canonaco, C. Mohr, A. S. H. Prévôt, J. D. Allan, L. Poulain, T. Petäjä, P. Tiitta, S. Carbone, A. Kiendler-Scharr, E. Nemitz, C. O'Dowd, E. Swietlicki, and S. N. Pandis

A detailed three-dimensional regional chemical transport model (Particulate Matter Comprehensive Air Quality Model with Extensions, PMCAMx) was applied over Europe, focusing on the formation and chemical transformation of organic matter. Three periods representative of different seasons were simulated, corresponding to intensive field campaigns. An extensive set of AMS measurements was used to evaluate the model and, using factor-analysis results, gain more insight into the sources and transformations of organic aerosol (OA). Overall, the agreement between predictions and measurements for OA concentration is encouraging, with the model reproducing two-thirds of the data (daily average mass concentrations) within a factor of 2. Oxygenated OA (OOA) is predicted to contribute 93% to total OA during May, 87% during winter and 96% during autumn, with the rest consisting of fresh primary OA (POA). Predicted OOA concentrations compare well with the observed OOA values for all periods, with an average fractional error of 0.53 and a bias equal to ?0.07 (mean error = 0.9 ?g m?3, mean bias = ?0.2 ?g m−3). The model systematically underpredicts fresh POA at most sites during late spring and autumn (mean bias up to ?0.8 ?g m−3). Based on results from a source apportionment algorithm running in parallel with PMCAMx, most of the POA originates from biomass burning (fires and residential wood combustion), and therefore biomass burning OA is most likely underestimated in the emission inventory. The sensitivity of POA predictions to the corresponding emissions' volatility distribution is discussed. The model performs well at all sites when the Positive Matrix Factorization (PMF)-estimated low-volatility OOA is compared against the OA with saturation concentrations of the OA surrogate species C* ≤ 0.1 ?g m?3 and semivolatile OOA against the OA with C* > 0.1 ?g m?3.

Posted on 3 September 2014 | 12:00 am


The contribution of fungal spores and bacteria to regional and global aerosol number and ice nucleation immersion freezing rates

The contribution of fungal spores and bacteria to regional and global aerosol number and ice nucleation immersion freezing rates

Atmospheric Chemistry and Physics, 14, 9051-9059, 2014

Author(s): D. V. Spracklen and C. L. Heald

Primary biological aerosol particles (PBAPs) may play an important role in aerosol–climate interactions, in particular by affecting ice formation in mixed phase clouds. However, the role of PBAPs is poorly understood because the sources and distribution of PBAPs in the atmosphere are not well quantified. Here we include emissions of fungal spores and bacteria in a global aerosol microphysics model and explore their contribution to concentrations of supermicron particle number, cloud condensation nuclei (CCN) and immersion freezing rates. Simulated surface annual mean concentrations of fungal spores are ~ 2.5 × 104 m?3 over continental midlatitudes and 1 × 105 m?3 over tropical forests. Simulated surface concentrations of bacteria are 2.5 × 104 m?3 over most continental regions and 5 × 104 m?3 over grasslands of central Asia and North America. These simulated surface number concentrations of fungal spores and bacteria are broadly in agreement with the limited available observations. We find that fungal spores and bacteria contribute 8 and 5% respectively to simulated continental surface mean supermicron number concentrations, but have very limited impact on CCN concentrations, altering regional concentrations by less than 1%. In agreement with previous global modelling studies, we find that fungal spores and bacteria contribute very little (3 × 10?3%, even when we assume upper limits for ice nucleation activity) to global average immersion freezing ice nucleation rates, which are dominated by soot and dust. However, at lower altitudes (400 to 600 hPa), where warmer temperatures mean that soot and dust may not nucleate ice, we find that PBAP controls the immersion freezing ice nucleation rate. This demonstrates that PBAPs can be of regional importance for IN formation, in agreement with case study observations.

Posted on 2 September 2014 | 12:00 am


Assessment of uncertainties of an aircraft-based mass balance approach for quantifying urban greenhouse gas emissions

Assessment of uncertainties of an aircraft-based mass balance approach for quantifying urban greenhouse gas emissions

Atmospheric Chemistry and Physics, 14, 9029-9050, 2014

Author(s): M. O. L. Cambaliza, P. B. Shepson, D. R. Caulton, B. Stirm, D. Samarov, K. R. Gurney, J. Turnbull, K. J. Davis, A. Possolo, A. Karion, C. Sweeney, B. Moser, A. Hendricks, T. Lauvaux, K. Mays, J. Whetstone, J. Huang, I. Razlivanov, N. L. Miles, and S. J. Richardson

Urban environments are the primary contributors to global anthropogenic carbon emissions. Because much of the growth in CO2 emissions will originate from cities, there is a need to develop, assess, and improve measurement and modeling strategies for quantifying and monitoring greenhouse gas emissions from large urban centers. In this study the uncertainties in an aircraft-based mass balance approach for quantifying carbon dioxide and methane emissions from an urban environment, focusing on Indianapolis, IN, USA, are described. The relatively level terrain of Indianapolis facilitated the application of mean wind fields in the mass balance approach. We investigate the uncertainties in our aircraft-based mass balance approach by (1) assessing the sensitivity of the measured flux to important measurement and analysis parameters including wind speed, background CO2 and CH4, boundary layer depth, and interpolation technique, and (2) determining the flux at two or more downwind distances from a point or area source (with relatively large source strengths such as solid waste facilities and a power generating station) in rapid succession, assuming that the emission flux is constant. When we quantify the precision in the approach by comparing the estimated emissions derived from measurements at two or more downwind distances from an area or point source, we find that the minimum and maximum repeatability were 12 and 52%, with an average of 31%. We suggest that improvements in the experimental design can be achieved by careful determination of the background concentration, monitoring the evolution of the boundary layer through the measurement period, and increasing the number of downwind horizontal transect measurements at multiple altitudes within the boundary layer.

Posted on 2 September 2014 | 12:00 am


One-year observations of size distribution characteristics of major aerosol constituents at a coastal receptor site in Hong Kong – Part 1: Inorganic ions and oxalate

One-year observations of size distribution characteristics of major aerosol constituents at a coastal receptor site in Hong Kong – Part 1: Inorganic ions and oxalate

Atmospheric Chemistry and Physics, 14, 9013-9027, 2014

Author(s): Q. Bian, X. H. H. Huang, and J. Z. Yu

Size distribution data of major aerosol constituents are essential in source apportioning of visibility degradation, testing and verification of air quality models incorporating aerosols. We report here 1-year observations of mass size distributions of major inorganic ions (sulfate, nitrate, chloride, ammonium, sodium, potassium, magnesium and calcium) and oxalate at a coastal suburban receptor site in Hong Kong, China. A total of 43 sets of size-segregated samples in the size range of 0.056–18 ?m were collected from March 2011 to February 2012. The size distributions of sulfate, ammonium, potassium and oxalate were characterized by a dominant droplet mode with a mass mean aerodynamic diameter (MMAD) in the range of ~ 0.7–0.9 ?m. Oxalate had a slightly larger MMAD than sulfate on days with temperatures above 22 °C as a result of the process of volatilization and repartitioning. Nitrate was mostly dominated by the coarse mode but enhanced presence in fine mode was detected on winter days with lower temperature and lower concentrations of sea salt and soil particles. This data set reveals an inversely proportional relationship between the fraction of nitrate in the fine mode and product of the sum of sodium and calcium in equivalent concentrations and the dissociation constant of ammonium nitrate (i.e., (1/([Na+] + 2[Ca2+]) × (1/Ke')) when Pn_fine is significant (> 10%). The seasonal variation observed for sea salt aerosol abundance, with lower values in summer and winter, is possibly linked with the lower marine salinities in these two seasons.

Positive matrix factorization was applied to estimate the relative contributions of local formation and transport to the observed ambient sulfate level through the use of the combined data sets of size-segregated sulfate and select gaseous air pollutants. On average, the regional/super-regional transport of air pollutants was the dominant source at this receptor site, especially on high-sulfate days while local formation processes contributed approximately 30% of the total sulfate. This work provides field-measurement-based evidence important for understanding both local photochemistry and regional/super-regional transport in order to properly simulate sulfate aerosols in air quality models.

Posted on 2 September 2014 | 12:00 am


On transition-zone water clouds

On transition-zone water clouds

Atmospheric Chemistry and Physics, 14, 9001-9012, 2014

Author(s): E. Hirsch, I. Koren, Z. Levin, O. Altaratz, and E. Agassi

A recent field campaign was conducted to measure the properties of thin, warm convective clouds forming under conditions of weak updrafts. During the campaign, short-lived clouds (on the order of minutes) with droplets' effective radius of 1–2 ?m and low liquid water path (~ 500 mg m–2) were measured. These low values are puzzling, since in most studies an effective radius of 4 ?m is reported to serve as the lower bound for clouds. A theoretical cloud model designed to resolve the droplet-activation process suggested conditions that favor the formation of such clouds. Here we show that these clouds, which mark the transition from haze to cloud, are highly sensitive to the magnitude of the initial perturbation that initiated them. We define these clouds as "transition-zone clouds". The existence of such clouds poses a key challenge for the analysis of atmospheric observations and models, since they "further smooth" the transition from dry aerosol through haze pockets to cumulus clouds.

Posted on 1 September 2014 | 12:00 am


How important are atmospheric depressions and mobile cyclones for emitting mineral dust aerosol in North Africa?

How important are atmospheric depressions and mobile cyclones for emitting mineral dust aerosol in North Africa?

Atmospheric Chemistry and Physics, 14, 8983-9000, 2014

Author(s): S. Fiedler, K. Schepanski, P. Knippertz, B. Heinold, and I. Tegen

This study presents the first quantitative estimate of the mineral dust emission associated with atmospheric depressions and mobile cyclones in North Africa. Atmospheric depressions are automatically tracked at 925 hPa based on ERA-Interim data from the European Centre for Medium-Range Weather Forecasts for 1989–2008. A set of filter criteria is applied to identify mobile cyclones, i.e. migrating and long-lived cyclones. The shorter term cyclone is used as a synonym for mobile cyclones. Dust emission is calculated with a dust emission model driven by 10 m winds and soil moisture from ERA-Interim. Emission peaks during winter and spring with spatial averages of 250–380 g m−2 per month. Comparison of the dust source activation frequency from the model against SEVIRI satellite observation shows a good agreement in the Bodélé Depression but differences in the north and west of North Africa. Depressions are abundant, particularly in summer when the Saharan heat low is situated over West Africa and during spring in the lee of the Atlas Mountains. Up to 90% (55% annually and spatially averaged) of dust emission occurs within 10 degrees of these depressions, with embedded mechanisms such as nocturnal low-level jets playing a role. Cyclones are rarer and occur primarily north of 20° N in spring in agreement with previous studies and over summertime West Africa consistent with near-surface signatures of African Easterly Waves. Dust emission within 10 degrees of cyclones peaks over Libya with up to 25% in spring. Despite the overall small contribution of 4% annually and spatially averaged, cyclones coincide with particularly intense dust emission events exceeding the climatological mean by a factor of four to eight. Soil moisture weakens dust emission during cyclone passage by about 10%.

Posted on 1 September 2014 | 12:00 am


Understanding the anthropogenic influence on formation of biogenic secondary organic aerosols in Denmark via analysis of organosulfates and related oxidation products

Understanding the anthropogenic influence on formation of biogenic secondary organic aerosols in Denmark via analysis of organosulfates and related oxidation products

Atmospheric Chemistry and Physics, 14, 8961-8981, 2014

Author(s): Q. T. Nguyen, M. K. Christensen, F. Cozzi, A. Zare, A. M. K. Hansen, K. Kristensen, T. E. Tulinius, H. H. Madsen, J. H. Christensen, J. Brandt, A. Massling, J. K. Nøjgaard, and M. Glasius

Anthropogenic emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) may affect concentration levels and composition of biogenic secondary organic aerosols (BSOA) through photochemical reactions with biogenic organic precursors to form organosulfates and nitrooxy organosulfates. We investigated this influence in a field study from 19 May to 22 June, 2011 at two sampling sites in Denmark. Within the study, we identified a substantial number of organic acids, organosulfates and nitrooxy organosulfates in the ambient urban curbside and semi-rural background air. A high degree of correlation in concentrations was found among a group of specific organic acids, organosulfates and nitrooxy organosulfates, which may originate from various precursors, suggesting a common mechanism or factor affecting their concentration levels at the sites. It was proposed that the formation of those species most likely occurred on a larger spatial scale, with the compounds being long-range transported to the sites on the days with the highest concentrations. The origin of the long-range transported aerosols was investigated using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model in addition to modeled emissions of related precursors, including isoprene and monoterpenes using the global Model of Emissions of Gases and Aerosols from Nature (MEGAN) and SO2 emissions using the European Monitoring and Evaluation Program (EMEP) database. The local impacts were also studied by examining the correlation between selected species, which showed significantly enhanced concentrations at the urban curbside site and the local concentrations of various gases, including SO2, ozone (O3), NOx, aerosol acidity and other meteorological conditions. This investigation showed that an inter-play of the local parameters such as the aerosol acidity, NOx, SO2, relative humidity (RH), temperature and global radiation seemed to affect the concentration level of those species, suggesting the influence of aqueous aerosol chemistry. The local impacts, however, seemed minor compared to the regional impacts. The total concentrations of organosulfates and nitrooxy organosulfates, on average, contributed to approximately 0.5–0.8% of PM1 mass at the two sampling sites.

Posted on 1 September 2014 | 12:00 am


Atmospheric occurrence, transport and deposition of polychlorinated biphenyls and hexachlorobenzene in the Mediterranean and Black seas

Atmospheric occurrence, transport and deposition of polychlorinated biphenyls and hexachlorobenzene in the Mediterranean and Black seas

Atmospheric Chemistry and Physics, 14, 8947-8959, 2014

Author(s): N. Berrojalbiz, J. Castro-Jiménez, G. Mariani, J. Wollgast, G. Hanke, and J. Dachs

The Mediterranean and Black seas are unique marine environments subject to important anthropogenic pressures due to direct and indirect loads of atmospheric inputs of organochlorine compounds (OCls) from primary and secondary sources. Here we report the results obtained during two east–west sampling cruises in June 2006 and May 2007 from Barcelona to Istanbul and Alexandria, respectively, where gas-phase and aerosol-phase samples were collected. Both matrices were analyzed for 41 polychlorinated biphenyls (PCBs), including dioxin-like congeners, and hexachlorobenzene (HCB). The values reported in this study for gas-phase HCB and ∑41PCB limit of detection (LOD) to 418.3 pg m?3 and from 81.99 to 931.6 pg m?3 respectively) are in the same range of those reported in former studies, possibly suggesting a limited decline in their atmospheric concentrations during the last decade for the Mediterranean region due to land-based OCl sources. There is a clear influence of the direction of the air mass on the atmospheric concentrations of PCBs, with higher concentrations when the air mass was from southern Europe, and the lowest concentrations for air masses coming from the SW Mediterranean and Atlantic Ocean. PCBs and HCB are close to air–water equilibrium for most sampling periods, thus resulting in low atmospheric deposition fluxes at open sea. This is consistent with the oligotrophic character of the Mediterranean Sea with a small influence of the biological pump capturing atmospheric PCBs. Therefore, degradation of gas-phase PCBs by OH radicals is estimated to be the main loss process of atmospheric PCBs during their transport over the Mediterranean Sea. Conversely, atmospheric residence times of HCB are predicted to be very long due to a lack of atmospheric degradation and low depositional fluxes due to concentrations at air–water equilibrium.

Posted on 1 September 2014 | 12:00 am


Kinetics of the reactions of isoprene-derived hydroxynitrates: gas phase epoxide formation and solution phase hydrolysis

Kinetics of the reactions of isoprene-derived hydroxynitrates: gas phase epoxide formation and solution phase hydrolysis

Atmospheric Chemistry and Physics, 14, 8933-8946, 2014

Author(s): M. I. Jacobs, W. J. Burke, and M. J. Elrod

Isoprene, the most abundant non-methane volatile organic compound (VOC) emitted into the atmosphere, is known to undergo gas phase oxidation to form eight different hydroxynitrate isomers in "high-NOx" environments. These hydroxynitrates are known to affect the global and regional formation of ozone and secondary organic aerosol (SOA), as well as affect the distribution of nitrogen. In the present study, we have synthesized three of the eight possible hydroxynitrates: 4-hydroxy-3-nitroxy isoprene (4,3-HNI) and E / Z-1-hydroxy-4-nitroxy isoprene (1,4-HNI). Oxidation of the 4,3-HNI isomer by the OH radical was monitored using a flow tube chemical ionization mass spectrometer (FT-CIMS), and its OH rate constant was determined to be (3.64 ± 0.41) × 10−11 cm3 molecule?1 s?1. The products of 4,3-HNI oxidation were monitored, and a mechanism to explain the products was developed. An isoprene epoxide (IEPOX) – a species important in SOA chemistry and thought to originate only from "low-NOx" isoprene oxidation – was found as a minor, but significant, product. Additionally, hydrolysis kinetics of the three synthesized isomers were monitored with nuclear magnetic resonance (NMR). The bulk, neutral solution hydrolysis rate constants for 4,3-HNI and the 1,4-HNI isomers were (1.59 ± 0.03) × 10−5 s?1 and (6.76 ± 0.09) × 10?3 s?1, respectively. The hydrolysis reactions of each isomer were found to be general acid-catalyzed. The reaction pathways, product yields and atmospheric implications for both the gas phase and aerosol phase reactions are discussed.

Posted on 1 September 2014 | 12:00 am


Influence of corona discharge on the ozone budget in the tropical free troposphere: a case study of deep convection during GABRIEL

Influence of corona discharge on the ozone budget in the tropical free troposphere: a case study of deep convection during GABRIEL

Atmospheric Chemistry and Physics, 14, 8917-8931, 2014

Author(s): H. Bozem, H. Fischer, C. Gurk, C. L. Schiller, U. Parchatka, R. Koenigstedt, A. Stickler, M. Martinez, H. Harder, D. Kubistin, J. Williams, G. Eerdekens, and J. Lelieveld

Convective redistribution of ozone and its precursors between the boundary layer (BL) and the free troposphere (FT) influences photochemistry, in particular in the middle and upper troposphere (UT). We present a case study of convective transport during the GABRIEL campaign over the tropical rain forest in Suriname in October 2005. During one measurement flight the inflow and outflow regions of a cumulonimbus cloud (Cb) have been characterized. We identified a distinct layer between 9 and 11 km altitude with enhanced mixing ratios of CO, O3, HOx, acetone and acetonitrile. The elevated O3 contradicts the expectation that convective transport brings low-ozone air from the boundary layer to the outflow region. Entrainment of ozone-rich air is estimated to account for 62% (range: 33–91%) of the observed O3. Ozone is enhanced by only 5–6% by photochemical production in the outflow due to enhanced NO from lightning, based on model calculations using observations including the first reported HOx measurements over the tropical rainforest. The "excess" ozone in the outflow is most probably due to direct production by corona discharge associated with lightning. We deduce a production rate of 5.12 × 1028 molecules O3 flash?1 (range: 9.89 × 1026–9.82 × 1028 molecules O3 flash?1), which is at the upper limit of the range reported previously.

Posted on 1 September 2014 | 12:00 am


Air–sea exchange and gas–particle partitioning of polycyclic aromatic hydrocarbons in the Mediterranean

Air–sea exchange and gas–particle partitioning of polycyclic aromatic hydrocarbons in the Mediterranean

Atmospheric Chemistry and Physics, 14, 8905-8915, 2014

Author(s): M. D. Mulder, A. Heil, P. Kuku?ka, J. Klánová, J. Kuta, R. Prokeš, F. Sprovieri, and G. Lammel

Polycyclic aromatic hydrocarbon (PAH) concentration in air of the central and eastern Mediterranean in summer 2010 was 1.45 (0.30–3.25) ng m?3 (sum of 25 PAHs), with 8 (1–17)% in the particulate phase, almost exclusively associated with particles < 0.25 ?m. The total deposition flux of particulate PAHs was 0.3–0.5 ? g m?2 yr?1. The diffusive air–sea exchange fluxes of fluoranthene and pyrene were mostly found net-depositional or close to phase equilibrium, while retene was net-volatilisational in a large sea region. Regional fire activity records in combination with box model simulations suggest that seasonal depositional input of retene from biomass burning into the surface waters during summer is followed by an annual reversal of air–sea exchange, while interannual variability is dominated by the variability of the fire season. One-third of primary retene sources to the sea region in the period 2005–2010 returned to the atmosphere as secondary emissions from surface seawaters. It is concluded that future negative emission trends or interannual variability of regional sources may trigger the sea to become a secondary PAH source through reversal of diffusive air–sea exchange.

Capsule: In late summer the seawater surface in the Mediterranean has turned into a temporary secondary source of PAH, obviously related to biomass burning in the region.

Posted on 1 September 2014 | 12:00 am


Long-term chemical characterization of tropical and marine aerosols at the Cape Verde Atmospheric Observatory (CVAO) from 2007 to 2011

Long-term chemical characterization of tropical and marine aerosols at the Cape Verde Atmospheric Observatory (CVAO) from 2007 to 2011

Atmospheric Chemistry and Physics, 14, 8883-8904, 2014

Author(s): K. W. Fomba, K. Müller, D. van Pinxteren, L. Poulain, M. van Pinxteren, and H. Herrmann

The first long-term aerosol sampling and chemical characterization results from measurements at the Cape Verde Atmospheric Observatory (CVAO) on the island of São Vicente are presented and are discussed with respect to air mass origin and seasonal trends. In total 671 samples were collected using a high-volume PM10 sampler on quartz fiber filters from January 2007 to December 2011. The samples were analyzed for their aerosol chemical composition, including their ionic and organic constituents. Back trajectory analyses showed that the aerosol at CVAO was strongly influenced by emissions from Europe and Africa, with the latter often responsible for high mineral dust loading. Sea salt and mineral dust dominated the aerosol mass and made up in total about 80% of the aerosol mass. The 5-year PM10 mean was 47.1 ± 55.5 ?g m?2, while the mineral dust and sea salt means were 27.9 ± 48.7 and 11.1 ± 5.5 ?g m?2, respectively. Non-sea-salt (nss) sulfate made up 62% of the total sulfate and originated from both long-range transport from Africa or Europe and marine sources. Strong seasonal variation was observed for the aerosol components. While nitrate showed no clear seasonal variation with an annual mean of 1.1 ± 0.6 ?g m?3, the aerosol mass, OC (organic carbon) and EC (elemental carbon), showed strong winter maxima due to strong influence of African air mass inflow. Additionally during summer, elevated concentrations of OM were observed originating from marine emissions. A summer maximum was observed for non-sea-salt sulfate and was connected to periods when air mass inflow was predominantly of marine origin, indicating that marine biogenic emissions were a significant source. Ammonium showed a distinct maximum in spring and coincided with ocean surface water chlorophyll a concentrations. Good correlations were also observed between nss-sulfate and oxalate during the summer and winter seasons, indicating a likely photochemical in-cloud processing of the marine and anthropogenic precursors of these species. High temporal variability was observed in both chloride and bromide depletion, differing significantly within the seasons, air mass history and Saharan dust concentration. Chloride (bromide) depletion varied from 8.8 ± 8.5% (62 ± 42%) in Saharan-dust-dominated air mass to 30 \textpm 12% (87 ± 11%) in polluted Europe air masses. During summer, bromide depletion often reached 100% in marine as well as in polluted continental samples. In addition to the influence of the aerosol acidic components, photochemistry was one of the main drivers of halogenide depletion during the summer; while during dust events, displacement reaction with nitric acid was found to be the dominant mechanism. Positive matrix factorization (PMF) analysis identified three major aerosol sources: sea salt, aged sea salt and long-range transport. The ionic budget was dominated by the first two of these factors, while the long-range transport factor could only account for about 14% of the total observed ionic mass.

Posted on 1 September 2014 | 12:00 am


Measurements of dust deposition velocity in a wind-tunnel experiment

Measurements of dust deposition velocity in a wind-tunnel experiment

Atmospheric Chemistry and Physics, 14, 8869-8882, 2014

Author(s): J. Zhang, Y. Shao, and N. Huang

In this study, we present the results of a wind-tunnel experiment on dust deposition. A new method is proposed to derive dust deposition velocity from PDA (particle dynamics analysis) particle-velocity and particle-size measurements. This method has the advantage that the motions of individual dust particles are directly observed and all relevant data for computing dust deposition velocity is collected using a single instrument, and thus the measurement uncertainties are reduced. The method is used in the wind-tunnel experiment to measure dust deposition velocities for different particle sizes, wind speeds and surface conditions. For sticky-smooth wood and water surfaces, the observed dust deposition velocities are compared with the predictions using a dust deposition scheme, and the entire data set is compared with the data found in the literature. From the wind-tunnel experiments, a relatively reliable data set of dust deposition velocities is obtained, which is valuable for the development and validation of dust deposition schemes.

Posted on 1 September 2014 | 12:00 am


The effects of energy paths and emission controls and standards on future trends in China's emissions of primary air pollutants

The effects of energy paths and emission controls and standards on future trends in China's emissions of primary air pollutants

Atmospheric Chemistry and Physics, 14, 8849-8868, 2014

Author(s): Y. Zhao, J. Zhang, and C. P. Nielsen

To examine the efficacy of China's actions to control atmospheric pollution, three levels of growth of energy consumption and three levels of implementation of emission controls are estimated, generating a total of nine combined activity-emission control scenarios that are then used to estimate trends of national emissions of primary air pollutants through 2030. The emission control strategies are expected to have more effects than the energy paths on the future emission trends for all the concerned pollutants. As recently promulgated national action plans of air pollution prevention and control (NAPAPPC) are implemented, China's anthropogenic pollutant emissions should decline. For example, the emissions of SO2, NOx, total suspended particles (TSP), PM10, and PM2.5 are estimated to decline 7, 20, 41, 34, and 31% from 2010 to 2030, respectively, in the "best guess" scenario that includes national commitment of energy saving policy and implementation of NAPAPPC. Should the issued/proposed emission standards be fully achieved, a less likely scenario, annual emissions would be further reduced, ranging from 17 (for primary PM2.5) to 29% (for NOx) declines in 2015, and the analogue numbers would be 12 and 24% in 2030. The uncertainties of emission projections result mainly from the uncertain operational conditions of swiftly proliferating air pollutant control devices and lack of detailed information about emission control plans by region. The predicted emission trends by sector and chemical species raise concerns about current pollution control strategies: the potential for emissions abatement in key sectors may be declining due to the near saturation of emission control devices use; risks of ecosystem acidification could rise because emissions of alkaline base cations may be declining faster than those of SO2; and radiative forcing could rise because emissions of positive-forcing carbonaceous aerosols may decline more slowly than those of SO2 emissions and thereby concentrations of negative-forcing sulfate particles. Expanded control of emissions of fine particles and carbonaceous aerosols from small industrial and residential sources is recommended, and a more comprehensive emission control strategy targeting a wider range of pollutants (volatile organic compounds, NH3 and CO, etc.) and taking account of more diverse environmental impacts is also urgently needed.

Posted on 1 September 2014 | 12:00 am





Other notes:



 Information about this site:


 
The author- or copyrights of the listed Internet pages are held by the respective authors or site operators, who are also responsible for the content of the presentations.
 
To see your page listed here: Send us an eMail! Condition: Subject-related content on chemistry, biochemistry and comparable academic disciplines!
Citation:
http://www.internetchemistry.com/rss/atmospheric-chemistry.php
Keywords:
Chronological list of recent articles on Chemistry, Atmospheric Chemistry, Atmospheric Chemistry and Physics.
Update:
28.09.2013


Internetchemistry ChemLin © 1996 - 2013 A. J.