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Atmospheric Chemistry and Physics - Current Research Articles



Current research articles: Atmospheric Chemistry

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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:



Satellite observations indicate substantial spatiotemporal variability in biomass burning NOx emission factors for South America

Satellite observations indicate substantial spatiotemporal variability in biomass burning NOx emission factors for South America

Atmospheric Chemistry and Physics, 14, 3929-3943, 2014

Author(s): P. Castellanos, K. F. Boersma, and G. R. van der Werf

Biomass burning is an important contributor to global total emissions of NOx (NO+NO2). Generally bottom-up fire emissions models calculate NOx emissions by multiplying fuel consumption estimates with static biome-specific emission factors, defined in units of grams of NO per kilogram of dry matter consumed. Emission factors are a significant source of uncertainty in bottom-up fire emissions modeling because relatively few observations are available to characterize the large spatial and temporal variability of burning conditions. In this paper we use NO2 tropospheric column observations from the Ozone Monitoring Instrument (OMI) from the year 2005 over South America to calculate monthly NOx emission factors for four fire types: deforestation, savanna/grassland, woodland, and agricultural waste burning. In general, the spatial patterns in NOx emission factors calculated in this work are consistent with emission factors derived from in situ measurements from the region but are more variable than published biome-specific global average emission factors widely used in bottom-up fire emissions inventories such as the Global Fire Emissions Database (GFED). Satellite-based NOx emission factors also indicate substantial temporal variability in burning conditions. Overall, we found that deforestation fires have the lowest NOx emission factors, on average 30% lower than the emission factors used in GFED v3. Agricultural fire NOx emission factors were the highest, on average a factor of 1.8 higher than GFED v3 values. For savanna, woodland, and deforestation fires, early dry season NOx emission factors were a factor of ~1.5–2 higher than late dry season emission factors. A minimum in the NOx emission factor seasonal cycle for deforestation fires occurred in August, the time period of severe drought in South America in 2005, supporting the hypothesis that prolonged dry spells may lead to an increase in the contribution of smoldering combustion from large-diameter fuels, offsetting the higher combustion efficiency of dryer fine fuels. We evaluated the OMI-derived NOx emission factors with SCIAMACHY NO2 tropospheric column observations and found improved model performance in regions dominated by fire emissions.

Posted on 17 April 2014 | 12:00 am


Climatology of new particle formation at Izaña mountain GAW observatory in the subtropical North Atlantic

Climatology of new particle formation at Izaña mountain GAW observatory in the subtropical North Atlantic

Atmospheric Chemistry and Physics, 14, 3865-3881, 2014

Author(s): M. I. García, S. Rodríguez, Y. González, and R. D. García

A climatology of new particle formation (NPF) events at high altitude in the subtropical North Atlantic is presented. A 4-year data set (June 2008–June 2012), which includes number size distributions (10–600 nm), reactive gases (SO2, NOx, and O3), several components of solar radiation and meteorological parameters, measured at Izaña Global Atmosphere Watch (GAW) observatory (2373 m above sea level; Tenerife, Canary Islands) was analysed. NPF is associated with the transport of gaseous precursors from the boundary layer by orographic buoyant upward flows that perturb the low free troposphere during daytime. On average, 30% of the days contained an NPF event. Mean values of the formation and growth rates during the study period were 0.46 cm?3 s?1 and 0.42 nm h?1, correspondingly. There is a clearly marked NPF season (May–August), when these events account for 50–60% of the days per month. Monthly mean values of the formation and growth rates exhibit higher values in this season, 0.49–0.92 cm?3 s?1 and 0.48–0.58 nm h?1, respectively. During NPF events, SO2, UV radiation and upslope winds showed higher values than during non-events. The overall data set indicates that SO2 plays a key role as precursor, although other species seem to contribute during some periods. Condensation of sulfuric acid vapour accounts for most of the measured particle growth during most of the year (~70%), except for some periods. In May, the highest mean growth rates (~0.6 nm h?1) and the lowest contribution of sulfuric acid (~13%) were measured, suggesting a significant involvement of other condensing vapours. The SO2 availability seems also to be the most influencing parameter in the year-to-year variability in the frequency of NPF events. The condensation sink showed similar features to other mountain sites, showing high values during NPF events. Summertime observations, when Izaña is within the Saharan Air Layer, suggest that dust particles may play a significant role acting as coagulation sink of freshly formed nucleation particles. The contribution of dust particles to the condensation sink of sulfuric acid vapours seems to be modest (~8% as average). Finally, we identified a set of NPF events in which two nucleation modes, which may evolve at different rates, occur simultaneously and for which further investigations are necessary.

Posted on 17 April 2014 | 12:00 am


Surface response to rain events throughout the West African monsoon

Surface response to rain events throughout the West African monsoon

Atmospheric Chemistry and Physics, 14, 3883-3898, 2014

Author(s): F. Lohou, L. Kergoat, F. Guichard, A. Boone, B. Cappelaere, J.-M. Cohard, J. Demarty, S. Galle, M. Grippa, C. Peugeot, D. Ramier, C. M. Taylor, and F. Timouk

This study analyses the response of the continental surface to rain events, taking advantage of the long-term near-surface measurements over different vegetation types at different latitudes, acquired during the African Monsoon Multidisciplinary Analysis (AMMA) by the AMMA-CATCH observing system. The simulated surface response by nine land surface models involved in AMMA Land Model Intercomparison Project (ALMIP), is compared to the observations. The surface response, described via the evaporative fraction (EF), evolves in two steps: the immediate surface response (corresponding to an increase of EF occurring immediately after the rain) and the surface recovery (characterized by a decrease of EF over several days after the rain). It is shown that, for all the experimental sites, the immediate surface response is mainly dependent on the soil moisture content and the recovery period follows an exponential relationship whose rate is strongly dependent on the vegetation type (from 1 day over bare soil to 70 days over forest) and plant functional type (below and above 10 days for annual and perennial plants, respectively). The ALMIP model ensemble depicts a broad range of relationships between EF and soil moisture, with the worst results for the drier sites (high latitudes). The land surface models tend to simulate a realistic surface recovery for vegetated sites, but a slower and more variable EF decrease is simulated over bare soil than observed.

Posted on 17 April 2014 | 12:00 am


Representing ozone extremes in European megacities: the importance of resolution in a global chemistry climate model

Representing ozone extremes in European megacities: the importance of resolution in a global chemistry climate model

Atmospheric Chemistry and Physics, 14, 3899-3912, 2014

Author(s): Z. S. Stock, M. R. Russo, and J. A. Pyle

The continuing growth of the world's urban population has led to an increasing number of cities with more than 10 million inhabitants. The higher emissions of pollutants, coupled to higher population density, makes predictions of air quality in these megacities of particular importance from both a science and a policy perspective. Global climate models are typically run at coarse resolution to enable both the efficient running of long time integrations, and the ability to run multiple future climate scenarios. However, when considering surface ozone concentrations at the local scale, coarse resolution can lead to inaccuracies arising from the highly nonlinear ozone chemistry and the sensitivity of ozone to the distribution of its precursors on smaller scales. In this study, we use UM-UKCA, a global atmospheric chemistry model, coupled to the UK Met Office Unified Model, to investigate the impact of model resolution on tropospheric ozone, ranging from global to local scales. We focus on the model's ability to represent the probability of high ozone concentrations in the summer and low ozone concentrations, associated with polluted megacity environments, in the winter, and how this varies with horizontal resolution.

We perform time-slice integrations with two model configurations at typical climate resolution (CR, ~150 km) and at a higher resolution (HR, ~40 km). The CR configuration leads to overestimation of ozone concentrations on both regional and local scales, while it gives broadly similar results to the HR configuration on the global scale. The HR configuration is found to produce a more realistic diurnal cycle of ozone concentrations and to give a better representation of the probability density function of ozone values in urban areas such as the megacities of London and Paris. We find the observed differences in model behaviour between CR and HR configurations to be largely caused by chemical differences during the winter and meteorological differences during the summer.

Posted on 17 April 2014 | 12:00 am


Characterization of uncertainties in atmospheric trace gas inversions using hierarchical Bayesian methods

Characterization of uncertainties in atmospheric trace gas inversions using hierarchical Bayesian methods

Atmospheric Chemistry and Physics, 14, 3855-3864, 2014

Author(s): A. L. Ganesan, M. Rigby, A. Zammit-Mangion, A. J. Manning, R. G. Prinn, P. J. Fraser, C. M. Harth, K.-R. Kim, P. B. Krummel, S. Li, J. Mühle, S. J. O'Doherty, S. Park, P. K. Salameh, L. P. Steele, and R. F. Weiss

We present a hierarchical Bayesian method for atmospheric trace gas inversions. This method is used to estimate emissions of trace gases as well as "hyper-parameters" that characterize the probability density functions (PDFs) of the a priori emissions and model-measurement covariances. By exploring the space of "uncertainties in uncertainties", we show that the hierarchical method results in a more complete estimation of emissions and their uncertainties than traditional Bayesian inversions, which rely heavily on expert judgment. We present an analysis that shows the effect of including hyper-parameters, which are themselves informed by the data, and show that this method can serve to reduce the effect of errors in assumptions made about the a priori emissions and model-measurement uncertainties. We then apply this method to the estimation of sulfur hexafluoride (SF6) emissions over 2012 for the regions surrounding four Advanced Global Atmospheric Gases Experiment (AGAGE) stations. We find that improper accounting of model representation uncertainties, in particular, can lead to the derivation of emissions and associated uncertainties that are unrealistic and show that those derived using the hierarchical method are likely to be more representative of the true uncertainties in the system. We demonstrate through this SF6 case study that this method is less sensitive to outliers in the data and to subjective assumptions about a priori emissions and model-measurement uncertainties than traditional methods.

Posted on 17 April 2014 | 12:00 am


On the variability of atmospheric 222Rn activity concentrations measured at Neumayer, coastal Antarctica

On the variability of atmospheric 222Rn activity concentrations measured at Neumayer, coastal Antarctica

Atmospheric Chemistry and Physics, 14, 3843-3853, 2014

Author(s): R. Weller, I. Levin, D. Schmithüsen, M. Nachbar, J. Asseng, and D. Wagenbach

We report on continuously measured 222Rn activity concentrations in near-surface air at Neumayer Station in the period 1995–2011. This 17-year record showed no long-term trend and has overall mean ± standard deviation of (0.019 ± 0.012) Bq m?3. A distinct and persistent seasonality could be distinguished with maximum values of (0.028 ± 0.013) Bq m?3 from January to March and minimum values of (0.015 ± 0.009) Bq m?3 from May to October. Elevated 222Rn activity concentrations were typically associated with air mass transport from the Antarctic Plateau. Our results do not support a relation between enhanced 222Rn activity concentrations at Neumayer and cyclonic activity or long-range transport from South America. The impact of oceanic 222Rn emissions could not be properly assessed but we tentatively identified regional sea ice extent (SIE) variability as a significant driver of the annual 222Rn cycle.

Posted on 17 April 2014 | 12:00 am


Long-term changes in the upper stratospheric ozone at Syowa, Antarctica

Long-term changes in the upper stratospheric ozone at Syowa, Antarctica

Atmospheric Chemistry and Physics, 14, 3945-3968, 2014

Author(s): K. Miyagawa, I. Petropavlovskikh, R. D. Evans, C. Long, J. Wild, G. L. Manney, and W. H. Daffer

Analyses of stratospheric ozone data determined from Dobson–Umkehr measurements since 1977 at the Syowa (69.0° S, 39.6° E), Antarctica, station show a significant decrease in ozone at altitudes higher than that of the 4 hPa pressure level during the 1980s and 1990s. Ozone values over Syowa have remained low since 2001. The time series of upper stratospheric ozone from the homogenized NOAA SBUV (Solar Backscatter Ultraviolet Instrument)(/2) 8.6 overpass data (±4°, 24 h) are in qualitative agreement with those from the Syowa station data. Ozone recovery during the austral spring over the Syowa station appears to be slower than predicted by the equivalent effective stratospheric chlorine (EESC) curve. The long-term changes in the station's equivalent latitude (indicative of vortex size/position in winter and spring) are derived from MERRA (Modern Era Retrospective-analysis for Research and Applications) reanalyses at ~ 2 and ~ 50 hPa. These data are used to attribute some of the upper and middle stratospheric ozone changes to the changes in vortex position relative to the station's location. In addition, high correlation of the Southern Hemisphere annular mode (SAM) with polar upper stratospheric ozone during years of maximum solar activity points toward a strong relationship between the strength of the Brewer–Dobson circulation and the polar stratospheric ozone recovery. In the lower stratosphere, ozone recovery attributable to CFCs (chlorofluorocarbons) is still not definitive, whereas the recovery of the upper stratosphere is slower than predicted. Further research indicates that dynamical and other chemical changes in the atmosphere are delaying detection of recovery over this station.

Posted on 17 April 2014 | 12:00 am


On the abundance and source contributions of dicarboxylic acids in size-resolved aerosol particles at continental sites in central Europe

On the abundance and source contributions of dicarboxylic acids in size-resolved aerosol particles at continental sites in central Europe

Atmospheric Chemistry and Physics, 14, 3913-3928, 2014

Author(s): D. van Pinxteren, C. Neusüß, and H. Herrmann

Dicarboxylic acids (DCAs) are among the most abundant organic compounds observed in atmospheric aerosol particles and have been extensively studied at many places around the world. The importance of the various primary sources and secondary formation pathways discussed in the literature is often difficult to assess from field studies, though. In the present study, a large data set of size-resolved DCA concentrations from several inland sites in Germany is combined with results from a recently developed approach of statistical back-trajectory analysis and additional data. Principal component analysis is then used to reveal the most important factors governing the abundance of DCAs in different particle size ranges. The two most important sources revealed are (i) photochemical formation during intense radiation days in polluted air masses, likely occurring in the gas phase on short timescales (gasSOA), and (ii) secondary reactions in anthropogenically influenced air masses, likely occurring in the aqueous phase on longer timescales (aqSOA). While the first source strongly impacts DCA concentrations mainly in small and large particles, the second one enhances accumulation mode DCAs and is responsible for the bulk of the observed concentrations. Primary sources were found to be minor (sea salt, soil resuspension) or non-existent (biomass burning, traffic). The results can be regarded as representative for typical central European continental conditions.

Posted on 17 April 2014 | 12:00 am


Quantifying water diffusion in high-viscosity and glassy aqueous solutions using a Raman isotope tracer method

Quantifying water diffusion in high-viscosity and glassy aqueous solutions using a Raman isotope tracer method

Atmospheric Chemistry and Physics, 14, 3817-3830, 2014

Author(s): H. C. Price, B. J. Murray, J. Mattsson, D. O'Sullivan, T. W. Wilson, K. J. Baustian, and L. G. Benning

Recent research suggests that under certain temperature and relative humidity conditions atmospheric aerosol may be present in the form of a glassy solid. In order to understand the impacts that this may have on aerosol–cloud interactions and atmospheric chemistry, knowledge of water diffusion within such aerosol particles is required. Here, a method is described in which Raman spectroscopy is used to observe D2O diffusion in high-viscosity aqueous solutions, enabling a quantitative assessment of water diffusion coefficients, Dwater, as a function of relative humidity. Results for sucrose solutions compare well with literature data at 23.5 ± 0.3 °C, and demonstrate that water diffusion is slow (Dwater ~5 × 10−17 m2 s−1), but not arrested, just below the glass transition at a water activity of 0.2. Room temperature water diffusion coefficients are also presented for aqueous levoglucosan and an aqueous mixture of raffinose, dicarboxylic acids and ammonium sulphate: at low humidity, diffusion is retarded but still occurs on millisecond to second timescales in atmospherically relevant-sized particles. The effect of gel formation on diffusion in magnesium sulfate solutions is shown to be markedly different from the gradual decrease in diffusion coefficients of highly viscous liquids. We show that using the Stokes–Einstein equation to determine diffusion timescales from viscosity leads to values which are more than 5 orders of magnitude too big, which emphasises the need to make measurements of diffusion coefficients. In addition, comparison of bounce fraction data for levoglucosan with measured diffusion data reveals that even when particles bounce the diffusion timescales for water are a fraction of a second for a 100 nm particle. This suggests a high bounce fraction does not necessarily indicate retarded water diffusion.

Posted on 16 April 2014 | 12:00 am


An MCM modeling study of nitryl chloride (ClNO2) impacts on oxidation, ozone production and nitrogen oxide partitioning in polluted continental outflow

An MCM modeling study of nitryl chloride (ClNO2) impacts on oxidation, ozone production and nitrogen oxide partitioning in polluted continental outflow

Atmospheric Chemistry and Physics, 14, 3789-3800, 2014

Author(s): T. P. Riedel, G. M. Wolfe, K. T. Danas, J. B. Gilman, W. C. Kuster, D. M. Bon, A. Vlasenko, S.-M. Li, E. J. Williams, B. M. Lerner, P. R. Veres, J. M. Roberts, J. S. Holloway, B. Lefer, S. S. Brown, and J. A. Thornton

Nitryl chloride (ClNO2) is produced at night by reactions of dinitrogen pentoxide (N2O5) on chloride containing surfaces. ClNO2 is photolyzed during the morning hours after sunrise to liberate highly reactive chlorine atoms (Cl·). This chemistry takes place primarily in polluted environments where the concentrations of N2O5 precursors (nitrogen oxide radicals and ozone) are high, though it likely occurs in remote regions at lower intensities. Recent field measurements have illustrated the potential importance of ClNO2 as a daytime Cl· source and a nighttime NOx reservoir. However, the fate of the Cl· and the overall impact of ClNO2 on regional photochemistry remain poorly constrained by measurements and models. To this end, we have incorporated ClNO2 production, photolysis, and subsequent Cl· reactions into an existing master chemical mechanism (MCM version 3.2) box model framework using observational constraints from the CalNex 2010 field study. Cl· reactions with a set of alkenes and alcohols, and the simplified multiphase chemistry of N2O5, ClNO2, HOCl, ClONO2, and Cl2, none of which are currently part of the MCM, have been added to the mechanism. The presence of ClNO2 produces significant changes to oxidants, ozone, and nitrogen oxide partitioning, relative to model runs excluding ClNO2 formation. From a nighttime maximum of 1.5 ppbv ClNO2, the daytime maximum Cl· concentration reaches 1 × 105 atoms cm?3 at 07:00 model time, reacting mostly with a large suite of volatile organic compounds (VOC) to produce 2.2 times more organic peroxy radicals in the morning than in the absence of ClNO2. In the presence of several ppbv of nitrogen oxide radicals (NOx = NO + NO2), these perturbations lead to similar enhancements in hydrogen oxide radicals (HOx = OH + HO2). Neglecting contributions from HONO, the total integrated daytime radical source is 17% larger when including ClNO2, which leads to a similar enhancement in integrated ozone production of 15%. Detectable levels (tens of pptv) of chlorine containing organic compounds are predicted to form as a result of Cl· addition to alkenes, which may be useful in identifying times of active Cl· chemistry.

Posted on 16 April 2014 | 12:00 am


A new data set of soil mineralogy for dust-cycle modeling

A new data set of soil mineralogy for dust-cycle modeling

Atmospheric Chemistry and Physics, 14, 3801-3816, 2014

Author(s): E. Journet, Y. Balkanski, and S. P. Harrison

The mineralogy of airborne dust affects the impact of dust particles on direct and indirect radiative forcing, on atmospheric chemistry and on biogeochemical cycling. It is determined partly by the mineralogy of the dust-source regions and partly by size-dependent fractionation during erosion and transport. Here we present a data set that characterizes the clay and silt-sized fractions of global soil units in terms of the abundance of 12 minerals that are important for dust–climate interactions: quartz, feldspars, illite, smectite, kaolinite, chlorite, vermiculite, mica, calcite, gypsum, hematite and goethite. The basic mineralogical information is derived from the literature, and is then expanded following explicit rules, in order to characterize as many soil units as possible. We present three alternative realizations of the mineralogical maps, taking the uncertainties in the mineralogical data into account. We examine the implications of the new database for calculations of the single scattering albedo of airborne dust and thus for dust radiative forcing.

Posted on 16 April 2014 | 12:00 am


Secondary organic aerosol formation during June 2010 in Central Europe: measurements and modelling studies with a mixed thermodynamic-kinetic approach

Secondary organic aerosol formation during June 2010 in Central Europe: measurements and modelling studies with a mixed thermodynamic-kinetic approach

Atmospheric Chemistry and Physics, 14, 3831-3842, 2014

Author(s): B. Langmann, K. Sellegri, and E. Freney

Until recently secondary organic carbon aerosol (SOA) mass concentrations have been systematically underestimated by three-dimensional atmospheric-chemistry-aerosol models. With a newly proposed concept of aging of organic vapours, more realistic model results for organic carbon aerosol mass concentrations can be achieved. Applying a mixed thermodynamic-kinetic approach for SOA formation shifted the aerosol size distribution towards particles in the cloud condensation nuclei size range, thereby emphasising the importance of SOA formation schemes for modelling realistic cloud and precipitation formation. The additional importance of hetero-molecular nucleation between H2SO4 and organic vapours remains to be evaluated in three-dimensional atmospheric-chemistry-aerosol models. Here a case study is presented focusing on Puy-de-Dôme, France in June 2010. The measurements indicate a considerable increase in SOA mass concentration during the measurement campaign, which could be reproduced by modelling using a simplified thermodynamic-kinetic approach for SOA formation and increased biogenic volatile organic compound (VOC) precursor emissions. Comparison with a thermodynamic SOA formation approach shows a huge improvement in modelled SOA mass concentration with the thermodynamic-kinetic approach for SOA formation. SOA mass concentration increases by a factor of up to 6 accompanied by a slight improvement of modelled particle size distribution. Even though nucleation events at Puy-de-Dôme were rare during the chosen period of investigation, a weak event in the boundary layer could be reproduced by the model in a sensitivity study when nucleation of low-volatile secondary organic vapour is included. Differences in the model results with and without nucleation of organic vapour are visible in the lower free troposphere over several days. Taking into account the nucleation of organic vapour leads to an increase in accumulation mode particles due to coagulation and condensational growth of nucleation and Aitken mode particles.

Posted on 16 April 2014 | 12:00 am


Numerical analysis of the chemical kinetic mechanisms of ozone depletion and halogen release in the polar troposphere

Numerical analysis of the chemical kinetic mechanisms of ozone depletion and halogen release in the polar troposphere

Atmospheric Chemistry and Physics, 14, 3771-3787, 2014

Author(s): L. Cao, H. Sihler, U. Platt, and E. Gutheil

The role of halogen species (e.g., Br, Cl) in the troposphere of polar regions has been investigated since the discovery of their importance for boundary layer ozone destruction in the polar spring about 25 years ago. Halogen species take part in an auto-catalytic chemical reaction cycle, which releases Br2 and BrCl from the sea salt aerosols, fresh sea ice or snowpack, leading to ozone depletion. In this study, three different chemical reaction schemes are investigated: a bromine-only reaction scheme, which then is subsequently extended to include nitrogen-containing compounds and chlorine species and corresponding chemical reactions. The importance of specific reactions and their rate constants is identified by a sensitivity analysis.

The heterogeneous reaction rates are parameterized by considering the aerodynamic resistance, a reactive surface ratio, ?, i.e., the ratio of reactive surface area to total ground surface area, and the boundary layer height, Lmix. It is found that for β = 1, a substantial ozone decrease occurs after five days and ozone depletion lasts for 40 h for Lmix = 200 m. For about β ≥ 20, the time required for major ozone depletion ([O3] < 4 ppb) to occur becomes independent of the height of the boundary layer, and for β = 100 it approaches two days, 28 h of which are attributable to the induction and 20 h to the depletion time.

In polar regions, a small amount of NOx may exist, which stems from nitrate contained in the snow, and may have a strong impact on the ozone depletion. Therefore, the role of nitrogen-containing species on the ozone depletion rate is studied. The results show that the NOx concentrations are influenced by different chemical reactions over different time periods. During ozone depletion, the reaction cycle involving the BrONO2 hydrolysis is dominant. A critical value of 0.0004 of the uptake coefficient of the BrONO2 hydrolysis reaction at the aerosol and saline surfaces is identified, beyond which the existence of NOx species accelerates the ozone depletion event, whereas for lower values, deceleration occurs.

Posted on 15 April 2014 | 12:00 am


Aerosol's optical and physical characteristics and direct radiative forcing during a shamal dust storm, a case study

Aerosol's optical and physical characteristics and direct radiative forcing during a shamal dust storm, a case study

Atmospheric Chemistry and Physics, 14, 3751-3769, 2014

Author(s): T. M. Saeed, H. Al-Dashti, and C. Spyrou

Dust aerosols are analyzed for their optical and physical properties during an episode of a dust storm that blew over Kuwait on 26 March 2003 when the military Operation Iraqi Freedom was in full swing. The intensity of the dust storm was such that it left a thick suspension of dust throughout the following day, 27 March. The synoptic sequence leading to the dust storm and the associated wind fields are discussed. Ground-based measurements of aerosol optical thickness reached 3.617 and 4.17 on 26 and 27 March respectively while the Ångstrom coefficient, α870/440, dropped to ?0.0234 and ?0.0318. Particulate matter concentration of 10 ?m diameter or less, PM10, peaked at 4800 ?g m?3 during dust storm hours of 26 March. Moderate Resolution Imaging Spectroradiometer (MODIS) retrieved aerosol optical depth (AOD) by Deep Blue algorithm and Total Ozone Mapping Spectrometer (TOMS) aerosol index (AI) exhibited high values. Latitude–longitude maps of AOD and AI were used to deduce source regions of dust transport over Kuwait. The vertical profile of the dust layer was simulated using the SKIRON atmospheric model. Instantaneous net direct radiative forcing is calculated at top of atmosphere (TOA) and surface level. The thick dust layer of 26 March resulted in cooling the TOA by ?60 Wm?2 and surface level by ?175 Wm?2 for a surface albedo of 0.35. Slightly higher values were obtained for 27 March due to the increase in aerosol optical thickness. Radiative heating/cooling rates in the shortwave and longwave bands were also examined. Shortwave heating rate reached a maximum value of 2 K day?1 between 3 and 5 km, dropped to 1.5 K day?1 at 6 km and diminished at 8 km. Longwave radiation initially heated the lower atmosphere by a maximum value of 0.2 K day?1 at surface level, declined sharply at increasing altitude and diminished at 4 km. Above 4 km longwave radiation started to cool the atmosphere slightly reaching a maximum rate of ?0.1 K day?1 at 6 km.

Posted on 14 April 2014 | 12:00 am


Ice particle habit and surface roughness derived from PARASOL polarization measurements

Ice particle habit and surface roughness derived from PARASOL polarization measurements

Atmospheric Chemistry and Physics, 14, 3739-3750, 2014

Author(s): B. H. Cole, P. Yang, B. A. Baum, J. Riedi, and L. C.-Labonnote

Ice clouds are an important element in the radiative balance of the earth's climate system, but their microphysical and optical properties still are not well constrained, especially ice particle habit and the degree of particle surface roughness. In situ observations have revealed common ice particle habits and evidence for surface roughness, but these observations are limited. An alternative is to infer the ice particle shape and surface roughness from satellite observations of polarized reflectivity since they are sensitive to both particle shape and degree of surface roughness. In this study an adding–doubling radiative transfer code is used to simulate polarized reflectivity for nine different ice habits and one habit mixture, along with 17 distinct levels of the surface roughness. A lookup table (LUT) is constructed from the simulation results and used to infer shape and surface roughness from PARASOL satellite polarized reflectivity data over the ocean. Globally, the retrievals yield a compact aggregate of columns as the most commonly retrieved ice habit. Analysis of PARASOL data from the tropics results in slightly more aggregates than in midlatitude or polar regions. Some level of surface roughness is inferred in nearly 70% of PARASOL data, with mean and median roughness near ? = 0.2 and 0.15, respectively. Tropical region analyses have 20% more pixels retrieved with particle surface roughness than in midlatitude or polar regions. The global asymmetry parameter inferred at a wavelength of 0.865 ?m has a mean value of 0.77 and a median value of 0.75.

Posted on 11 April 2014 | 12:00 am


Technical Note: An empirical algorithm estimating dry deposition velocity of fine, coarse and giant particles

Technical Note: An empirical algorithm estimating dry deposition velocity of fine, coarse and giant particles

Atmospheric Chemistry and Physics, 14, 3729-3737, 2014

Author(s): L. Zhang and Z. He

An empirical algorithm is developed for calculating bulk dry deposition velocity (Vd) of fine (PM2.5) – particles having a diameter of ≤ 2.5 ?m), coarse (PM2.5−10 – particles having a diameter of 2.5–10 ?m), and giant (PM10+ – particles having a diameter of > 10 ?m) atmospheric particles. The algorithm is developed from an empirical fit of Vd data calculated using the size-resolved Vd scheme of Zhang et al. (2001) with assumed lognormal size distributions of PM2.5, PM2.5−10 and PM10+. In the new algorithm, the surface deposition velocity (Vds) is parameterized as a simple linear function of friction velocity (u*) for PM2.5 and as a polynomial function of u* for both PM2.5−10 and PM10+ over all the 26 land use categories (LUCs). An adjustment factor as an exponential function of u* and leaf area index (LAI) is also applied to Vds of PM2.5−10 and PM10+ over 9 of the 26 LUCs that have variable LAI. Constant gravitational settling velocities are provided for PM2.5, PM2.5−10 and PM10+. Aerodynamic resistance between a reference height and the surface can be calculated using available analytical formulas from the literature. The bulk Vd of PM2.5, PM2.5−10 and PM10+ at the reference height can then be calculated by combining the gravitational settling velocity, aerodynamic resistance and the parameterized Vds. Vd values calculated using the new algorithm are within ±20% of those using the original size-resolved scheme for fine, coarse and giant particles. Uncertainties in Vd values from the new algorithm due to the pre-assumed size distributions are on the order of 20% for fine particles and on the order of a factor of 2.0 for coarse and giant particles. The new algorithm provides an alternative approach for calculating Vd of bulk aerosol particles. Vd of any particulate species can be simply estimated using this scheme as long as the mass fractions in fine, coarse and giant particles are known or can be assumed.

Posted on 11 April 2014 | 12:00 am


Inferring regional sources and sinks of atmospheric CO2 from GOSAT XCO2 data

Inferring regional sources and sinks of atmospheric CO2 from GOSAT XCO2 data

Atmospheric Chemistry and Physics, 14, 3703-3727, 2014

Author(s): F. Deng, D. B. A. Jones, D. K. Henze, N. Bousserez, K. W. Bowman, J. B. Fisher, R. Nassar, C. O'Dell, D. Wunch, P. O. Wennberg, E. A. Kort, S. C. Wofsy, T. Blumenstock, N. M. Deutscher, D. W. T. Griffith, F. Hase, P. Heikkinen, V. Sherlock, K. Strong, R. Sussmann, and T. Warneke

We have examined the utility of retrieved column-averaged, dry-air mole fractions of CO2 (XCO2) from the Greenhouse Gases Observing Satellite (GOSAT) for quantifying monthly, regional flux estimates of CO2, using the GEOS-Chem four-dimensional variational (4D-Var) data assimilation system. We focused on assessing the potential impact of biases in the GOSAT CO2 data on the regional flux estimates. Using different screening and bias correction approaches, we selected three different subsets of the GOSAT XCO2 data for the 4D-Var inversion analyses, and found that the inferred global fluxes were consistent across the three XCO2 inversions. However, the GOSAT observational coverage was a challenge for the regional flux estimates. In the northern extratropics, the inversions were more sensitive to North American fluxes than to European and Asian fluxes due to the lack of observations over Eurasia in winter and over eastern and southern Asia in summer. The regional flux estimates were also sensitive to the treatment of the residual bias in the GOSAT XCO2 data. The largest differences obtained were for temperate North America and temperate South America, for which the largest spread between the inversions was 1.02 and 0.96 Pg C, respectively. In the case of temperate North America, one inversion suggested a strong source, whereas the second and third XCO2 inversions produced a weak and strong sink, respectively. Despite the discrepancies in the regional flux estimates between the three XCO2 inversions, the a posteriori CO2 distributions were in good agreement (with a mean difference between the three inversions of typically less than 0.5 ppm) with independent data from the Total Carbon Column Observing Network (TCCON), the surface flask network, and from the HIAPER Pole-to-Pole Observations (HIPPO) aircraft campaign. The discrepancy in the regional flux estimates from the different inversions, despite the agreement of the global flux estimates suggests the need for additional work to determine the minimum spatial scales at which we can reliably quantify the fluxes using GOSAT XCO2. The fact that the a posteriori CO2 from the different inversions were in good agreement with the independent data although the regional flux estimates differed significantly, suggests that innovative ways of exploiting existing data sets, and possibly additional observations, are needed to better evaluate the inferred regional flux estimates.

Posted on 11 April 2014 | 12:00 am


Oligomer formation within secondary organic aerosols: equilibrium and dynamic considerations

Oligomer formation within secondary organic aerosols: equilibrium and dynamic considerations

Atmospheric Chemistry and Physics, 14, 3691-3701, 2014

Author(s): E. R. Trump and N. M Donahue

We present a model based on the volatility basis set to consider the potential influence of oligomer content on volatility driven secondary organic aerosol (SOA) yields. The implications for aerosol evaporation studies, including dilution, chamber thermo-equilibration, and thermodenuder studies, are also considered. A simplified description of oligomer formation reproduces essentially all of the broad classes of equilibrium and dynamical observations related to SOA formation and evaporation: significant oligomer content may be consistent with mass yields that increase with organic aerosol mass concentration; reversible oligomerization can explain the hysteresis between the rate of SOA formation and its evaporation rate upon dilution; and the model is consistent with both chamber thermo-equilibration studies and thermodenuder studies of SOA evaporation.

Posted on 10 April 2014 | 12:00 am


Multi-decadal aerosol variations from 1980 to 2009: a perspective from observations and a global model

Multi-decadal aerosol variations from 1980 to 2009: a perspective from observations and a global model

Atmospheric Chemistry and Physics, 14, 3657-3690, 2014

Author(s): M. Chin, T. Diehl, Q. Tan, J. M. Prospero, R. A. Kahn, L. A. Remer, H. Yu, A. M. Sayer, H. Bian, I. V. Geogdzhayev, B. N. Holben, S. G. Howell, B. J. Huebert, N. C. Hsu, D. Kim, T. L. Kucsera, R. C. Levy, M. I. Mishchenko, X. Pan, P. K. Quinn, G. L. Schuster, D. G. Streets, S. A. Strode, O. Torres, and X.-P. Zhao

Aerosol variations and trends over different land and ocean regions from 1980 to 2009 are analyzed with the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model and observations from multiple satellite sensors and available ground-based networks. Excluding time periods with large volcanic influence, aerosol optical depth (AOD) and surface concentration over polluted land regions generally vary with anthropogenic emissions, but the magnitude of this association can be dampened by the presence of natural aerosols, especially dust. Over the 30-year period in this study, the largest reduction in aerosol levels occurs over Europe, where AOD has decreased by 40–60% on average and surface sulfate concentrations have declined by a factor of up to 3–4. In contrast, East Asia and South Asia show AOD increases, but the relatively high level of dust aerosols in Asia reduces the correlation between AOD and pollutant emission trends. Over major dust source regions, model analysis indicates that the change of dust emissions over the Sahara and Sahel has been predominantly driven by the change of near-surface wind speed, but over Central Asia it has been largely influenced by the change of the surface wetness. The decreasing dust trend in the North African dust outflow region of the tropical North Atlantic and the receptor sites of Barbados and Miami is closely associated with an increase of the sea surface temperature in the North Atlantic. This temperature increase may drive the decrease of the wind velocity over North Africa, which reduces the dust emission, and the increase of precipitation over the tropical North Atlantic, which enhances dust removal during transport. Despite significant trends over some major continental source regions, the model-calculated global annual average AOD shows little change over land and ocean in the past three decades, because opposite trends in different land regions cancel each other out in the global average, and changes over large open oceans are negligible. This highlights the necessity for regional-scale assessment of aerosols and their climate impacts, as global-scale average values can obscure important regional changes.

Posted on 10 April 2014 | 12:00 am


Improved satellite retrievals of NO2 and SO2 over the Canadian oil sands and comparisons with surface measurements

Improved satellite retrievals of NO2 and SO2 over the Canadian oil sands and comparisons with surface measurements

Atmospheric Chemistry and Physics, 14, 3637-3656, 2014

Author(s): C. A. McLinden, V. Fioletov, K. F. Boersma, S. K. Kharol, N. Krotkov, L. Lamsal, P. A. Makar, R. V. Martin, J. P. Veefkind, and K. Yang

Satellite remote sensing is increasingly being used to monitor air quality over localized sources such as the Canadian oil sands. Following an initial study, significantly low biases have been identified in current NO2 and SO2 retrieval products from the Ozone Monitoring Instrument (OMI) satellite sensor over this location resulting from a combination of its rapid development and small spatial scale. Air mass factors (AMFs) used to convert line-of-sight "slant" columns to vertical columns were re-calculated for this region based on updated and higher resolution input information including absorber profiles from a regional-scale (15 km × 15 km resolution) air quality model, higher spatial and temporal resolution surface reflectivity, and an improved treatment of snow. The overall impact of these new Environment Canada (EC) AMFs led to substantial increases in the peak NO2 and SO2 average vertical column density (VCD), occurring over an area of intensive surface mining, by factors of 2 and 1.4, respectively, relative to estimates made with previous AMFs. Comparisons are made with long-term averages of NO2 and SO2 (2005–2011) from in situ surface monitors by using the air quality model to map the OMI VCDs to surface concentrations. This new OMI-EC product is able to capture the spatial distribution of the in situ instruments (slopes of 0.65 to 1.0, correlation coefficients of >0.9). The concentration absolute values from surface network observations were in reasonable agreement, with OMI-EC NO2 and SO2 biased low by roughly 30%. Several complications were addressed including correction for the interference effect in the surface NO2 instruments and smoothing and clear-sky biases in the OMI measurements. Overall these results highlight the importance of using input information that accounts for the spatial and temporal variability of the location of interest when performing retrievals.

Posted on 9 April 2014 | 12:00 am





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