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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:



The thermodynamic structure of summer Arctic stratocumulus and the dynamic coupling to the surface

The thermodynamic structure of summer Arctic stratocumulus and the dynamic coupling to the surface

Atmospheric Chemistry and Physics, 14, 12573-12592, 2014

Author(s): G. Sotiropoulou, J. Sedlar, M. Tjernström, M. D. Shupe, I. M. Brooks, and P. O. G. Persson

The vertical structure of Arctic low-level clouds and Arctic boundary layer is studied, using observations from ASCOS (Arctic Summer Cloud Ocean Study), in the central Arctic, in late summer 2008. Two general types of cloud structures are examined: the "neutrally stratified" and "stably stratified" clouds. Neutrally stratified are mixed-phase clouds where radiative-cooling near cloud top produces turbulence that generates a cloud-driven mixed layer. When this layer mixes with the surface-generated turbulence, the cloud layer is coupled to the surface, whereas when such an interaction does not occur, it remains decoupled; the latter state is most frequently observed. The decoupled clouds are usually higher compared to the coupled; differences in thickness or cloud water properties between the two cases are however not found. The surface fluxes are also very similar for both states. The decoupled clouds exhibit a bimodal thermodynamic structure, depending on the depth of the sub-cloud mixed layer (SCML): clouds with shallower SCMLs are disconnected from the surface by weak inversions, whereas those that lay over a deeper SCML are associated with stronger inversions at the decoupling height. Neutrally stratified clouds generally precipitate; the evaporation/sublimation of precipitation often enhances the decoupling state. Finally, stably stratified clouds are usually lower, geometrically and optically thinner, non-precipitating liquid-water clouds, not containing enough liquid to drive efficient mixing through cloud-top cooling.

Posted on 28 November 2014 | 12:00 am


Analysis of elevated springtime levels of Peroxyacetyl nitrate (PAN) at the high Alpine research sites Jungfraujoch and Zugspitze

Analysis of elevated springtime levels of Peroxyacetyl nitrate (PAN) at the high Alpine research sites Jungfraujoch and Zugspitze

Atmospheric Chemistry and Physics, 14, 12553-12571, 2014

Author(s): S. Pandey Deolal, S. Henne, L. Ries, S. Gilge, U. Weers, M. Steinbacher, J. Staehelin, and T. Peter

The largest atmospheric peroxyacetyl nitrate (PAN) mole fractions at remote surface sites in the Northern Hemisphere are commonly observed during the months April and May. Different formation mechanisms for this seasonal maximum have previously been suggested: hemispheric-scale production from precursors accumulated during the winter months, increased springtime transport from up-wind continents or increased regional-scale production in the atmospheric boundary layer from recent emissions. The two high Alpine research sites Jungfraujoch (Switzerland) and Zugspitze (Germany) exhibit a distinct and consistent springtime PAN maximum. Since these sites intermittently sample air masses of free-tropospheric and boundary layer origin, they are ideally suited to identify the above-mentioned PAN formation processes and attribute local observations to these. Here we present a detailed analysis of PAN observations and meteorological conditions during May 2008 when PAN levels were especially elevated at both sites. The highest PAN concentrations were connected with anticyclonic conditions, which persisted in May 2008 for about 10 days with north-easterly advection towards the sites. A backward dispersion model analysis showed that elevated PAN concentrations were caused by the combination of favourable photochemical production conditions and large precursor concentrations in the European atmospheric boundary layer. The results suggest that the largest PAN values in spring 2008 at both sites were attributable to regional-scale photochemical production of PAN in the (relatively cold) planetary boundary layer from European precursors, whereas the contribution of inter-continental transport or free-tropospheric build-up was of smaller importance for these sites.

Posted on 28 November 2014 | 12:00 am


Low temperatures enhance organic nitrate formation: evidence from observations in the 2012 Uintah Basin Winter Ozone Study

Low temperatures enhance organic nitrate formation: evidence from observations in the 2012 Uintah Basin Winter Ozone Study

Atmospheric Chemistry and Physics, 14, 12441-12454, 2014

Author(s): L. Lee, P. J. Wooldridge, J. B. Gilman, C. Warneke, J. de Gouw, and R. C. Cohen

Nitrogen dioxide (NO2) and total alkyl nitrates (?ANs) were measured using thermal dissociation laser-induced fluorescence during the 2012 Uintah Basin Winter Ozone Study (UBWOS) in Utah, USA. The observed NO2 concentration was highest before sunrise and lowest in the late afternoon, suggestive of a persistent local source of NO2 coupled with turbulent mixing out of the boundary layer. In contrast, ?ANs co-varied with solar radiation with a noontime maximum, indicating that local photochemical production combined with rapid mixing and/or deposition was the dominant factor in determining the ?AN concentrations. We calculate that ?ANs were a large fraction (~60%) of the HOx free radical chain termination and show that the temperature dependence of the alkyl nitrate yields enhances the role of ?ANs in local chemistry during winter by comparison to what would occur in the warmer temperatures of summer.

Posted on 27 November 2014 | 12:00 am


Competition between water uptake and ice nucleation by glassy organic aerosol particles

Competition between water uptake and ice nucleation by glassy organic aerosol particles

Atmospheric Chemistry and Physics, 14, 12513-12531, 2014

Author(s): T. Berkemeier, M. Shiraiwa, U. Pöschl, and T. Koop

Organic aerosol particles play a key role in climate by serving as nuclei for clouds and precipitation. Their sources and composition are highly variable, and their phase state ranges from liquid to solid under atmospheric conditions, affecting the pathway of activation to cloud droplets and ice crystals. Due to slow diffusion of water in the particle phase, organic particles may deviate in phase and morphology from their thermodynamic equilibrium state, hampering the prediction of their influence on cloud formation. We overcome this problem by combining a novel semi-empirical method for estimation of water diffusivity with a kinetic flux model that explicitly treats water diffusion. We estimate timescales for particle deliquescence as well as various ice nucleation pathways for a wide variety of organic substances, including secondary organic aerosol (SOA) from the oxidation of isoprene, ?-pinene, naphthalene, and dodecane. The simulations show that, in typical atmospheric updrafts, glassy states and solid/liquid core-shell morphologies can persist for long enough that heterogeneous ice nucleation in the deposition and immersion mode can dominate over homogeneous ice nucleation. Such competition depends strongly on ambient temperature and relative humidity as well as humidification rate and particle size. Due to differences in glass transition temperature, hygroscopicity and atomic O / C ratio of the different SOA, naphthalene SOA particles have the highest potential to act as heterogeneous ice nuclei. Our findings demonstrate that kinetic limitations of water diffusion into organic aerosol particles are likely to be encountered under atmospheric conditions and can strongly affect ice nucleation pathways. For the incorporation of ice nucleation by organic aerosol particles into atmospheric models, our results demonstrate a demand for model formalisms that account for the effects of molecular diffusion and not only describe ice nucleation onsets as a function of temperature and relative humidity but also include updraft velocity, particle size and composition.

Posted on 27 November 2014 | 12:00 am


Chemistry and mineralogy of clay minerals in Asian and Saharan dusts and the implications for iron supply to the oceans

Chemistry and mineralogy of clay minerals in Asian and Saharan dusts and the implications for iron supply to the oceans

Atmospheric Chemistry and Physics, 14, 12415-12428, 2014

Author(s): G. Y. Jeong and E. P. Achterberg

Mineral dust supplied to remote ocean regions stimulates phytoplankton growth through delivery of micronutrients, notably iron (Fe). Although attention is usually paid to Fe (hydr)oxides as major sources of available Fe, Fe-bearing clay minerals are typically the dominant phase in mineral dust. The mineralogy and chemistry of clay minerals in dust particles, however, are largely unknown. We conducted microscopic identification and chemical analysis of the clay minerals in Asian and Saharan dust particles. Cross-sectional slices of dust particles were prepared by focused ion beam (FIB) techniques and analyzed by transmission electron microscopy (TEM) combined with energy dispersive X-ray spectroscopy (EDXS). TEM images of FIB slices revealed that clay minerals occurred as either nano-thin platelets or relatively thick plates. Chemical compositions and lattice fringes of the nano-thin platelets suggested that they included illite, smectite, illite–smectite mixed layers, and their nanoscale mixtures (illite–smectite series clay minerals, ISCMs) which could not be resolved with an electron microbeam. EDXS chemical analysis of the clay mineral grains revealed that the average Fe content was 5.8% in nano-thin ISCM platelets assuming 14% H2O, while the Fe content of illite and chlorite was 2.8 and 14.8%, respectively. In addition, TEM and EDXS analyses were performed on clay mineral grains dispersed and loaded on micro-grids. The average Fe content of clay mineral grains was 6.7 and 5.4% in Asian and Saharan dusts, respectively. A comparative X-ray diffraction analysis of bulk dusts showed that Saharan dust was more enriched in clay minerals than Asian dust, while Asian dust was more enriched in chlorite. Clay minerals, in particular nanocrystalline ISCMs and Fe-rich chlorite, are probably important sources of Fe to remote marine ecosystems. Further detailed analyses of the mineralogy and chemistry of clay minerals in global mineral dusts are required to evaluate the inputs of Fe to surface ocean microbial communities.

Posted on 27 November 2014 | 12:00 am


Stratospheric lifetime ratio of CFC-11 and CFC-12 from satellite and model climatologies

Stratospheric lifetime ratio of CFC-11 and CFC-12 from satellite and model climatologies

Atmospheric Chemistry and Physics, 14, 12479-12497, 2014

Author(s): L. Hoffmann, C. M. Hoppe, R. Müller, G. S. Dutton, J. C. Gille, S. Griessbach, A. Jones, C. I. Meyer, R. Spang, C. M. Volk, and K. A. Walker

Chlorofluorocarbons (CFCs) play a key role in stratospheric ozone loss and are strong infrared absorbers that contribute to global warming. The stratospheric lifetimes of CFCs are a measure of their stratospheric loss rates that are needed to determine global warming and ozone depletion potentials. We applied the tracer–tracer correlation approach to zonal mean climatologies from satellite measurements and model data to assess the lifetimes of CFCl3 (CFC-11) and CF2Cl2 (CFC-12). We present estimates of the CFC-11/CFC-12 lifetime ratio and the absolute lifetime of CFC-12, based on a reference lifetime of 52 years for CFC-11. We analyzed climatologies from three satellite missions, the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS), the HIgh Resolution Dynamics Limb Sounder (HIRDLS), and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). We found a CFC-11/CFC-12 lifetime ratio of 0.47±0.08 and a CFC-12 lifetime of 112(96–133) years for ACE-FTS, a ratio of 0.46±0.07 and a lifetime of 113(97–134) years for HIRDLS, and a ratio of 0.46±0.08 and a lifetime of 114(98–136) years for MIPAS. The error-weighted, combined CFC-11/CFC-12 lifetime ratio is 0.46±0.04 and the CFC-12 lifetime estimate is 113(103–124) years. These results agree with the recent Stratosphere-troposphere Processes And their Role in Climate (SPARC) reassessment, which recommends lifetimes of 52(43–67) years and 102(88–122) years, respectively. Having smaller uncertainties than the results from other recent studies, our estimates can help to better constrain CFC-11 and CFC-12 lifetime recommendations in future scientific studies and assessments. Furthermore, the satellite observations were used to validate first simulation results from a new coupled model system, which integrates a Lagrangian chemistry transport model into a climate model. For the coupled model we found a CFC-11/CFC-12 lifetime ratio of 0.48±0.07 and a CFC-12 lifetime of 110(95–129) years, based on a 10-year perpetual run. Closely reproducing the satellite observations, the new model system will likely become a useful tool to assess the impact of advective transport, mixing, and photochemistry as well as climatological variability on the stratospheric lifetimes of long-lived tracers.

Posted on 27 November 2014 | 12:00 am


Modelled black carbon radiative forcing and atmospheric lifetime in AeroCom Phase II constrained by aircraft observations

Modelled black carbon radiative forcing and atmospheric lifetime in AeroCom Phase II constrained by aircraft observations

Atmospheric Chemistry and Physics, 14, 12465-12477, 2014

Author(s): B. H. Samset, G. Myhre, A. Herber, Y. Kondo, S.-M. Li, N. Moteki, M. Koike, N. Oshima, J. P. Schwarz, Y. Balkanski, S. E. Bauer, N. Bellouin, T. K. Berntsen, H. Bian, M. Chin, T. Diehl, R. C. Easter, S. J. Ghan, T. Iversen, A. Kirkevåg, J.-F. Lamarque, G. Lin, X. Liu, J. E. Penner, M. Schulz, Ø. Seland, R. B. Skeie, P. Stier, T. Takemura, K. Tsigaridis, and K. Zhang

Atmospheric black carbon (BC) absorbs solar radiation, and exacerbates global warming through exerting positive radiative forcing (RF). However, the contribution of BC to ongoing changes in global climate is under debate. Anthropogenic BC emissions, and the resulting distribution of BC concentration, are highly uncertain. In particular, long-range transport and processes affecting BC atmospheric lifetime are poorly understood. Here we discuss whether recent assessments may have overestimated present-day BC radiative forcing in remote regions. We compare vertical profiles of BC concentration from four recent aircraft measurement campaigns to simulations by 13 aerosol models participating in the AeroCom Phase II intercomparison. An atmospheric lifetime of BC of less than 5 days is shown to be essential for reproducing observations in remote ocean regions, in line with other recent studies. Adjusting model results to measurements in remote regions, and at high altitudes, leads to a 25% reduction in AeroCom Phase II median direct BC forcing, from fossil fuel and biofuel burning, over the industrial era. The sensitivity of modelled forcing to BC vertical profile and lifetime highlights an urgent need for further flight campaigns, close to sources and in remote regions, to provide improved quantification of BC effects for use in climate policy.

Posted on 27 November 2014 | 12:00 am


Modeling of gaseous methylamines in the global atmosphere: impacts of oxidation and aerosol uptake

Modeling of gaseous methylamines in the global atmosphere: impacts of oxidation and aerosol uptake

Atmospheric Chemistry and Physics, 14, 12455-12464, 2014

Author(s): F. Yu and G. Luo

Gaseous amines have attracted increasing attention due to their potential role in enhancing particle nucleation and growth and affecting secondary organic aerosol formation. Here we study with a chemistry transport model the global distributions of the most common and abundant amines in the air: monomethylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA). We show that gas phase oxidation and aerosol uptakes are dominant sinks for these methylamines. The oxidation alone (i.e., no aerosol uptake) leads to methylamine lifetimes of 5–10 h in most parts of low and middle latitude regions. The uptake by secondary species with uptake coefficient (?) of 0.03 (corresponding to the uptake by sulfuric acid particles) reduces the lifetime by ~30% over oceans and much more over the major continents, resulting in a methylamine lifetime of less than 1–2 h over central Europe, eastern Asia, and eastern US. With the estimated global emission flux, from the literature, our simulations indicate that [DMA] in the model surface layer over major continents is generally in the range of 0.1–2 ppt (parts per trillion) when ? = 0.03 and 0.2–10 ppt when ? = 0, and decreases quickly with altitude. [DMA] over oceans is below 0.05 ppt and over polar regions it is below 0.01 ppt. The simulated [MMA] is about a factor of ~2.5 higher while [TMA] is a factor of ~8 higher than [DMA]. The modeled concentrations of methylamines are substantially lower than the limited observed values available, with normalized mean bias ranging from ?57 (? = 0) to ?88% (? = 0.03) for MMA and TMA, and from ?78 (? = 0) to ?93% (? = 0.03) for DMA.

Posted on 27 November 2014 | 12:00 am


Air quality simulations of wildfires in the Pacific Northwest evaluated with surface and satellite observations during the summers of 2007 and 2008

Air quality simulations of wildfires in the Pacific Northwest evaluated with surface and satellite observations during the summers of 2007 and 2008

Atmospheric Chemistry and Physics, 14, 12533-12551, 2014

Author(s): F. L. Herron-Thorpe, G. H. Mount, L. K. Emmons, B. K. Lamb, D. A. Jaffe, N. L. Wigder, S. H. Chung, R. Zhang, M. D. Woelfle, and J. K. Vaughan

Evaluation of a regional air quality forecasting system for the Pacific Northwest was carried out using a suite of surface and satellite observations. Wildfire events for the 2007 and 2008 fire seasons were simulated using the Air Information Report for Public Access and Community Tracking v.3 (AIRPACT-3) framework utilizing the Community Multi-scale Air Quality (CMAQ) model. Fire emissions were simulated using the BlueSky framework with fire locations determined by the Satellite Mapping Automated Reanalysis Tool for Fire Incident Reconciliation (SMARTFIRE). Plume rise was simulated using two different methods: the Fire Emission Production Simulator (FEPS) and the Sparse Matrix Operator Kernel Emissions (SMOKE) model. Predicted plume top heights were compared to the Cloud-Aerosol LIDAR with Orthogonal Polarization (CALIOP) instrument aboard the Cloud Aerosol LIDAR and Infrared Pathfinder Satellite Observation (CALIPSO) satellite. Carbon monoxide predictions were compared to the Atmospheric InfraRed Sounder (AIRS) instrument aboard the Aqua satellite. Horizontal distributions of column aerosol optical depth (AOD) were compared to retrievals by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument aboard the Aqua satellite. Model tropospheric nitrogen dioxide distributions were compared to retrievals from the Ozone Monitoring Instrument (OMI) aboard the Aura satellite. Surface ozone and PM2.5 predictions were compared to surface observations. The AIRPACT-3 model captured the location and transport direction of fire events well, but sometimes missed the timing of fire events and overall underestimated the PM2.5 impact of wildfire events at surface monitor locations. During the 2007 (2008) fire period, the fractional biases (FBs) of AIRPACT-3 for various pollutant observations included: average 24 h PM2.5 FB = ?33% (?27%); maximum daily average 8 h ozone FB = ?8% (+1%); AOD FB = ?61% (?53%); total column CO FB = ?10% (?5%); and tropospheric column NO2 FB = ?39% (?28%). The bias in total column CO is within the range of expected error. Fractional biases of AIRPACT-3 plume tops were found to be ?46% when compared in terms of above mean sea level, but only ?28% when compared in terms of above ground level, partly due to the under-estimation of AIRPACT-3 ground height in complex terrain that results from the 12 km grid-cell smoothing. We conclude that aerosol predictions were too low for locations greater than ~100–300 km downwind from wildfire sources and that model predictions are likely under-predicting secondary organic aerosol (SOA) production, due to a combination of very low volatile organic compound (VOC) emission factors used in the United States Forest Service Consume model, an incomplete speciation of VOC to SOA precursors in SMOKE, and under-prediction by the SOA parameterization within CMAQ.

Posted on 27 November 2014 | 12:00 am


A new parameterization of particle dry deposition over rough surfaces

A new parameterization of particle dry deposition over rough surfaces

Atmospheric Chemistry and Physics, 14, 12429-12440, 2014

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

In existing particle dry deposition schemes, the effects of gravity and surface roughness elements on particle motion are often poorly represented. In this study, we propose a new scheme to overcome such deficiencies. Particle deposition velocity is a function of aerodynamic, surface-collection and gravitational resistances. In this study, the effect of gravitation settling is treated analytically. More importantly, the new scheme takes into consideration the impacts of roughness elements on turbulent particle diffusion and surface particle collection. A relationship between aerodynamic and surface-collection processes is established by using an analogy between drag partition and deposition-flux partition. The scheme is then tested against a wind-tunnel data set for four different surfaces and a good agreement between the scheme predictions and the observations is found. The sensitivity of the scheme to the input parameters is tested. Important factors which affect particle deposition in different particle size ranges are identified. The scheme shows good capacity for modeling particle deposition over rough surfaces.

Posted on 27 November 2014 | 12:00 am


Variations of cloud condensation nuclei (CCN) and aerosol activity during fog–haze episode: a case study from Shanghai

Variations of cloud condensation nuclei (CCN) and aerosol activity during fog–haze episode: a case study from Shanghai

Atmospheric Chemistry and Physics, 14, 12499-12512, 2014

Author(s): C. Leng, Q. Zhang, D. Zhang, C. Xu, T. Cheng, R. Zhang, J. Tao, J. Chen, S. Zha, Y. Zhang, X. Li, L. Kong, and W. Gao

Measurements of cloud condensation nuclei (CCN), condensation nuclei (CN) and aerosol chemical composition were performed simultaneously at an urban site in Shanghai from 6 to 9 November 2010. The variations of CCN number concentration (NCCN) and aerosol activity (activated aerosol fraction, NCCN/NCN) were examined during a fog–haze co-occurring event. Anthropogenic pollutants emitted from vehicles and unfavorable meteorological conditions such as low planetary boundary layer (PBL) height exerted a great influence on PM2.5 and black carbon (BC) loadings. NCCN at 0.2% supersaturation (SS) mostly fell in the range of 994 to 6268 cm?3, and the corresponding NCCN/NCN varied between 0.09 and 0.57. NCCN and NCCN/NCN usually were usually higher in the hazy case due to increased aerosol concentration in the accumulation mode (100–500 nm), and lower in the foggy–hazy and clear cases. The BC mass concentration posed a strong positive effect on NCCN in the foggy–hazy and hazy cases, whereas it poorly correlated with NCCN in the clear case. NCCN/NCN was weakly related with BC in both foggy–hazy and hazy cases. By using a simplified particle hygroscopicity (κ), the calculated critical dry size (CDS) of activated aerosol did not exceed 130 nm at 0.2% SS in spite of diverse aerosol chemical compositions. The predicted NCCN at 0.2% SS was very successful compared with the observed NCCN in clear case (R2=0.96) and foggy–hazy/hazy cases (R2=0.91). In addition, their corresponding ratios of predicted to observed NCCNwere on average 0.95 and 0.92, respectively. More organic matter is possibly responsible for this closure difference between foggy–hazy/hazy and clear cases. These results reveal that the particulate pollutant burden exerts a significant impact on NCCN, especially NCCN/NCN promotes effectively during the polluted periods.

Posted on 27 November 2014 | 12:00 am


Regional climate model assessment of the urban land-surface forcing over central Europe

Regional climate model assessment of the urban land-surface forcing over central Europe

Atmospheric Chemistry and Physics, 14, 12393-12413, 2014

Author(s): P. Huszar, T. Halenka, M. Belda, M. Zak, K. Sindelarova, and J. Miksovsky

For the purpose of qualifying and quantifying the climate impact of cities and urban surfaces in general on climate of central Europe, the surface parameterization in regional climate model RegCM4 has been extended with the Single-layer Urban Canopy Model (SLUCM). A set of experiments was performed over the period of 2005–2009 for central Europe, either without considering urban surfaces or with the SLUCM treatment. Results show a statistically significant impact of urbanized surfaces on temperature (up to 1.5 K increase in summer) as well as on the boundary layer height (increases up to 50 m). Urbanization further influences surface wind with a winter decrease up to ?0.6 m s?1, though both increases and decreases were detected in summer depending on the location relative to the cities and daytime (changes up to 0.3 m s?1). Urban surfaces significantly reduce the humidity over the surface. This impacts the simulated summer precipitation rate, showing a decrease over cities of up to ?2 mm day?1. Significant temperature increases are simulated over higher altitudes as well, not only within the urban canopy layer. With the urban parameterization, the climate model better describes the diurnal temperature variation, reducing the cold afternoon and evening bias of RegCM4.

Sensitivity experiments were carried out to quantify the response of the meteorological conditions to changes in the parameters specific to the urban environment, such as street width, building height, albedo of the roofs and anthropogenic heat release. The results proved to be rather robust and the choice of the key SLUCM parameters impacts them only slightly (mainly temperature, boundary layer height and wind velocity). Statistically significant impacts are modelled not only over large urbanized areas, but the influence of the cities is also evident over rural areas without major urban surfaces. It is shown that this is the result of the combined effect of the distant influence of the cities and the influence of the minor local urban surface coverage.

Posted on 26 November 2014 | 12:00 am


Measurements of OH and RO2 radicals at Dome C, East Antarctica

Measurements of OH and RO2 radicals at Dome C, East Antarctica

Atmospheric Chemistry and Physics, 14, 12373-12392, 2014

Author(s): A. Kukui, M. Legrand, S. Preunkert, M. M. Frey, R. Loisil, J. Gil Roca, B. Jourdain, M. D. King, J. L. France, and G. Ancellet

Concentrations of OH radicals and the sum of peroxy radicals, RO2, were measured in the boundary layer for the first time on the East Antarctic Plateau at the Concordia Station (Dome C, 75.10° S, 123.31° E) during the austral summer 2011/2012. The median concentrations of OH and RO2 radicals were 3.1 × 106 molecule cm?3 and 9.9 × 106 molecule cm?3, respectively. These values are comparable to those observed at the South Pole, confirming that the elevated oxidative capacity of the Antarctic atmospheric boundary layer found at the South Pole is not restricted to the South Pole but common over the high Antarctic plateau. At Concordia, the concentration of radicals showed distinct diurnal profiles with the median maximum of 5.2 × 106 molecule cm?3 at 11:00 and the median minimum of 1.1 × 106 molecule cm?3 at 01:00 for OH radicals and 1.7 × 108 molecule cm?3 and 2.5 × 107 molecule cm?3 for RO2 radicals at 13:00 and 23:00, respectively (all times are local times). Concurrent measurements of O3, HONO, NO, NO2, HCHO and H2O2 demonstrated that the major primary source of OH and RO2 radicals at Dome C was the photolysis of HONO, HCHO and H2O2, with the photolysis of HONO contributing ~75% of total primary radical production. However, photochemical modelling with accounting for all these radical sources overestimates the concentrations of OH and RO2 radicals by a factor of 2 compared to field observations. Neglecting the net OH production from HONO in the photochemical modelling results in an underestimation of the concentrations of OH and RO2 radicals by a factor of 2. To explain the observations of radicals in this case an additional source of OH equivalent to about (25–35)% of measured photolysis of HONO is required. Even with a factor of 5 reduction in the concentrations of HONO, the photolysis of HONO represents the major primary radical source at Dome C. To account for a possibility of an overestimation of NO2 observed at Dome C the calculations were also performed with NO2 concentrations estimated by assuming steady-state NO2 / NO ratios. In this case the net radical production from the photolysis of HONO should be reduced by a factor of 5 or completely removed based on the photochemical budget of OH or 0-D modelling, respectively. Another major factor leading to the large concentration of OH radicals measured at Dome C was large concentrations of NO molecules and fast recycling of peroxy radicals to OH radicals.

Posted on 26 November 2014 | 12:00 am


Particle surface area dependence of mineral dust in immersion freezing mode: investigations with freely suspended drops in an acoustic levitator and a vertical wind tunnel

Particle surface area dependence of mineral dust in immersion freezing mode: investigations with freely suspended drops in an acoustic levitator and a vertical wind tunnel

Atmospheric Chemistry and Physics, 14, 12343-12355, 2014

Author(s): K. Diehl, M. Debertshäuser, O. Eppers, H. Schmithüsen, S. K. Mitra, and S. Borrmann

The heterogeneous freezing temperatures of supercooled drops were measured using an acoustic levitator. This technique allows one to freely suspend single drops in the air without any wall contact. Heterogeneous nucleation by two types of illite (illite IMt1 and illite NX) and a montmorillonite sample was investigated in the immersion mode. Drops of 1 mm in radius were monitored by a video camera while cooled down to ?28 °C to simulate freezing within the tropospheric temperature range. The surface temperature of the drops was contact-free, determined with an infrared thermometer; the onset of freezing was indicated by a sudden increase of the drop surface temperature. For comparison, measurements with one particle type (illite NX) were additionally performed in the Mainz vertical wind tunnel with drops of 340 ?m radius freely suspended. Immersion freezing was observed in a temperature range between ?13 and ?26 °C as a function of particle type and particle surface area immersed in the drops. Isothermal experiments in the wind tunnel indicated that after the cooling stage freezing still proceeds, at least during the investigated time period of 30 s. The results were evaluated by applying two descriptions of heterogeneous freezing, the stochastic and the singular model. Although the wind tunnel results do not support the time-independence of the freezing process both models are applicable for comparing the results from the two experimental techniques.

Posted on 25 November 2014 | 12:00 am


Enhancements of the refractory submicron aerosol fraction in the Arctic polar vortex: feature or exception?

Enhancements of the refractory submicron aerosol fraction in the Arctic polar vortex: feature or exception?

Atmospheric Chemistry and Physics, 14, 12319-12342, 2014

Author(s): R. Weigel, C. M. Volk, K. Kandler, E. Hösen, G. Günther, B. Vogel, J.-U. Grooß, S. Khaykin, G. V. Belyaev, and S. Borrmann

In situ measurements with a four-channel stratospheric condensation particle counter (CPC) were conducted at up to 20 km altitude on board the aircraft M-55 Geophysica from Kiruna, Sweden, in January through March (EUPLEX 2003, RECONCILE 2010) and in December (ESSenCe 2011). During all campaigns air masses from the upper stratosphere and mesosphere were subsiding inside the Arctic winter vortex, thus initializing a transport of refractory aerosol into the lower stratosphere (? < 500 K). The strength and extent of this downward transport varied between the years depending on the dynamical evolution of the vortex. Inside the vortex and at potential temperatures ? ? 450 K around 11 submicron particles per cm3 were generally detected. Up to 8 of these 11 particles per cm3 were found to contain thermo-stable (at 250 °C) residuals with diameters of 10 nm to about 1 ?m. Particle mixing ratios (150 mg?1) and fractions of non-volatile particles (75% of totally detected particles) exhibited highest values in air masses having the lowest content of nitrous oxide (70 nmol mol?1 of N2O). This indicates that refractory aerosol originates from the upper stratosphere or the mesosphere. Derived from the mixing ratio of the simultaneously measured long-lived tracer N2O, an empirical index serves to differentiate probed air masses according to their origin: inside the vortex, the vortex edge region, or outside the vortex. Previously observed high fractions of refractory submicron aerosol in the 2003 Arctic vortex were ascribed to unusually strong subsidence during that winter. However, measurements under perturbed vortex conditions in 2010 and during early winter in December 2011 revealed similarly high values. Thus, the abundance of refractory aerosol in the lower stratosphere within the Arctic vortices appears to be a regular feature rather than the exception. During December, the import from aloft into the lower stratosphere appears to be developing; thereafter the abundance of refractory aerosol inside the vortex reaches its highest levels in March. The correlations of refractory aerosol with N2O suggest that, apart from mean subsidence, diabatic dispersion inside the vortex significantly contributes to the transport of particles to the Arctic lower stratosphere. A measurement-based estimate of the total mass of refractory aerosol inside the vortex is provided for each campaign. Based on the derived increase of particle mass in the lower stratospheric vortex (100–67 hPa pressure altitude) by a factor of 4.5 between early and late winter, we estimate the total mass of mesospheric particles deposited over the winter 2009/2010 in the entire Arctic vortex to range between 77 × 103 and 375 × 106 kg. This estimate is compared with the expected atmospheric influx of meteoritic material (110 ± 55 × 103 kg per day). Such estimates at present still hold considerable uncertainties, which are discussed in this article. Nevertheless, the results enable placing constraints on the shape of the so far unknown size distribution of refractory aerosol within the vortex.

Posted on 25 November 2014 | 12:00 am


Mesoscopic surface roughness of ice crystals pervasive across a wide range of ice crystal conditions

Mesoscopic surface roughness of ice crystals pervasive across a wide range of ice crystal conditions

Atmospheric Chemistry and Physics, 14, 12357-12371, 2014

Author(s): N. B. Magee, A. Miller, M. Amaral, and A. Cumiskey

Here we show high-magnification images of hexagonal ice crystals acquired by environmental scanning electron microscopy (ESEM). Most ice crystals were grown and sublimated in the water vapor environment of an FEI-Quanta-200 ESEM, but crystals grown in a laboratory diffusion chamber were also transferred intact and imaged via ESEM. All of these images display prominent mesoscopic topography including linear striations, ridges, islands, steps, peaks, pits, and crevasses; the roughness is not observed to be confined to prism facets. The observations represent the most highly magnified images of ice surfaces yet reported and expand the range of conditions in which rough surface features are known to be conspicuous. Microscale surface topography is seen to be ubiquitously present at temperatures ranging from ?10 °C to ?40 °C, in supersaturated and subsaturated conditions, on all crystal facets, and irrespective of substrate. Despite the constant presence of surface roughness, the patterns of roughness are observed to be dramatically different between growing and sublimating crystals, and transferred crystals also display qualitatively different patterns of roughness. Crystals are also demonstrated to sometimes exhibit inhibited growth in moderately supersaturated conditions following exposure to near-equilibrium conditions, a phenomenon interpreted as evidence of 2-D nucleation. New knowledge about the characteristics of these features could affect the fundamental understanding of ice surfaces and their physical parameterization in the context of satellite retrievals and cloud modeling. Links to supplemental videos of ice growth and sublimation are provided.

Posted on 25 November 2014 | 12:00 am


CCN activity of size-selected aerosol at a Pacific coastal location

CCN activity of size-selected aerosol at a Pacific coastal location

Atmospheric Chemistry and Physics, 14, 12307-12317, 2014

Author(s): J. D. Yakobi-Hancock, L. A. Ladino, A. K. Bertram, J. A. Huffman, K. Jones, W. R. Leaitch, R. H. Mason, C. L. Schiller, D. Toom-Sauntry, J. P. S. Wong, and J. P. D. Abbatt

As one aspect of the NETwork on Climate and Aerosols: addressing key uncertainties in Remote Canadian Environments (NETCARE), measurements of the cloud condensation nucleation properties of 50 and 100 nm aerosol particles were conducted at Ucluelet on the west coast of Vancouver Island in August 2013. The overall hygroscopicity parameter of the aerosol (κambient) exhibited a wide variation, ranging from 0.14 ± 0.05 to 1.08 ± 0.40 (where the uncertainty represents the systematic error). The highest ? values arose when the organic-to-sulfate ratio of the aerosol was lowest and when winds arrived from the west after transport through the marine boundary layer. The average κambient during this time was 0.57 ± 0.16, where the uncertainty represents the standard deviation. At most other times, the air was predominantly influenced by both marine and continental emissions, which had lower average PM1 κambient values (max value, 0.41 ± 0.08). The two-day average aerosol ionic composition also showed variation, but was consistently acidic and dominated by ammonium (18–56% by mole) and sulfate (19–41% by mole), with only minor levels of sodium or chloride. Average κorg (hygroscopicity parameter for the aerosol's organic component) values were estimated using PM1 aerosol composition data and by assuming that the ratio of aerosol organic to sulfate mass is related directly to the composition of the size-selected particles.

Posted on 24 November 2014 | 12:00 am


Modeling of HCHO and CHOCHO at a semi-rural site in southern China during the PRIDE-PRD2006 campaign

Modeling of HCHO and CHOCHO at a semi-rural site in southern China during the PRIDE-PRD2006 campaign

Atmospheric Chemistry and Physics, 14, 12291-12305, 2014

Author(s): X. Li, F. Rohrer, T. Brauers, A. Hofzumahaus, K. Lu, M. Shao, Y. H. Zhang, and A. Wahner

HCHO and CHOCHO are important trace gases in the atmosphere, serving as tracers of VOC oxidations. In the past decade, high concentrations of HCHO and CHOCHO have been observed for the Pearl River Delta (PRD) region in southern China. In this study, we performed box model simulations of HCHO and CHOCHO at a semi-rural site in the PRD, focusing on understanding their sources and sinks and factors influencing the CHOCHO to HCHO ratio (RGF). The model was constrained by the simultaneous measurements of trace gases and radicals. Isoprene oxidation by OH radicals is the major pathway forming HCHO, followed by degradations of alkenes, aromatics, and alkanes. The production of CHOCHO is dominated by isoprene and aromatic degradation; contributions from other NMHCs are of minor importance. Compared to the measurement results, the model predicts significant higher HCHO and CHOCHO concentrations. Sensitivity studies suggest that fresh emissions of precursor VOCs, uptake of HCHO and CHOCHO by aerosols, fast vertical transport, and uncertainties in the treatment of dry deposition all have the potential to contribute significantly to this discrepancy. Our study indicates that, in addition to chemical considerations (i.e., VOC composition, OH and NOx levels), atmospheric physical processes (e.g., transport, dilution, deposition) make it difficult to use the CHOCHO to HCHO ratio as an indicator for the origin of air mass composition.

Posted on 21 November 2014 | 12:00 am


Recent trends in aerosol optical properties derived from AERONET measurements

Recent trends in aerosol optical properties derived from AERONET measurements

Atmospheric Chemistry and Physics, 14, 12271-12289, 2014

Author(s): J. Li, B. E. Carlson, O. Dubovik, and A. A. Lacis

The Aerosol Robotic Network (AERONET) has been providing high-quality retrievals of aerosol optical properties from the surface at worldwide locations for more than a decade. Many sites have continuous and consistent records for more than 10 years, which enables the investigation of long-term trends in aerosol properties at these locations. In this study, we present the results of a trend analysis at selected stations with long data records. In addition to commonly studied parameters such as aerosol optical depth (AOD) and Ångström exponent (AE), we also focus on inversion products including absorption aerosol optical depth (ABS), single-scattering albedo (SSA) and the absorption Ångström exponent (AAE). Level 2.0 quality assured data are the primary source. However, due to the scarcity of level 2.0 inversion products resulting from the strict AOD quality control threshold, we have also analyzed level 1.5 data, with some quality control screening to provide a reference for global results. Two statistical methods are used to detect and estimate the trend: the Mann–Kendall test associated with Sen's slope and linear least-squares fitting. The results of these statistical tests agree well in terms of the significance of the trend for the majority of the cases. The results indicate that Europe and North America experienced a uniform decrease in AOD, while significant (>90%) increases in these two parameters are found for North India and the Arabian Peninsula. The AE trends turn out to be different for North America and Europe, with increases for the former and decreases for the latter, suggesting opposite changes in fine/coarse-mode fraction. For level 2.0 inversion parameters, Beijing and Kanpur both experienced an increase in SSA. Beijing also shows a reduction in ABS, while the SSA increase for Kanpur is mainly due the increase in scattering aerosols. Increased absorption and reduced SSA are found at Solar_Village. At level 1.5, most European and North American sites also show positive SSA and negative ABS trends, although the data are more uncertain. The AAE trends are less spatially coherent due to large uncertainties, except for a robust increase at three sites in West Africa, which suggests a possible reduction in black carbon. Overall, the trends do not exhibit obvious seasonality for the majority of parameters and stations.

Posted on 21 November 2014 | 12:00 am


Missing SO2 oxidant in the coastal atmosphere? – observations from high-resolution measurements of OH and atmospheric sulfur compounds

Missing SO2 oxidant in the coastal atmosphere? – observations from high-resolution measurements of OH and atmospheric sulfur compounds

Atmospheric Chemistry and Physics, 14, 12209-12223, 2014

Author(s): H. Berresheim, M. Adam, C. Monahan, C. O'Dowd, J. M. C. Plane, B. Bohn, and F. Rohrer

Diurnal and seasonal variations of gaseous sulfuric acid (H2SO4) and methane sulfonic acid (MSA) were measured in NE Atlantic air at the Mace Head atmospheric research station during the years 2010 and 2011. The measurements utilized selected-ion chemical ionization mass spectrometry (SI/CIMS) with a detection limit for both compounds of 4.3 × 104 cm?3 at 5 min signal integration. The H2SO4 and MSA gas-phase concentrations were analyzed in conjunction with the condensational sink for both compounds derived from 3 nm to 10 ?m (aerodynamic diameter) aerosol size distributions. Accommodation coefficients of 1.0 for H2SO4 and 0.12 for MSA were assumed, leading to estimated atmospheric lifetimes on the order of 7 and 25 min, respectively. With the SI/CIMS instrument in OH measurement mode alternating between OH signal and background (non-OH) signal, evidence was obtained for the presence of one or more unknown oxidants of SO2 in addition to OH. Depending on the nature of the oxidant(s), its ambient concentration may be enhanced in the CIMS inlet system by additional production. The apparent unknown SO2 oxidant was additionally confirmed by direct measurements of SO2 in conjunction with calculated H2SO4 concentrations. The calculated H2SO4 concentrations were consistently lower than the measured concentrations by a factor of 4.7 ± 2.4 when considering the oxidation of SO2 by OH as the only source of H2SO4. Both the OH and the background signal were also observed to increase significantly during daytime aerosol nucleation events, independent of the ozone photolysis frequency, J(O1D), and were followed by peaks in both H2SO4 and MSA concentrations. This suggests a strong relation between the unknown oxidant(s), OH chemistry, and the atmospheric photolysis and photooxidation of biogenic iodine compounds. As to the identity of the atmospheric SO2 oxidant(s), we have been able to exclude ClO, BrO, IO, and OIO as possible candidates based on {ab initio} calculations. Never\-theless, IO could contribute significantly to the observed CIMS background signal. A detailed analysis of this CIMS background signal in context with recently published kinetic data currently suggests that Criegee intermediates (CIs) produced from ozonolysis of alkenes play no significant role for SO2 oxidation in the marine atmosphere at Mace Head. On the other hand, SO2 oxidation by small CIs such as CH2OO produced photolytically or possibly in the photochemical degradation of methane is consistent with our observations. In addition, H2SO4 formation from dimethyl sulfide oxidation via SO3 as an intermediate instead of SO2 also appears to be a viable explanation. Both pathways need to be further explored.

Posted on 20 November 2014 | 12:00 am


Chemical composition and mass size distribution of PM1 at an elevated site in central east China

Chemical composition and mass size distribution of PM1 at an elevated site in central east China

Atmospheric Chemistry and Physics, 14, 12237-12249, 2014

Author(s): Y. M. Zhang, X. Y. Zhang, J. Y. Sun, G. Y. Hu, X. J. Shen, Y. Q. Wang, T. T. Wang, D. Z. Wang, and Y. Zhao

Size-resolved aerosol chemical compositions were measured continuously for 1.5 years from June 2010 to January 2012 with an aerosol mass spectrometer (AMS) to characterize the mass and size distributions (MSDs) of major chemical components in submicron particles (approximately PM1) at Mountain Tai (Mt. Tai), an elevated site in central east China. The annual mean mass concentrations of organic, sulfate, nitrate, ammonium, and chloride were 11.2, 9.2, 7.2, 5.8, and 0.95 ?g m?3, respectively, which are much higher than those at most mountain sites in the USA and Europe, but lower than those at the nearby surface rural sites in China. A clear seasonality was observed for all major components throughout the study, with low concentration in fall and high in summer, and is believed to be caused by seasonal variations in planetary boundary layer (PBL) height, near surface pollutant concentrations and regional transport processes. Air masses were classified into categories impacted by PBL, lower free troposphere (LFT), new particle formation (NPF), in-cloud processes, and polluted aerosols. Organics dominated the PM1 mass during the NPF episodes, while sulfate contributed most to PM1 in cloud events. The average MSDs of particles between 30 and 1000 nm during the entire study for organics, sulfate, nitrate, and ammonium were approximately log-normal with mass median diameters (MMDs) of 539, 585, 542, and 545 nm, respectively. These values are slightly larger than those observed at ground sites within the North China Plain (NCP), likely due to the relative aged and well-mixed aerosol masses at Mt. Tai. There were no obvious differences in MMDs during the PBL, LFT, in-cloud and polluted episodes, but smaller MMDs, especially for organics, were observed during the NPF events. During the PBL, NPF, and polluted episodes, organics accounted for major proportions at smaller modes, and reached 70% at 100–200 nm particles in the polluted events. In cloud episodes, inorganics contributed 70% to the whole size range dominated by sulfate, which contributed 40% to small particles (100–200 nm), while organics occupied 20%, indicating that sulfate is a critical chemical component in cloud formation. Seven clusters of air masses were classified based on 72 h back-trajectory analysis. The majority of the regionally dispersed aerosols were found to be contributed from short distance mixed aerosols, mostly originated from the south with organics and sulfate as major components. Air masses from long range transport always brought clean and dry aerosols which resulted in low concentrations at Mt. Tai. AMS-PMF (positive matrix factorization) was employed to resolve the subtype of organics. Oxygenic organics aerosols (OAs) occupied 49, 56, 51, and 41% of OAs in the four seasons respectively, demonstrating that most OA were oxidized in summer due to strong photochemical reactions. Biomass burning OAs (BBOAs) accounted for 34% of OA in summer, mainly from field burning of agriculture residues, and coal combustion OAs (CCOAs) accounted for 22% of OA in winter from heating.

Posted on 20 November 2014 | 12:00 am


Relations between erythemal UV dose, global solar radiation, total ozone column and aerosol optical depth at Uccle, Belgium

Relations between erythemal UV dose, global solar radiation, total ozone column and aerosol optical depth at Uccle, Belgium

Atmospheric Chemistry and Physics, 14, 12251-12270, 2014

Author(s): V. De Bock, H. De Backer, R. Van Malderen, A. Mangold, and A. Delcloo

At Uccle, Belgium, a long time series (1991–2013) of simultaneous measurements of erythemal ultraviolet (UV) dose (Sery), global solar radiation (Sg), total ozone column (Q_{O3}$) and aerosol optical depth (τaer) (at 320.1 nm) is available, which allows for an extensive study of the changes in the variables over time. Linear trends were determined for the different monthly anomalies time series. Sery, Sg and QO3 all increase by respectively 7, 4 and 3% per decade. τaer shows an insignificant negative trend of ?8% per decade. These trends agree with results found in the literature for sites with comparable latitudes. A change-point analysis, which determines whether there is a significant change in the mean of the time series, is applied to the monthly anomalies time series of the variables. Only for Sery and QO3, was a significant change point present in the time series around February 1998 and March 1998, respectively. The change point in QO3 corresponds with results found in the literature, where the change in ozone levels around 1997 is attributed to the recovery of ozone. A multiple linear regression (MLR) analysis is applied to the data in order to study the influence of Sg, QO3 and τaer on Sery. Together these parameters are able to explain 94% of the variation in Sery. Most of the variation (56%) in Sery is explained by Sg. The regression model performs well, with a slight tendency to underestimate the measured Sery values and with a mean absolute bias error (MABE) of 18%. However, in winter, negative Sery are modeled. Applying the MLR to the individual seasons solves this issue. The seasonal models have an adjusted R2 value higher than 0.8 and the correlation between modeled and measured Sery values is higher than 0.9 for each season. The summer model gives the best performance, with an absolute mean error of only 6%. However, the seasonal regression models do not always represent reality, where an increase in Sery is accompanied with an increase in QO3 and a decrease in τaer. In all seasonal models, Sg is the factor that contributes the most to the variation in Sery, so there is no doubt about the necessity to include this factor in the regression models. The individual contribution of τaer to Sery is very low, and for this reason it seems unnecessary to include τaer in the MLR analysis. Including QO3, however, is justified to increase the adjusted R2 and to decrease the MABE of the model.

Posted on 20 November 2014 | 12:00 am


Cirrus and water vapour transport in the tropical tropopause layer – Part 2: Roles of ice nucleation and sedimentation, cloud dynamics, and moisture conditions

Cirrus and water vapour transport in the tropical tropopause layer – Part 2: Roles of ice nucleation and sedimentation, cloud dynamics, and moisture conditions

Atmospheric Chemistry and Physics, 14, 12225-12236, 2014

Author(s): T. Dinh, S. Fueglistaler, D. Durran, and T. Ackerman

A high-resolution, two-dimensional numerical model is used to study the moisture redistribution following homogeneous ice nucleation induced by Kelvin waves in the tropical tropopause layer (TTL). We compare results for dry/moist initial conditions and three levels of complexity for the representation of cloud processes: complete microphysics and cloud radiative effects, likewise but without radiative effects, and instantaneous removal of moisture in excess of saturation upon nucleation.

Cloud evolution and moisture redistribution are found to be sensitive to initial conditions and cloud processes. Ice sedimentation leads to a downward flux of water, whereas the cloud radiative heating induces upward advection of the cloudy air. The latter results in an upward (downward) flux of water vapour if the cloudy air is moister (drier) than the environment, which is typically when the environment is subsaturated (supersaturated).

Only a fraction (~25% or less) of the cloud experiences nucleation. Post-nucleation processes (ice depositional growth, sedimentation, and sublimation) are important to cloud morphology, and both dehydrated and hydrated layers may be indicators of TTL cirrus occurrence. The calculation with instantaneous removal of moisture not only misses the hydration but also underestimates dehydration due to (i) nucleation before reaching the minimum saturation mixing ratio, and (ii) lack of moisture removal from sedimenting ice particles below the nucleation level.

The sensitivity to initial conditions and cloud processes suggests that it is difficult to reach generic, quantitative estimates of cloud-induced moisture redistribution on the basis of case-by-case calculations.

Posted on 20 November 2014 | 12:00 am


Aerosol indirect effects on continental low-level clouds over Sweden and Finland

Aerosol indirect effects on continental low-level clouds over Sweden and Finland

Atmospheric Chemistry and Physics, 14, 12167-12179, 2014

Author(s): M. K. Sporre, E. Swietlicki, P. Glantz, and M. Kulmala

Aerosol effects on low-level clouds over the Nordic Countries are investigated by combining in situ ground-based aerosol measurements with remote sensing data of clouds and precipitation. Ten years of number size distribution data from two aerosol measurement stations (Vavihill, Sweden and Hyytiälä, Finland) provide aerosol number concentrations in the atmospheric boundary layer. This is combined with cloud satellite data from the Moderate Resolution Imaging Spectroradiometer and weather radar data from the Baltic Sea Experiment. Also, how the meteorological conditions affect the clouds is investigated using reanalysis data from the European Centre for Medium-Range Weather Forecasts.

The cloud droplet effective radius is found to decrease when the aerosol number concentration increases, while the cloud optical thickness does not vary with boundary layer aerosol number concentrations. Furthermore, the aerosol–cloud interaction parameter (ACI), a measure of how the effective radius is influenced by the number concentration of cloud active particles, is found to be somewhere between 0.10 and 0.18 and the magnitude of the ACI is greatest when the number concentration of particles with a diameter larger than 130 nm is used. Lower precipitation intensity in the weather radar images is associated with higher aerosol number concentrations. In addition, at Hyytiälä the particle number concentrations is generally higher for non-precipitating cases than for precipitating cases. The apparent absence of the first indirect effect of aerosols on low-level clouds over land raises questions regarding the magnitude of the indirect aerosol radiative forcing.

Posted on 19 November 2014 | 12:00 am


Atmospheric amines and ammonia measured with a chemical ionization mass spectrometer (CIMS)

Atmospheric amines and ammonia measured with a chemical ionization mass spectrometer (CIMS)

Atmospheric Chemistry and Physics, 14, 12181-12194, 2014

Author(s): Y. You, V. P. Kanawade, J. A. de Gouw, A. B. Guenther, S. Madronich, M. R. Sierra-Hernández, M. Lawler, J. N. Smith, S. Takahama, G. Ruggeri, A. Koss, K. Olson, K. Baumann, R. J. Weber, A. Nenes, H. Guo, E. S. Edgerton, L. Porcelli, W. H. Brune, A. H. Goldstein, and S.-H. Lee

We report measurements of ambient amines and ammonia with a fast response chemical ionization mass spectrometer (CIMS) in a southeastern US forest and a moderately polluted midwestern site during the summer. At the forest site, mostly C3-amines (from pptv to tens of pptv) and ammonia (up to 2 ppbv) were detected, and they both showed temperature dependencies. Aerosol-phase amines measured thermal-desorption chemical ionization mass spectrometer (TDCIMS) showed a higher mass fraction in the evening with cooler temperatures and lower in the afternoon with warmer temperatures, a trend opposite to the gas-phase amines. Concentrations of aerosol-phase primary amines measured with Fourier transform infrared spectroscopy (FTIR) from micron and submicron particles were 2 orders of magnitude higher than the gas-phase amines. These results indicate that gas to particle conversion is one of the major processes that control the ambient amine concentrations at this forest site. Temperature dependencies of C3-amines and ammonia also imply reversible processes of evaporation of these nitrogen-containing compounds from soil surfaces in daytime and deposition to soil surfaces at nighttime. During the transported biomass burning plume events, various amines (C1–C6) appeared at the pptv level, indicating that biomass burning is a substantial source of amines in the southeastern US. At the moderately polluted Kent site, there were higher concentrations of C1- to C6-amines (pptv to tens of pptv) and ammonia (up to 6 ppbv). C1- to C3-amines and ammonia were well correlated with the ambient temperature. C4- to C6-amines showed frequent spikes during the nighttime, suggesting that they were emitted from local sources. These abundant amines and ammonia may in part explain the frequent new particle formation events reported from Kent. Higher amine concentrations measured at the polluted site than at the rural forested site highlight the importance of constraining anthropogenic emission sources of amines.

Posted on 19 November 2014 | 12:00 am


Injection of mineral dust into the free troposphere during fire events observed with polarization lidar at Limassol, Cyprus

Injection of mineral dust into the free troposphere during fire events observed with polarization lidar at Limassol, Cyprus

Atmospheric Chemistry and Physics, 14, 12155-12165, 2014

Author(s): A. Nisantzi, R. E. Mamouri, A. Ansmann, and D. Hadjimitsis

Four-year observations (2010–2014) with EARLINET polarization lidar and AERONET sun/sky photometer at Limassol (34.7° N, 33° E), Cyprus, were used to study the soil dust content in lofted fire smoke plumes advected from Turkey. This first systematic attempt to characterize less than 3-day-old smoke plumes in terms of particle linear depolarization ratio (PDR), measured with lidar, contributes to the more general effort to properly describe the life cycle of free-tropospheric smoke–dust mixtures from the emission event to phases of long-range transport (> 4 days after emission). We found significant PDR differences with values from 9 to 18% in lofted aerosol layers when Turkish fires contributed to the aerosol burden and of 3–13 % when Turkish fires were absent. High Ångström exponents of 1.4–2.2 during all these events with lofted smoke layers, occurring between 1 and 3 km height, suggest the absence of a pronounced particle coarse mode. When plotted vs. travel time (spatial distance between Limassol and last fire area), PDR decreased strongly from initial values around 16–18% (1 day travel) to 4–8% after 4 days of travel caused by deposition processes. This behavior was found to be in close agreement with findings described in the literature. Computation of particle extinction coefficient and mass concentrations, derived from the lidar observations, separately for fine-mode dust, coarse-mode dust, and non-dust aerosol components show extinction-related dust fractions on the order of 10% (for PDR =4%, travel times > 4 days) and 50% (PDR =15%, 1 day travel time) and respective mass-related dust fractions of 25% (PDR =4%) to 80% (PDR =15%). Biomass burning should therefore be considered as another source of free tropospheric soil dust.

Posted on 19 November 2014 | 12:00 am


OH-initiated heterogeneous oxidation of tris-2-butoxyethyl phosphate: implications for its fate in the atmosphere

OH-initiated heterogeneous oxidation of tris-2-butoxyethyl phosphate: implications for its fate in the atmosphere

Atmospheric Chemistry and Physics, 14, 12195-12207, 2014

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

A particle-phase relative rates technique is used to investigate the heterogeneous reaction between OH radicals and tris-2-butoxyethyl phosphate (TBEP) at 298 K by combining aerosol time-of-flight mass spectrometry (C-ToF-MS) data and positive matrix factor (PMF) analysis. The derived second-order rate constants (k2) for the heterogeneous loss of TBEP is (4.44 ± 0.45) × 10−12 cm3 molecule?1 s?1, from which an approximate particle-phase lifetime was estimated to be 2.6 (2.3–2.9) days. However, large differences in the rate constants for TBEP relative to a reference compound were observed when comparing internally and externally mixed TBEP/organic particles, and upon changes in the RH. The heterogeneous degradation of TBEP was found to be depressed or enhanced depending upon the particle mixing state and phase, highlighting the complexity of heterogeneous oxidation in the atmosphere. The effect of gas-particle partitioning on the estimated overall lifetime (gas + particle) for several organophosphate esters (OPEs) was also examined through the explicit modeling of this process. The overall atmospheric lifetimes of TBEP, tris-2-ethylhexyl phosphate (TEHP) and tris-1,3-dichloro-2-propyl phosphate (TDCPP) were estimated to be 1.9, 1.9 and 2.4 days respectively, and are highly dependent upon particle size. These results demonstrate that modeling the atmospheric fate of particle-phase toxic compounds for the purpose of risk assessment must include the gas-particle partitioning process, and in the future include the effect of other particulate components on the evaporation kinetics and/or the heterogeneous loss rates.

Posted on 19 November 2014 | 12:00 am


Reactivity of stabilized Criegee intermediates (sCIs) from isoprene and monoterpene ozonolysis toward SO2 and organic acids

Reactivity of stabilized Criegee intermediates (sCIs) from isoprene and monoterpene ozonolysis toward SO2 and organic acids

Atmospheric Chemistry and Physics, 14, 12143-12153, 2014

Author(s): M. Sipilä, T. Jokinen, T. Berndt, S. Richters, R. Makkonen, N. M. Donahue, R. L. Mauldin III, T. Kurtén, P. Paasonen, N. Sarnela, M. Ehn, H. Junninen, M. P. Rissanen, J. Thornton, F. Stratmann, H. Herrmann, D. R. Worsnop, M. Kulmala, V.-M. Kerminen, and T. Petäjä

Oxidation processes in Earth's atmosphere are tightly connected to many environmental and human health issues and are essential drivers for biogeochemistry. Until the recent discovery of the atmospheric relevance of the reaction of stabilized Criegee intermediates (sCIs) with SO2, atmospheric oxidation processes were thought to be dominated by a few main oxidants: ozone, hydroxyl radicals (OH), nitrate radicals and, e.g. over oceans, halogen atoms such as chlorine. Here, we report results from laboratory experiments at 293 K and atmospheric pressure focusing on sCI formation from the ozonolysis of isoprene and the most abundant monoterpenes (?-pinene and limonene), and subsequent reactions of the resulting sCIs with SO2 producing sulfuric acid (H2SO4). The measured total sCI yields were (0.15 ± 0.07), (0.27 ± 0.12) and (0.58 ± 0.26) for ?-pinene, limonene and isoprene, respectively. The ratio between the rate coefficient for the sCI loss (including thermal decomposition and the reaction with water vapour) and the rate coefficient for the reaction of sCI with SO2, k(loss) /k(sCI + SO2), was determined at relative humidities of 10 and 50%. Observed values represent the average reactivity of all sCIs produced from the individual alkene used in the ozonolysis. For the monoterpene-derived sCIs, the relative rate coefficients k(loss) / k(sCI + SO2) were in the range (2.0–2.4) × 1012 molecules cm?3 and nearly independent of the relative humidity. This fact points to a minor importance of the sCI + H2O reaction in the case of the sCI arising from ?-pinene and limonene. For the isoprene sCIs, however, the ratio k(loss) / k(sCI + SO2) was strongly dependent on the relative humidity. To explore whether sCIs could have a more general role in atmospheric oxidation, we investigated as an example the reactivity of acetone oxide (sCI from the ozonolysis of 2,3-dimethyl-2-butene) toward small organic acids, i.e. formic and acetic acid. Acetone oxide was found to react faster with the organic acids than with SO2; k(sCI + acid) / k(sCI + SO2) = (2.8 ± 0.3) for formic acid, and k(sCI + acid) / k(sCI + SO2) = (3.4 ± 0.2) for acetic acid. This finding indicates that sCIs can play a role in the formation and loss of other atmospheric constituents besides SO2.

Posted on 19 November 2014 | 12:00 am


Measurements of the aerosol chemical composition and mixing state in the Po Valley using multiple spectroscopic techniques

Measurements of the aerosol chemical composition and mixing state in the Po Valley using multiple spectroscopic techniques

Atmospheric Chemistry and Physics, 14, 12109-12132, 2014

Author(s): S. Decesari, J. Allan, C. Plass-Duelmer, B. J. Williams, M. Paglione, M. C. Facchini, C. O'Dowd, R. M. Harrison, J. K. Gietl, H. Coe, L. Giulianelli, G. P. Gobbi, C. Lanconelli, C. Carbone, D. Worsnop, A. T. Lambe, A. T. Ahern, F. Moretti, E. Tagliavini, T. Elste, S. Gilge, Y. Zhang, and M. Dall'Osto

The use of co-located multiple spectroscopic techniques can provide detailed information on the atmospheric processes regulating aerosol chemical composition and mixing state. So far, field campaigns heavily equipped with aerosol mass spectrometers have been carried out mainly in large conurbations and in areas directly affected by their outflow, whereas lesser efforts have been dedicated to continental areas characterised by a less dense urbanisation. We present here the results obtained at a background site in the Po Valley, Italy, in summer 2009. For the first time in Europe, six state-of-the-art spectrometric techniques were used in parallel: aerosol time-of-flight mass spectrometer (ATOFMS), two aerosol mass spectrometers (high-resolution time-of-flight aerosol mass spectrometer – HR-ToF-AMS and soot particle aerosol mass spectrometer – SP-AMS), thermal desorption aerosol gas chromatography (TAG), chemical ionisation mass spectrometry (CIMS) and (offline) proton nuclear magnetic resonance (1H-NMR) spectroscopy. The results indicate that, under high-pressure conditions, atmospheric stratification at night and early morning hours led to the accumulation of aerosols produced by anthropogenic sources distributed over the Po Valley plain. Such aerosols include primary components such as black carbon (BC), secondary semivolatile compounds such as ammonium nitrate and amines and a class of monocarboxylic acids which correspond to the AMS cooking organic aerosol (COA) already identified in urban areas. In daytime, the entrainment of aged air masses in the mixing layer is responsible for the accumulation of low-volatility oxygenated organic aerosol (LV-OOA) and also for the recycling of non-volatile primary species such as black carbon. According to organic aerosol source apportionment, anthropogenic aerosols accumulating in the lower layers overnight accounted for 38% of organic aerosol mass on average, another 21% was accounted for by aerosols recirculated in residual layers but still originating in northern Italy, while a substantial fraction (41%) was due to the most aged aerosols imported from transalpine areas. The different meteorological regimes also affected the BC mixing state: in periods of enhanced stagnation and recirculation of pollutants, the number fraction of the BC-containing particles determined by ATOFMS was 75% of the total, while in the days of enhanced ventilation of the planetary boundary layer (PBL), such fraction was significantly lower (50%) because of the relative greater influence of non-BC-containing aerosol local sources in the Po Valley. Overall, a full internal mixing between BC and the non-refractory aerosol chemical components was not observed during the experiment in this environment.

Posted on 18 November 2014 | 12:00 am


Characteristics and sources of gravity waves observed in noctilucent cloud over Norway

Characteristics and sources of gravity waves observed in noctilucent cloud over Norway

Atmospheric Chemistry and Physics, 14, 12133-12142, 2014

Author(s): T. D. Demissie, P. J. Espy, N. H. Kleinknecht, M. Hatlen, N. Kaifler, and G. Baumgarten

Four years of noctilucent cloud (NLC) images from an automated digital camera in Trondheim and results from a ray-tracing model are used to extend the climatology of gravity waves to higher latitudes and to identify their sources during summertime. The climatology of the summertime gravity waves detected in NLC between 64 and 74° N is similar to that observed between 60 and 64° N by Pautet et al. (2011). The direction of propagation of gravity waves observed in the NLC north of 64° N is a continuation of the north and northeast propagation as observed in south of 64° N. However, a unique population of fast, short wavelength waves propagating towards the SW is observed in the NLC, which is consistent with transverse instabilities generated in situ by breaking gravity waves (Fritts and Alexander, 2003). The relative amplitude of the waves observed in the NLC Mie scatter have been combined with ray-tracing results to show that waves propagating from near the tropopause, rather than those resulting from secondary generation in the stratosphere or mesosphere, are more likely to be the sources of the prominent wave structures observed in the NLC. The coastal region of Norway along the latitude of 70° N is identified as the primary source region of the waves generated near the tropopause.

Posted on 18 November 2014 | 12:00 am


Ground-based aerosol characterization during the South American Biomass Burning Analysis (SAMBBA) field experiment

Ground-based aerosol characterization during the South American Biomass Burning Analysis (SAMBBA) field experiment

Atmospheric Chemistry and Physics, 14, 12069-12083, 2014

Author(s): J. Brito, L. V. Rizzo, W. T. Morgan, H. Coe, B. Johnson, J. Haywood, K. Longo, S. Freitas, M. O. Andreae, and P. Artaxo

This paper investigates the physical and chemical characteristics of aerosols at ground level at a site heavily impacted by biomass burning. The site is located near Porto Velho, Rondônia, in the southwestern part of the Brazilian Amazon rainforest, and was selected for the deployment of a large suite of instruments, among them an Aerosol Chemical Speciation Monitor. Our measurements were made during the South American Biomass Burning Analysis (SAMBBA) field experiment, which consisted of a combination of aircraft and ground-based measurements over Brazil, aimed to investigate the impacts of biomass burning emissions on climate, air quality, and numerical weather prediction over South America. The campaign took place during the dry season and the transition to the wet season in September/October 2012.

During most of the campaign, the site was impacted by regional biomass burning pollution (average CO mixing ratio of 0.6 ppm), occasionally superimposed by intense (up to 2 ppm of CO), freshly emitted biomass burning plumes. Aerosol number concentrations ranged from ~1000 cm−3 to peaks of up to 35 000 cm−3 (during biomass burning (BB) events, corresponding to an average submicron mass mean concentrations of 13.7 μg m−3 and peak concentrations close to 100 μg m−3. Organic aerosol strongly dominated the submicron non-refractory composition, with an average concentration of 11.4 μg m−3. The inorganic species, NH4, SO4, NO3, and Cl, were observed, on average, at concentrations of 0.44, 0.34, 0.19, and 0.01 μg m−3, respectively. Equivalent black carbon (BCe) ranged from 0.2 to 5.5 μg m−3, with an average concentration of 1.3 μg m−3. During BB peaks, organics accounted for over 90% of total mass (submicron non-refractory plus BCe), among the highest values described in the literature.

We examined the ageing of biomass burning organic aerosol (BBOA) using the changes in the H : C and O : C ratios, and found that throughout most of the aerosol processing (O : C ≅ 0.25 to O : C ≅ 0.6), no remarkable change is observed in the H : C ratio (~1.35). Such a result contrasts strongly with previous observations of chemical ageing of both urban and Amazonian biogenic aerosols. At higher levels of processing (O : C > 0.6), the H : C ratio changes with a H : C / O : C slope of ?0.5, possibly due to the development of a combination of BB (H : C / O : C slope = 0) and biogenic (H : C /O :C slope =−1) organic aerosol (OA). An analysis of the ΔOA /ΔCO mass ratios yields very little enhancement in the OA loading with atmospheric processing, consistent with previous observations. These results indicate that negligible secondary organic aerosol (SOA) formation occurs throughout the observed BB plume processing, or that SOA formation is almost entirely balanced by OA volatilization.

Positive matrix factorization (PMF) of the organic aerosol spectra resulted in three factors: fresh BBOA, aged BBOA, and low-volatility oxygenated organic aerosol (LV-OOA). Analysis of the diurnal patterns and correlation with external markers indicates that during the first part of the campaign, OA concentrations are impacted by local fire plumes with some chemical processing occurring in the near-surface layer. During the second part of the campaign, long-range transport of BB plumes above the surface layer, as well as potential SOAs formed aloft, dominates OA concentrations at our ground-based sampling site.

This manuscript describes the first ground-based deployment of the aerosol mass spectrometry at a site heavily impacted by biomass burning in the Amazon region, allowing a deeper understanding of aerosol life cycle in this important ecosystem.

Posted on 18 November 2014 | 12:00 am


Spatial and temporal variability of sources of ambient fine particulate matter (PM2.5) in California

Spatial and temporal variability of sources of ambient fine particulate matter (PM2.5) in California

Atmospheric Chemistry and Physics, 14, 12085-12097, 2014

Author(s): S. Hasheminassab, N. Daher, A. Saffari, D. Wang, B. D. Ostro, and C. Sioutas

To identify major sources of ambient fine particulate matter (PM2.5, dp < 2.5 μm) and quantify their contributions in the state of California, a positive matrix factorization (PMF) receptor model was applied on Speciation Trends Network (STN) data, collected between 2002 and 2007 at eight distinct sampling locations, including El Cajon, Rubidoux, Los Angeles, Simi Valley, Bakersfield, Fresno, San Jose, and Sacramento. Between five to nine sources of fine PM were identified at each sampling site, several of which were common among multiple locations. Secondary aerosols, including secondary ammonium nitrate and ammonium sulfate, were the most abundant contributor to ambient PM2.5 mass at all sampling sites, except for San Jose, with an annual average cumulative contribution of 26 to 63%, across the state. On an annual average basis, vehicular emissions (including both diesel and gasoline vehicles) were the largest primary source of fine PM at all sampling sites in southern California (17–18% of total mass), whereas in Fresno and San Jose, biomass burning was the most dominant primary contributor to ambient PM2.5 (27 and 35% of total mass, respectively), in general agreement with the results of previous source apportionment studies in California. In Bakersfield and Sacramento, vehicular emissions and biomass burning displayed relatively equal annual contributions to ambient PM2.5 mass (12 and 25%, respectively). Other commonly identified sources at all sites included aged and fresh sea salt and soil, which contributed to 0.5–13%, 2–27%, and 1–19% of the total mass, respectively, across all sites and seasons. In addition, a few minor sources were identified exclusively at some of the sites (e.g., chlorine sources, sulfate-bearing road dust, and different types of industrial emissions). These sources overall accounted for a small fraction of the total PM mass across the sampling locations (1 to 15%, on an annual average basis).

Posted on 18 November 2014 | 12:00 am


Lidar observation of the 2011 Puyehue-Cordón Caulle volcanic aerosols at Lauder, New Zealand

Lidar observation of the 2011 Puyehue-Cordón Caulle volcanic aerosols at Lauder, New Zealand

Atmospheric Chemistry and Physics, 14, 12099-12108, 2014

Author(s): K. Nakamae, O. Uchino, I. Morino, B. Liley, T. Sakai, T. Nagai, and T. Yokota

On 4 June 2011, the Puyehue-Cordón Caulle volcanic complex (40.6° S, 72.1° W) in Chile erupted violently and injected volcanic aerosols into the atmosphere. For the safety of civil aviation, continuous lidar observations were made at Lauder, New Zealand (45.0° S, 169.7° E), from 11 June through 6 July 2011. The purpose of our study is to quantify the influence of the volcanic ejections from large eruptions, and we use the data from the ground-based lidar observation. We analyzed lidar data at a wavelength of 532 nm and derived the backscattering ratio and depolarization ratio profiles. During June and July, within the altitude range of 10–15 km, the volcanic aerosols had high depolarization ratios (20–35%), an indication of non-spherical volcanic ash particles. The time series of the backscattering ratio during continuous observations had three peaks occurring at about 12-day intervals: 26.7 at 11.2 km on 11 June, 18.1 at 12.0 km on 23 June, and 5.3 at 11.1 km on 6 July. The optical depth of the volcanic aerosols was 0.45 on 11 June, when the continuous lidar observation started, 0.31 on 23 June, and 0.12 on 6 July. The depolarization ratio values remained high up to a month after the eruption, and the small wavelength exponent calculated from the backscattering coefficients at 532 nm and 1064 nm suggests that a major constituent of the volcanic aerosols was large, non-spherical particles. The presence of volcanic ash in the stratosphere might affect the error in Greenhouse gases Observing SATellite (GOSAT) XCO2 retrieval using the 1.6 ?m band. We briefly discuss the influence of the increased aerosols on GOSAT products.

Posted on 18 November 2014 | 12:00 am


The impact of aerosol hygroscopic growth on the single-scattering albedo and its application on the NO2 photolysis rate coefficient

The impact of aerosol hygroscopic growth on the single-scattering albedo and its application on the NO2 photolysis rate coefficient

Atmospheric Chemistry and Physics, 14, 12055-12067, 2014

Author(s): J. C. Tao, C. S. Zhao, N. Ma, and P. F. Liu

Hygroscopic growth of aerosol particles can significantly affect their single-scattering albedo (?), and consequently alters the aerosol effect on tropospheric photochemistry. In this study, the impact of aerosol hygroscopic growth on ? and its application to the NO2 photolysis rate coefficient (JNO2) are investigated for a typical aerosol particle population in the North China Plain (NCP). The variations of aerosol optical properties with relative humidity (RH) are calculated using a Mie theory aerosol optical model, on the basis of field measurements of number–size distribution and hygroscopic growth factor (at RH values above 90%) from the 2009 HaChi (Haze in China) project. Results demonstrate that ambient ? has pronouncedly different diurnal patterns from ? measured at dry state, and is highly sensitive to the ambient RHs. Ambient ? in the NCP can be described by a dry state ? value of 0.863, increasing with the RH following a characteristic RH dependence curve. A Monte Carlo simulation shows that the uncertainty of ? from the propagation of uncertainties in the input parameters decreases from 0.03 (at dry state) to 0.015 (RHs > 90%). The impact of hygroscopic growth on ? is further applied in the calculation of the radiative transfer process. Hygroscopic growth of the studied aerosol particle population generally inhibits the photolysis of NO2 at the ground level, whereas accelerates it above the moist planetary boundary layer. Compared with dry state, the calculated JNO2 at RH of 98% at the height of 1 km increases by 30.4%, because of the enhancement of ultraviolet radiation by the humidified scattering-dominant aerosol particles. The increase of JNO2 due to the aerosol hygroscopic growth above the upper boundary layer may affect the tropospheric photochemical processes and this needs to be taken into account in the atmospheric chemical models.

Posted on 17 November 2014 | 12:00 am


Assimilation of lidar signals: application to aerosol forecasting in the western Mediterranean basin

Assimilation of lidar signals: application to aerosol forecasting in the western Mediterranean basin

Atmospheric Chemistry and Physics, 14, 12031-12053, 2014

Author(s): Y. Wang, K. N. Sartelet, M. Bocquet, P. Chazette, M. Sicard, G. D'Amico, J. F. Léon, L. Alados-Arboledas, A. Amodeo, P. Augustin, J. Bach, L. Belegante, I. Binietoglou, X. Bush, A. Comerón, H. Delbarre, D. García-Vízcaino, J. L. Guerrero-Rascado, M. Hervo, M. Iarlori, P. Kokkalis, D. Lange, F. Molero, N. Montoux, A. Muñoz, C. Muñoz, D. Nicolae, A. Papayannis, G. Pappalardo, J. Preissler, V. Rizi, F. Rocadenbosch, K. Sellegri, F. Wagner, and F. Dulac

This paper presents a new application of assimilating lidar signals to aerosol forecasting. It aims at investigating the impact of a ground-based lidar network on the analysis and short-term forecasts of aerosols through a case study in the Mediterranean basin. To do so, we employ a data assimilation (DA) algorithm based on the optimal interpolation method developed in the Polair3D chemistry transport model (CTM) of the Polyphemus air quality modelling platform. We assimilate hourly averaged normalised range-corrected lidar signals (PR2) retrieved from a 72 h period of intensive and continuous measurements performed in July 2012 by ground-based lidar systems of the European Aerosol Research Lidar Network (EARLINET) integrated into the Aerosols, Clouds, and Trace gases Research InfraStructure (ACTRIS) network and an additional system in Corsica deployed in the framework of the pre-ChArMEx (Chemistry-Aerosol Mediterranean Experiment)/TRAQA (TRAnsport à longue distance et Qualité de l'Air) campaign. This lidar campaign was dedicated to demonstrating the potential operationality of a research network like EARLINET and the potential usefulness of assimilation of lidar signals to aerosol forecasts. Particles with an aerodynamic diameter lower than 2.5 ?m (PM2.5) and those with an aerodynamic diameter higher than 2.5 ?m but lower than 10 ?m (PM10–2.5) are analysed separately using the lidar observations at each DA step. First, we study the spatial and temporal influences of the assimilation of lidar signals on aerosol forecasting. We conduct sensitivity studies on algorithmic parameters, e.g. the horizontal correlation length (Lh) used in the background error covariance matrix (50 km, 100 km or 200 km), the altitudes at which DA is performed (0.75–3.5 km, 1.0–3.5 km or 1.5–3.5 km a.g.l.) and the assimilation period length (12 h or 24 h). We find that DA with Lh = 100 km and assimilation from 1.0 to 3.5 km a.g.l. during a 12 h assimilation period length leads to the best scores for PM10 and PM2.5 during the forecast period with reference to available measurements from surface networks. Secondly, the aerosol simulation results without and with lidar DA using the optimal parameters (Lh = 100 km, an assimilation altitude range from 1.0 to 3.5 km a.g.l. and a 12 h DA period) are evaluated using the level 2.0 (cloud-screened and quality-assured) aerosol optical depth (AOD) data from AERONET, and mass concentration measurements (PM10 or PM2.5) from the French air quality (BDQA) network and the EMEP-Spain/Portugal network. The results show that the simulation with DA leads to better scores than the one without DA for PM2.5, PM10and AOD. Additionally, the comparison of model results to evaluation data indicates that the temporal impact of assimilating lidar signals is longer than 36 h after the assimilation period.

Posted on 17 November 2014 | 12:00 am


Methane sulfonic acid-enhanced formation of molecular clusters of sulfuric acid and dimethyl amine

Methane sulfonic acid-enhanced formation of molecular clusters of sulfuric acid and dimethyl amine

Atmospheric Chemistry and Physics, 14, 12023-12030, 2014

Author(s): N. Bork, J. Elm, T. Olenius, and H. Vehkamäki

Over oceans and in coastal regions, methane sulfonic acid (MSA) is present in substantial concentrations in aerosols and in the gas phase. We present an investigation into the effect of MSA on sulfuric acid- and dimethyl amine (DMA)-based cluster formation rates. From systematic conformational scans and well-tested ab initio methods, we optimise the structures of all MSAx (H2SO4)yDMAz clusters where x + y ≤ 3 and z ≤ 2. The resulting thermodynamic data are used in the Atmospheric Cluster Dynamics Code, and the effect of MSA is evaluated by comparing ternary MSA–H2SO4–DMA cluster formation rates to binary H2SO4–DMA cluster formation rates. Within the range of atmospherically relevant MSA concentrations, we find that MSA may increase cluster formation rates by up to 1 order of magnitude, although typically, the increase will be less than 300 % at 258 K, less than 100 % at 278 K and less than 15 % at 298 K. The results are rationalised by a detailed analysis of the main growth paths of the clusters. We find that MSA-enhanced clustering involves clusters containing one MSA molecule, while clusters containing more than one MSA molecule do not contribute significantly to the growth.

Posted on 17 November 2014 | 12:00 am


Impact of the representation of marine stratocumulus clouds on the anthropogenic aerosol effect

Impact of the representation of marine stratocumulus clouds on the anthropogenic aerosol effect

Atmospheric Chemistry and Physics, 14, 11997-12022, 2014

Author(s): D. Neubauer, U. Lohmann, C. Hoose, and M. G. Frontoso

Stratocumulus clouds are important for climate as they reflect large amounts of solar radiation back into space. However they are difficult to simulate in global climate models because they form under a sharp inversion and are thin. A comparison of model simulations with the ECHAM6-HAM2 global aerosol climate model to observations, reanalysis and literature data revealed too strong turbulent mixing at the top of stratocumulus clouds and a lack of vertical resolution. Further reasons for cloud biases in stratocumulus regions are the too "active" shallow convection scheme, the cloud cover scheme and possibly too low subsidence rates.

To address some of these issues and improve the representation of stratocumulus clouds, we made three distinct changes to ECHAM6-HAM2. With a "sharp" stability function in the turbulent mixing scheme we have observed, similar to previous studies, increases in stratocumulus cloud cover and liquid water path. With an increased vertical resolution in the lower troposphere in ECHAM6-HAM2 the stratocumulus clouds form higher up in the atmosphere and their vertical extent agrees better with reanalysis data. The recently implemented in-cloud aerosol processing in stratiform clouds is used to improve the aerosol representation in the model.

Including the improvements also affects the anthropogenic aerosol effect. In-cloud aerosol processing in ECHAM6-HAM2 leads to a decrease in the anthropogenic aerosol effect in the global annual mean from ?1.19 Wm?2 in the reference simulation to ?1.08 Wm?2, while using a "sharp" stability function leads to an increase to ?1.34 Wm?2. The results from the simulations with increased vertical resolution are diverse but increase the anthropogenic aerosol effect to ?2.08 Wm?2 at 47 levels and ?2.30 Wm?2 at 95 levels.

Posted on 14 November 2014 | 12:00 am


Forest canopy interactions with nucleation mode particles

Forest canopy interactions with nucleation mode particles

Atmospheric Chemistry and Physics, 14, 11985-11996, 2014

Author(s): S. C. Pryor, K. E. Hornsby, and K. A. Novick

Ultrafine particle size distributions through a deciduous forest canopy indicate that nucleation mode particle concentrations decline with depth into the canopy, such that number concentrations at the bottom of the canopy are an average of 16% lower than those at the top. However, growth rates of nucleation mode particles (diameters 6–30 nm) are invariant with height within the canopy, which implies that the semi-volatile gases contributing to their growth are comparatively well-mixed through the canopy. Growth rates of nucleation mode particles during a meteorological drought year (2012) were substantially lower than during a meteorologically normal year with high soil water potential (2013). This may reflect suppression of actual biogenic volatile organic compound (BVOC) emissions by drought and thus a reduction in the production of condensable products during the drought-affected vegetation season. This hypothesis is supported by evidence that growth rates during the normal year exhibit a positive correlation with emissions of BVOC modeled on observed forest composition, leaf area index, temperature and photosynthetically active radiation (PAR), but particle growth rates during the drought-affected vegetation season are not correlated with modeled BVOC emissions. These data thus provide indirect evidence that drought stress in forests may reduce BVOC emissions and limit growth of nucleation mode particles to climate-relevant sizes.

Posted on 14 November 2014 | 12:00 am


Forecasting global atmospheric CO2

Forecasting global atmospheric CO2

Atmospheric Chemistry and Physics, 14, 11959-11983, 2014

Author(s): A. Agustí-Panareda, S. Massart, F. Chevallier, S. Boussetta, G. Balsamo, A. Beljaars, P. Ciais, N. M. Deutscher, R. Engelen, L. Jones, R. Kivi, J.-D. Paris, V.-H. Peuch, V. Sherlock, A. T. Vermeulen, P. O. Wennberg, and D. Wunch

A new global atmospheric carbon dioxide (CO2) real-time forecast is now available as part of the pre-operational Monitoring of Atmospheric Composition and Climate – Interim Implementation (MACC-II) service using the infrastructure of the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS). One of the strengths of the CO2 forecasting system is that the land surface, including vegetation CO2 fluxes, is modelled online within the IFS. Other CO2 fluxes are prescribed from inventories and from off-line statistical and physical models. The CO2 forecast also benefits from the transport modelling from a state-of-the-art numerical weather prediction (NWP) system initialized daily with a wealth of meteorological observations. This paper describes the capability of the forecast in modelling the variability of CO2 on different temporal and spatial scales compared to observations. The modulation of the amplitude of the CO2 diurnal cycle by near-surface winds and boundary layer height is generally well represented in the forecast. The CO2 forecast also has high skill in simulating day-to-day synoptic variability. In the atmospheric boundary layer, this skill is significantly enhanced by modelling the day-to-day variability of the CO2 fluxes from vegetation compared to using equivalent monthly mean fluxes with a diurnal cycle. However, biases in the modelled CO2 fluxes also lead to accumulating errors in the CO2 forecast. These biases vary with season with an underestimation of the amplitude of the seasonal cycle both for the CO2 fluxes compared to total optimized fluxes and the atmospheric CO2 compared to observations. The largest biases in the atmospheric CO2 forecast are found in spring, corresponding to the onset of the growing season in the Northern Hemisphere. In the future, the forecast will be re-initialized regularly with atmospheric CO2 analyses based on the assimilation of CO2 products retrieved from satellite measurements and CO2 in situ observations, as they become available in near-real time. In this way, the accumulation of errors in the atmospheric CO2 forecast will be reduced. Improvements in the CO2 forecast are also expected with the continuous developments in the operational IFS.

Posted on 14 November 2014 | 12:00 am


Satellite observations of stratospheric carbonyl fluoride

Satellite observations of stratospheric carbonyl fluoride

Atmospheric Chemistry and Physics, 14, 11915-11933, 2014

Author(s): J. J. Harrison, M. P. Chipperfield, A. Dudhia, S. Cai, S. Dhomse, C. D. Boone, and P. F. Bernath

The vast majority of emissions of fluorine-containing molecules are anthropogenic in nature, e.g. chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs). These molecules slowly degrade in the atmosphere, leading to the formation of HF, COF2, and COClF, which are the main fluorine-containing species in the stratosphere. Ultimately both COF2 and COClF further degrade to form HF, an almost permanent reservoir of stratospheric fluorine due to its extreme stability. Carbonyl fluoride (COF2) is the second-most abundant stratospheric "inorganic" fluorine reservoir, with main sources being the atmospheric degradation of CFC-12 (CCl2F2), HCFC-22 (CHF2Cl), and CFC-113 (CF2ClCFCl2).

This work reports the first global distributions of carbonyl fluoride in the Earth's atmosphere using infrared satellite remote-sensing measurements by the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS), which has been recording atmospheric spectra since 2004, and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument, which recorded thermal emission atmospheric spectra between 2002 and 2012. The observations reveal a high degree of seasonal and latitudinal variability over the course of a year. These have been compared with the output of SLIMCAT, a state-of-the-art three-dimensional chemical transport model. In general the observations agree well with each other, although MIPAS is biased high by as much as ~30%, and compare well with SLIMCAT.

Between January 2004 and September 2010 COF2 grew most rapidly at altitudes above ~25 km in the southern latitudes and at altitudes below ~25 km in the northern latitudes, whereas it declined most rapidly in the tropics. These variations are attributed to changes in stratospheric dynamics over the observation period. The overall COF2 global trend over this period is calculated as 0.85 ± 0.34 (MIPAS), 0.30 ± 0.44 (ACE), and 0.88% year?1 (SLIMCAT).

Posted on 13 November 2014 | 12:00 am


Chemical climatology of the southeastern United States, 1999–2013

Chemical climatology of the southeastern United States, 1999–2013

Atmospheric Chemistry and Physics, 14, 11893-11914, 2014

Author(s): G. M. Hidy, C. L. Blanchard, K. Baumann, E. Edgerton, S. Tanenbaum, S. Shaw, E. Knipping, I. Tombach, J. Jansen, and J. Walters

A series of experiments (the Southern Oxidant and Aerosol Study – SOAS) took place in central Alabama in June–July, 2013 as part of the broader Southern Atmosphere Study (SAS). These projects were aimed at studying oxidant photochemistry and formation and impacts of aerosols at a detailed process level in a location where high biogenic organic vapor emissions interact with anthropogenic emissions, and the atmospheric chemistry occurs in a subtropical climate in North America. The majority of the ground-based experiments were located at the Southeastern Aerosol Research and Characterization (SEARCH) Centreville (CTR) site near Brent, Alabama, where extensive, unique aerometric measurements of trace gases and particles and meteorology were made beginning in the early 1990s through 2013. The SEARCH network data permits a characterization of the temporal and spatial context of the SOAS findings. Our earlier analyses of emissions and air quality trends are extended through 2013 to provide a perspective for continued decline in ambient concentrations, and the implications of these changes to regional sulfur oxide, nitrogen–ozone, and carbon chemistry. The narrative supports the SAS program in terms of long-term average chemistry (chemical climatology) and short-term comparisons of early summer average spatial variability across the southeastern US at high temporal (hourly) resolution. The long-term measurements show that the SOAS experiments took place during the second wettest and coolest year in the 2000–2013 period, with lower than average solar radiation. The pollution levels at CTR and other SEARCH sites were the lowest since full measurements began in 1999. Changes in anthropogenic gas and particle emissions between 1999 and 2013 account for the decline in pollutant concentrations at the monitoring sites in the region. The data provide an opportunity to contrast SOAS results with temporally and spatially variable conditions in support of the development of tests for the robustness of SOAS findings.

Posted on 13 November 2014 | 12:00 am


The distribution and trends of fog and haze in the North China Plain over the past 30 years

The distribution and trends of fog and haze in the North China Plain over the past 30 years

Atmospheric Chemistry and Physics, 14, 11949-11958, 2014

Author(s): G. Q. Fu, W. Y. Xu, R. F. Yang, J. B. Li, and C. S. Zhao

Frequent low visibility, haze and fog events were found in the North China Plain (NCP). Data throughout the NCP during the past 30 years were examined to determine the horizontal distribution and decadal trends of low visibility, haze and fog events. The impact of meteorological factors such as wind and relative humidity (RH) on those events was investigated. Results reveal distinct distributions of haze and fog days, due to their different formation mechanisms. Low visibility, haze and fog days all display increasing trends of before 1995, a steady stage during the period 1995–2003 and a drastically drop thereafter. All three events occurred most frequently during the heating season. Benefiting from emission control measures, haze and fog both show decreasing trends in winter during the past 3 decades, while summertime haze displays continuous increasing trends. The distribution of wind speed and wind direction as well as the topography within the NCP has determinative impacts on the distribution of haze and fog. Weakened south-easterly winds in the southern part of the NCP have resulted in high pollutant concentrations and frequent haze events along the foot of the Taihang Mountains. The orographically generated boundary layer wind convergence line in the central area of the southern NCP is responsible for the frequent fog events in this region. Wind speed has been decreasing throughout the entire southern NCP, resulting in more stable atmospheric conditions and weaker dispersion abilities, calling for harder efforts to control emissions to prevent haze events. Haze events are strongly influenced by the ambient RH. RH values associated with haze days are evidently increasing, suggesting that an increasing fraction of haze events are caused by the hygroscopic growth of aerosols, rather than simply by high aerosol loadings.

Posted on 13 November 2014 | 12:00 am


The effects of aerosols on water cloud microphysics and macrophysics based on satellite-retrieved data over East Asia and the North Pacific

The effects of aerosols on water cloud microphysics and macrophysics based on satellite-retrieved data over East Asia and the North Pacific

Atmospheric Chemistry and Physics, 14, 11935-11948, 2014

Author(s): T. Michibata, K. Kawamoto, and T. Takemura

This study examines the characteristics of the microphysics and macrophysics of water clouds from East Asia to the North Pacific, using data from active CloudSat radar measurements and passive MODerate-resolution Imaging Spectroradiometer (MODIS) retrievals. Our goals are to clarify differences in microphysics and macrophysics between land and oceanic clouds, seasonal differences unique to the midlatitudes, characteristics of the drizzling process, and cloud vertical structure. In pristine oceanic areas, fractional occurrences of cloud optical thickness (COT) and cloud droplet effective radius (CDR) increase systematically with an increase in drizzle intensity, but these characteristics of the COT and CDR transition are less evident in polluted land areas. In addition, regional and seasonal differences are identified in terms of drizzle intensity as a function of the liquid water path (LWP) and cloud droplet number concentration (Nc). The correlations between drizzle intensity and LWP, and between drizzle intensity and Nc, are both more robust over oceanic areas than over land areas. We also demonstrate regional and seasonal characteristics of the cloud vertical structure. Our results suggest that aerosol–cloud interaction mainly occurs around the cloud base in polluted land areas during the winter season. In addition, a difference between polluted and pristine areas in the efficiency of cloud droplet growth is confirmed. These results suggest that water clouds over the midlatitudes exhibit a different drizzle system to those over the tropics.

Posted on 13 November 2014 | 12:00 am


Airborne verification of CALIPSO products over the Amazon: a case study of daytime observations in a complex atmospheric scene

Airborne verification of CALIPSO products over the Amazon: a case study of daytime observations in a complex atmospheric scene

Atmospheric Chemistry and Physics, 14, 11871-11881, 2014

Author(s): F. Marenco, V. Amiridis, E. Marinou, A. Tsekeri, and J. Pelon

A daytime underflight of CALIPSO with the Facility for Airborne Atmospheric Measurements was performed on 20 September 2012 in the Amazon region of Brazil, during the biomass burning season. The scene is dominated by a thin elevated layer (aerosol optical depth (AOD) 0.03 at 532 nm) and a moderately turbid boundary layer (AOD ~ 0.2 at 532 nm). The boundary layer is topped with small broken stratocumulus clouds. In this complex scene, a comparison of observations from the airborne and spaceborne lidars reveals a few discrepancies. The CALIPSO detection scheme tends to miss the elevated thin layer, and also shows several gaps (~ 30%) in the boundary layer. The small clouds are not correctly removed from the signals; this can cause the CALIPSO aerosol subtype to oscillate between smoke and polluted dust and may introduce distortion in the aerosol retrieval scheme. The magnitude of the average extinction coefficient estimated from CALIPSO Level 2 data in the boundary layer is as expected, when compared to the aircraft lidar and accounting for wavelength scaling. However, when the gaps in aerosol detection mentioned above are accounted for, we are left with an overall estimate of AOD for this particular scene that is of the order of two thirds of that determined with the airborne lidar.

Posted on 12 November 2014 | 12:00 am


Biogenic SOA formation through gas-phase oxidation and gas-to-particle partitioning – a comparison between process models of varying complexity

Biogenic SOA formation through gas-phase oxidation and gas-to-particle partitioning – a comparison between process models of varying complexity

Atmospheric Chemistry and Physics, 14, 11853-11869, 2014

Author(s): E. Hermansson, P. Roldin, A. Rusanen, D. Mogensen, N. Kivekäs, R. Väänänen, M. Boy, and E. Swietlicki

Biogenic volatile organic compounds (BVOCs) emitted by vegetation play an important role for aerosol mass loadings since the oxidation products of these compounds can take part in the formation and growth of secondary organic aerosols (SOA). The concentrations and properties of BVOCs and their oxidation products in the atmosphere are poorly characterized, which leads to high uncertainties in modeled SOA mass and properties. In this study, the formation of SOA has been modeled along an air-mass trajectory over northern European boreal forest using two aerosol dynamics box models where the prediction of the condensable organics from the gas-phase oxidation of BVOC is handled with schemes of varying complexity. The use of box model simulations along an air-mass trajectory allows us to compare, under atmospheric relevant conditions, different model parameterizations and their effect on SOA formation. The result of the study shows that the modeled mass concentration of SOA is highly dependent on the organic oxidation scheme used to predict oxidation products. A near-explicit treatment of organic gas-phase oxidation (Master Chemical Mechanism version 3.2) was compared to oxidation schemes that use the volatility basis set (VBS) approach. The resulting SOA mass modeled with different VBS schemes varies by a factor of about 7 depending on how the first-generation oxidation products are parameterized and how they subsequently age (e.g., how fast the gas-phase oxidation products react with the OH radical, how they respond to temperature changes, and if they are allowed to fragment during the aging process). Since the VBS approach is frequently used in regional and global climate models due to its relatively simple treatment of the oxidation products compared to near-explicit oxidation schemes, a better understanding of the above-mentioned processes is needed. Based on the results of this study, fragmentation should be included in order to obtain a realistic SOA formation. Furthermore, compared to the most commonly used VBS schemes, the near-explicit method produces less – but more oxidized – SOA.

Posted on 12 November 2014 | 12:00 am


Assessment and application of clustering techniques to atmospheric particle number size distribution for the purpose of source apportionment

Assessment and application of clustering techniques to atmospheric particle number size distribution for the purpose of source apportionment

Atmospheric Chemistry and Physics, 14, 11883-11892, 2014

Author(s): F. Salimi, Z. Ristovski, M. Mazaheri, R. Laiman, L. R. Crilley, C. He, S. Clifford, and L. Morawska

Long-term measurements of particle number size distribution (PNSD) produce a very large number of observations and their analysis requires an efficient approach in order to produce results in the least possible time and with maximum accuracy. Clustering techniques are a family of sophisticated methods that have been recently employed to analyse PNSD data; however, very little information is available comparing the performance of different clustering techniques on PNSD data. This study aims to apply several clustering techniques (i.e. K means, PAM, CLARA and SOM) to PNSD data, in order to identify and apply the optimum technique to PNSD data measured at 25 sites across Brisbane, Australia. A new method, based on the Generalised Additive Model (GAM) with a basis of penalised B-splines, was proposed to parameterise the PNSD data and the temporal weight of each cluster was also estimated using the GAM. In addition, each cluster was associated with its possible source based on the results of this parameterisation, together with the characteristics of each cluster. The performances of four clustering techniques were compared using the Dunn index and Silhouette width validation values and the K means technique was found to have the highest performance, with five clusters being the optimum. Therefore, five clusters were found within the data using the K means technique. The diurnal occurrence of each cluster was used together with other air quality parameters, temporal trends and the physical properties of each cluster, in order to attribute each cluster to its source and origin. The five clusters were attributed to three major sources and origins, including regional background particles, photochemically induced nucleated particles and vehicle generated particles. Overall, clustering was found to be an effective technique for attributing each particle size spectrum to its source and the GAM was suitable to parameterise the PNSD data. These two techniques can help researchers immensely in analysing PNSD data for characterisation and source apportionment purposes.

Posted on 12 November 2014 | 12:00 am


HO2NO2 and HNO3 in the coastal Antarctic winter night: a "lab-in-the-field" experiment

HO2NO2 and HNO3 in the coastal Antarctic winter night: a "lab-in-the-field" experiment

Atmospheric Chemistry and Physics, 14, 11843-11851, 2014

Author(s): A. E. Jones, N. Brough, P. S. Anderson, and E. W. Wolff

Observations of peroxynitric acid (HO2NO2) and nitric acid (HNO3) were made during a 4 month period of Antarctic winter darkness at the coastal Antarctic research station, Halley. Mixing ratios of HNO3 ranged from instrumental detection limits to ~8 parts per trillion by volume (pptv), and of HO2NO2 from detection limits to ~5 pptv; the average ratio of HNO3 : HO2NO2 was 2.0(± 0.6) : 1, with HNO3 always present at greater mixing ratios than HO2NO2 during the winter darkness. An extremely strong association existed for the entire measurement period between mixing ratios of the respective trace gases and temperature: for HO2NO2, R2 = 0.72, and for HNO3, R2 = 0.70. We focus on three cases with considerable variation in temperature, where wind speeds were low and constant, such that, with the lack of photochemistry, changes in mixing ratio were likely to be driven by physical mechanisms alone. We derived enthalpies of adsorption (ΔHads) for these three cases. The average ΔHads for HNO3 was ?42 ± 2 kJ mol?1 and for HO2NO2 was ?56 ± 1 kJ mol?1; these values are extremely close to those derived in laboratory studies. This exercise demonstrates (i) that adsorption to/desorption from the snow pack should be taken into account when addressing budgets of boundary layer HO2NO2 and HNO3 at any snow-covered site, and (ii) that Antarctic winter can be used as a natural "laboratory in the field" for testing data on physical exchange mechanisms.

Posted on 12 November 2014 | 12:00 am


On the relationship between responses in cloud water and precipitation to changes in aerosol

On the relationship between responses in cloud water and precipitation to changes in aerosol

Atmospheric Chemistry and Physics, 14, 11817-11831, 2014

Author(s): Z. J. Lebo and G. Feingold

Climate models continue to exhibit strong sensitivity to the representation of aerosol effects on cloud reflectance and cloud amount. This paper evaluates a proposed method to constrain modeled cloud liquid water path (LWP) adjustments in response to changes in aerosol concentration Na using observations of precipitation susceptibility. Recent climate modeling has suggested a linear relationship between relative LWP responses to relative changes in Na, i.e., dln LWP / dln Na, and the precipitation frequency susceptibility Spop, which is defined as the relative change in the probability of precipitation for a relative change in Na. Using large-eddy simulations (LES) of marine stratocumulus and trade wind cumulus clouds, we show that these two cloud regimes exhibit qualitatively different relationships between ? and Spop; in stratocumulus clouds, ? increases with Spop, while in trade wind cumulus, ? decreases with Spop. The LES-derived relationship for marine stratocumulus is qualitatively similar but quantitatively different than that derived from climate model simulations of oceanic clouds aggregated over much larger spatial scales. We explore possible reasons for variability in these relationships, including the selected precipitation threshold and the various definitions of precipitation susceptibility that are currently in use. Because aerosol–cloud–precipitation interactions are inherently small-scale processes, we recommend that when deriving the relationship between ? and Spop, careful attention be given to the cloud regime, the scale, and the extent of aggregation of the model output or the observed data.

Posted on 11 November 2014 | 12:00 am


A meta-analysis of particle water uptake reconciliation studies

A meta-analysis of particle water uptake reconciliation studies

Atmospheric Chemistry and Physics, 14, 11833-11841, 2014

Author(s): J. D. Whitehead, M. Irwin, J. D. Allan, N. Good, and G. McFiggans

Water uptake by aerosol particles controls their ability to form cloud droplets, and reconciliation between different techniques for examining cloud condensation nuclei (CCN) properties is important to our understanding of these processes and our ability to measure and predict them. Reconciliation between measurements of sub-saturated and supersaturated aerosol particle water uptake was attempted at a wide range of locations between 2007 and 2013. The agreement in derived number of CCN (NCCN or particle hygroscopicity was mixed across the projects, with some data sets showing poor agreement across all supersaturations and others agreeing within errors for at least some of the supersaturation range. The degree of reconciliation did not seem to depend on the environment in which the measurements were taken. The discrepancies can only be attributable to differences in the chemical behaviour of aerosols and gases in each instrument, leading to under- or overestimated growth factors and/or CCN counts, though poorer reconciliation at lower supersaturations can be attributed to uncertainties in the size distribution at the threshold diameter found at these supersaturations. From a single instrument, the variability in NCCN calculated using particle hygroscopicity or size distribution averaged across a project demonstrates a greater sensitivity to variation in the size distribution than chemical composition in most of the experiments. However, the discrepancies between instruments indicate a strong requirement for reliable quantification of CCN in line with an improved understanding of the physical processes involved in their measurement. Without understanding the reason for discrepancies in the measurements, it is questionable whether quantification of CCN behaviour is meaningful.

Posted on 11 November 2014 | 12:00 am


Aerosol characterization at the Saharan AERONET site Tamanrasset

Aerosol characterization at the Saharan AERONET site Tamanrasset

Atmospheric Chemistry and Physics, 14, 11753-11773, 2014

Author(s): C. Guirado, E. Cuevas, V. E. Cachorro, C. Toledano, S. Alonso-Pérez, J. J. Bustos, S. Basart, P. M. Romero, C. Camino, M. Mimouni, L. Zeudmi, P. Goloub, J. M. Baldasano, and A. M. de Frutos

More than 2 years of columnar atmospheric aerosol measurements (2006–2009) at the Tamanrasset site (22.79° N, 5.53° E, 1377 m a.s.l.), in the heart of the Sahara, are analysed. Aerosol Robotic Network (AERONET) level 2.0 data were used. The KCICLO (K is the name of a constant and ciclo means cycle in Spanish) method was applied to a part of the level 1.5 data series to improve the quality of the results. The annual variability of aerosol optical depth (AOD) and Ångström exponent (AE) has been found to be strongly linked to the convective boundary layer (CBL) thermodynamic features. The dry-cool season (autumn and winter) is characterized by a shallow CBL and very low mean turbidity (AOD ~ 0.09 at 440 nm, AE ~ 0.62). The wet-hot season (spring and summer) is dominated by high turbidity of coarse dust particles (AE ~ 0.28, AOD ~ 0.39 at 440 nm) and a deep CBL. The aerosol-type characterization shows desert mineral dust as the prevailing aerosol. Both pure Saharan dust and very clear sky conditions are observed depending on the season. However, several case studies indicate an anthropogenic fine mode contribution from the industrial areas in Libya and Algeria. The concentration weighted trajectory (CWT) source apportionment method was used to identify potential sources of air masses arriving at Tamanrasset at several heights for each season. Microphysical and optical properties and precipitable water vapour were also investigated.

Posted on 11 November 2014 | 12:00 am


The response of the equatorial tropospheric ozone to the Madden–Julian Oscillation in TES satellite observations and CAM-chem model simulation

The response of the equatorial tropospheric ozone to the Madden–Julian Oscillation in TES satellite observations and CAM-chem model simulation

Atmospheric Chemistry and Physics, 14, 11775-11790, 2014

Author(s): W. Sun, P. Hess, and B. Tian

The Madden–Julian Oscillation (MJO) is the dominant form of the atmospheric intra-seasonal oscillation, manifested by slow eastward movement (about 5 m s?1) of tropical deep convection. This study investigates the MJO's impact on equatorial tropospheric ozone (10° N–10° S) in satellite observations and chemical transport model (CTM) simulations. For the satellite observations, we analyze the Tropospheric Emission Spectrometer (TES) level-2 ozone profile data for the period of January 2004 to June 2009. For the CTM simulations, we run the Community Atmosphere Model with chemistry (CAM-chem) driven by the Goddard Earth Observing System Model, Version 5 (GEOS-5)-analyzed meteorological fields for the same data period as the TES measurements. Our analysis indicates that the behavior of the total tropospheric column (TTC) ozone at the intra-seasonal timescale is different from that of the total column ozone, with the signal in the equatorial region comparable with that in the subtropics. The model-simulated and satellite-measured ozone anomalies agree in their general pattern and amplitude when examined in the vertical cross section (the average spatial correlation coefficient among the eight phases is 0.63), with an eastward propagation signature at a similar phase speed as the convective anomalies (5 m s?1). The model ozone anomalies on the intra-seasonal timescale are about 5 times larger when lightning emissions of NOx are included in the simulation than when they are not. Nevertheless, large-scale advection is the primary driving force for the ozone anomalies associated with the MJO. The variability related to the MJO for ozone reaches up to 47% of the total variability (ranging from daily to interannual), indicating that the MJO should be accounted for in simulating ozone perturbations in the tropics.

Posted on 11 November 2014 | 12:00 am


De praeceptis ferendis: good practice in multi-model ensembles

De praeceptis ferendis: good practice in multi-model ensembles

Atmospheric Chemistry and Physics, 14, 11791-11815, 2014

Author(s): I. Kioutsioukis and S. Galmarini

Ensembles of air quality models have been formally and empirically shown to outperform single models in many cases. Evidence suggests that ensemble error is reduced when the members form a diverse and accurate ensemble. Diversity and accuracy are hence two factors that should be taken care of while designing ensembles in order for them to provide better predictions. Theoretical aspects like the bias–variance–covariance decomposition and the accuracy–diversity decomposition are linked together and support the importance of creating ensemble that incorporates both these elements. Hence, the common practice of unconditional averaging of models without prior manipulation limits the advantages of ensemble averaging. We demonstrate the importance of ensemble accuracy and diversity through an inter-comparison of ensemble products for which a sound mathematical framework exists, and provide specific recommendations for model selection and weighting for multi-model ensembles. The sophisticated ensemble averaging techniques, following proper training, were shown to have higher skill across all distribution bins compared to solely ensemble averaging forecasts.

Posted on 11 November 2014 | 12:00 am


Global modelling of direct and indirect effects of sea spray aerosol using a source function encapsulating wave state

Global modelling of direct and indirect effects of sea spray aerosol using a source function encapsulating wave state

Atmospheric Chemistry and Physics, 14, 11731-11752, 2014

Author(s): A.-I. Partanen, E. M. Dunne, T. Bergman, A. Laakso, H. Kokkola, J. Ovadnevaite, L. Sogacheva, D. Baisnée, J. Sciare, A. Manders, C. O'Dowd, G. de Leeuw, and H. Korhonen

Recently developed parameterizations for the sea spray aerosol source flux, encapsulating wave state, and its organic fraction were incorporated into the aerosol–climate model ECHAM-HAMMOZ to investigate the direct and indirect radiative effects of sea spray aerosol particles. Our simulated global sea salt emission of 805 Tg yr−1 (uncertainty range 378–1233 Tg yr−1) was much lower than typically found in previous studies. Modelled sea salt and sodium ion concentrations agreed relatively well with measurements in the smaller size ranges at Mace Head (annual normalized mean model bias ?13% for particles with vacuum aerodynamic diameter Dva < 1 μm), Point Reyes (?29% for particles with aerodynamic diameter Da < 2.5 μm) and Amsterdam Island (?52% for particles with Da < 1 μm) but the larger sizes were overestimated (899% for particles with 2.5 μm < Da < 10 μm) at Amsterdam Island. This suggests that at least the high end of the previous estimates of sea spray mass emissions is unrealistic. On the other hand, the model clearly underestimated the observed concentrations of organic or total carbonaceous aerosol at Mace Head (?82%) and Amsterdam Island (?68%). The large overestimation (212%) of organic matter at Point Reyes was due to the contribution of continental sources. At the remote Amsterdam Island site, the organic concentration was underestimated especially in the biologically active months, suggesting a need to improve the parameterization of the organic sea spray fraction. Globally, the satellite-retrieved AOD over the oceans, using PARASOL data, was underestimated by the model (means over ocean 0.16 and 0.10, respectively); however, in the pristine region around Amsterdam Island the measured AOD fell well within the simulated uncertainty range. The simulated sea spray aerosol contribution to the indirect radiative effect was positive (0.3 W m−2), in contrast to previous studies. This positive effect was ascribed to the tendency of sea salt aerosol to suppress both the in-cloud supersaturation and the formation of cloud condensation nuclei from sulfate. These effects can be accounted for only in models with sufficiently detailed aerosol microphysics and physics-based parameterizations of cloud activation. However, due to a strong negative direct effect, the simulated effective radiative forcing (total radiative) effect was ?0.2 W m−2. The simulated radiative effects of the primary marine organic emissions were small, with a direct effect of 0.03 W m−2 and an indirect effect of ?0.07 W m−2.

Posted on 7 November 2014 | 12:00 am


Analysis of nucleation events in the European boundary layer using the regional aerosol–climate model REMO-HAM with a solar radiation-driven OH-proxy

Analysis of nucleation events in the European boundary layer using the regional aerosol–climate model REMO-HAM with a solar radiation-driven OH-proxy

Atmospheric Chemistry and Physics, 14, 11711-11729, 2014

Author(s): J.-P. Pietikäinen, S. Mikkonen, A. Hamed, A. I. Hienola, W. Birmili, M. Kulmala, and A. Laaksonen

This work describes improvements in the regional aerosol–climate model REMO-HAM in order to simulate more realistically the process of atmospheric new particle formation (NPF). A new scheme was implemented to simulate OH radical concentrations using a proxy approach based on observations and also accounting for the effects of clouds upon OH concentrations. Second, the nucleation rate calculation was modified to directly simulate the formation rates of 3 nm particles, which removes some unnecessary steps in the formation rate calculations used earlier in the model. Using the updated model version, NPF over Europe was simulated for the periods 2003–2004 and 2008–2009. The statistics of the simulated particle formation events were subsequently compared to observations from 13 ground-based measurement sites. The new model shows improved agreement with the observed NPF rates compared to former versions and can simulate the event statistics realistically for most parts of Europe.

Posted on 7 November 2014 | 12:00 am


Biases in modeled surface snow BC mixing ratios in prescribed-aerosol climate model runs

Biases in modeled surface snow BC mixing ratios in prescribed-aerosol climate model runs

Atmospheric Chemistry and Physics, 14, 11697-11709, 2014

Author(s): S. J. Doherty, C. M. Bitz, and M. G. Flanner

Black carbon (BC) in snow lowers its albedo, increasing the absorption of sunlight, leading to positive radiative forcing, climate warming and earlier snowmelt. A series of recent studies have used prescribed-aerosol deposition flux fields in climate model runs to assess the forcing by black carbon in snow. In these studies, the prescribed mass deposition flux of BC to surface snow is decoupled from the mass deposition flux of snow water to the surface. Here we compare prognostic- and prescribed-aerosol runs and use a series of offline calculations to show that the prescribed-aerosol approach results, on average, in a factor of about 1.5–2.5 high bias in annual-mean surface snow BC mixing ratios in three key regions for snow albedo forcing by BC: Greenland, Eurasia and North America. These biases will propagate directly to positive biases in snow and surface albedo reduction by BC. The bias is shown be due to coupling snowfall that varies on meteorological timescales (daily or shorter) with prescribed BC mass deposition fluxes that are more temporally and spatially smooth. The result is physically non-realistic mixing ratios of BC in surface snow. We suggest that an alternative approach would be to prescribe BC mass mixing ratios in snowfall, rather than BC mass fluxes, and we show that this produces more physically realistic BC mixing ratios in snowfall and in the surface snow layer.

Posted on 7 November 2014 | 12:00 am


Lidar-observed enhancement of aerosols in the upper troposphere and lower stratosphere over the Tibetan Plateau induced by the Nabro volcano eruption

Lidar-observed enhancement of aerosols in the upper troposphere and lower stratosphere over the Tibetan Plateau induced by the Nabro volcano eruption

Atmospheric Chemistry and Physics, 14, 11687-11696, 2014

Author(s): Q. S. He, C. C. Li, J. Z. Ma, H. Q. Wang, X. L. Yan, J. Lu, Z. R. Liang, and G. M. Qi

Vertical profiles of aerosol extinction coefficients were measured by a micro-pulse lidar at Naqu (31.5° N, 92.1° E; 4508 m a.m.s.l.), a meteorological station located on the central part of the Tibetan Plateau during summer 2011. Observations show a persistent maximum in aerosol extinction coefficients in the upper troposphere–lower stratosphere (UTLS). These aerosol layers were generally located at an altitude of 18–19 km a.m.s.l., 1–2 km higher than the tropopause, with broad layer depth ranging approximately 3–4 km and scattering ratio of 4–9. Daily averaged aerosol optical depths (AODs) of the enhanced aerosol layers in UTLS over the Tibetan Plateau varied from 0.007 to 0.030, in agreement with globally averaged levels of 0.018 ± 0.009 at 532 nm from previous observations, but the percentage contributions of the enhanced aerosol layers to the total AOD over the Tibetan Plateau are higher than those observed elsewhere. The aerosol layers in UTLS wore off gradually with the reducing intensity of the Asian monsoon over the Tibetan Plateau at the end of August. The eruption of Nabro volcano on 13 June 2011 is considered an important factor to explain the enhancement of tropopause aerosols observed this summer over the Tibetan Plateau.

Posted on 6 November 2014 | 12:00 am


A pathway analysis of global aerosol processes

A pathway analysis of global aerosol processes

Atmospheric Chemistry and Physics, 14, 11657-11686, 2014

Author(s): N. A. J. Schutgens and P. Stier

We present a detailed budget of the changes in atmospheric aerosol mass and numbers due to various processes: emission (including instant condensation of soluble biogenic emissions), nucleation, coagulation, H2SO4 condensation and in-cloud production, aging and deposition. The budget is created from monthly averaged tracer tendencies calculated by the global aerosol model ECHAM5.5-HAM2 and allows us to investigate process contributions at various length-scales and timescales. As a result, we show in unprecedented detail what processes drive the evolution of aerosol. In particular, we show that the processes that affect aerosol masses are quite different from those that affect aerosol numbers. Condensation of H2SO4 gas onto pre-existing particles is an important process, dominating the growth of small particles in the nucleation mode to the Aitken mode and the aging of hydrophobic matter. Together with in-cloud production of H2SO4, it significantly contributes to (and often dominates) the mass burden (and hence composition) of the hydrophilic Aitken and accumulation mode particles. Particle growth itself is the leading source of number densities in the hydrophilic Aitken and accumulation modes, with their hydrophobic counterparts contributing (even locally) relatively little. As expected, the coarse mode is dominated by primary emissions and mostly decoupled from the smaller modes. Our analysis also suggests that coagulation serves mainly as a loss process for number densities and that, relative to other processes, it is a rather unimportant contributor to composition changes of aerosol. The analysis is extended with sensitivity studies where the impact of a lower model resolution or pre-industrial emissions is shown to be small. We discuss the use of the current budget for model simplification, prioritization of model improvements, identification of potential structural model errors and model evaluation against observations.

Posted on 6 November 2014 | 12:00 am


Observations of rapid aerosol optical depth enhancements in the vicinity of polluted cumulus clouds

Observations of rapid aerosol optical depth enhancements in the vicinity of polluted cumulus clouds

Atmospheric Chemistry and Physics, 14, 11633-11656, 2014

Author(s): T. F. Eck, B. N. Holben, J. S. Reid, A. Arola, R. A. Ferrare, C. A. Hostetler, S. N. Crumeyrolle, T. A. Berkoff, E. J. Welton, S. Lolli, A. Lyapustin, Y. Wang, J. S. Schafer, D. M. Giles, B. E. Anderson, K. L. Thornhill, P. Minnis, K. E. Pickering, C. P. Loughner, A. Smirnov, and A. Sinyuk

During the July 2011 Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) field experiment in Maryland, significant enhancements in Aerosol Robotic Network (AERONET) sun–sky radiometer measured aerosol optical depth (AOD) were observed in the immediate vicinity of non-precipitating cumulus clouds on some days. Both measured Ångström exponents and aerosol size distribution retrievals made before, during and after cumulus development often suggest little change in fine mode particle size; therefore, implying possible new particle formation in addition to cloud processing and humidification of existing particles. In addition to sun–sky radiometer measurements of large enhancements of fine mode AOD, lidar measurements made from both ground-based and aircraft-based instruments during the experiment also measured large increases in aerosol signal at altitudes associated with the presence of fair weather cumulus clouds. These data show modifications of the aerosol vertical profile as a result of the aerosol enhancements at and below cloud altitudes. The airborne lidar data were utilized to estimate the spatial extent of these aerosol enhancements, finding increased AOD, backscatter and extinction out to 2.5 km distance from the cloud edge. Furthermore, in situ measurements made from aircraft vertical profiles over an AERONET site during the experiment also showed large increases in aerosol scattering and aerosol volume after cloud formation as compared to before. The 15-year AERONET database of AOD measurements at the Goddard Space Flight Center (GSFC), Maryland site, was investigated in order to obtain a climatological perspective of this phenomenon of AOD enhancement. Analysis of the diurnal cycle of AOD in summer showed significant increases in AOD from morning to late afternoon, corresponding to the diurnal cycle of cumulus development.

Posted on 6 November 2014 | 12:00 am


Evaluation of OMI operational standard NO2 column retrievals using in situ and surface-based NO2 observations

Evaluation of OMI operational standard NO2 column retrievals using in situ and surface-based NO2 observations

Atmospheric Chemistry and Physics, 14, 11587-11609, 2014

Author(s): L. N. Lamsal, N. A. Krotkov, E. A. Celarier, W. H. Swartz, K. E. Pickering, E. J. Bucsela, J. F. Gleason, R. V. Martin, S. Philip, H. Irie, A. Cede, J. Herman, A. Weinheimer, J. J. Szykman, and T. N. Knepp

We assess the standard operational nitrogen dioxide (NO2) data product (OMNO2, version 2.1) retrieved from the Ozone Monitoring Instrument (OMI) onboard NASA's Aura satellite using a combination of aircraft and surface in~situ measurements as well as ground-based column measurements at several locations and a bottom-up NOx emission inventory over the continental US. Despite considerable sampling differences, NO2 vertical column densities from OMI are modestly correlated (r = 0.3–0.8) with in situ measurements of tropospheric NO2 from aircraft, ground-based observations of NO2 columns from MAX-DOAS and Pandora instruments, in situ surface NO2 measurements from photolytic converter instruments, and a bottom-up NOx emission inventory. Overall, OMI retrievals tend to be lower in urban regions and higher in remote areas, but generally agree with other measurements to within ± 20%. No consistent seasonal bias is evident. Contrasting results between different data sets reveal complexities behind NO2 validation. Since validation data sets are scarce and are limited in space and time, validation of the global product is still limited in scope by spatial and temporal coverage and retrieval conditions. Monthly mean vertical NO2 profile shapes from the Global Modeling Initiative (GMI) chemistry-transport model (CTM) used in the OMI retrievals are highly consistent with in situ aircraft measurements, but these measured profiles exhibit considerable day-to-day variation, affecting the retrieved daily NO2 columns by up to 40%. This assessment of OMI tropospheric NO2 columns, together with the comparison of OMI-retrieved and model-simulated NO2 columns, could offer diagnostic evaluation of the model.

Posted on 5 November 2014 | 12:00 am


Evolution of aerosol chemistry in Xi'an, inland China, during the dust storm period of 2013 – Part 1: Sources, chemical forms and formation mechanisms of nitrate and sulfate

Evolution of aerosol chemistry in Xi'an, inland China, during the dust storm period of 2013 – Part 1: Sources, chemical forms and formation mechanisms of nitrate and sulfate

Atmospheric Chemistry and Physics, 14, 11571-11585, 2014

Author(s): G. H. Wang, C. L. Cheng, Y. Huang, J. Tao, Y. Q. Ren, F. Wu, J. J. Meng, J. J. Li, Y. T. Cheng, J. J. Cao, S. X. Liu, T. Zhang, R. Zhang, and Y. B. Chen

A total suspended particulate (TSP) sample was collected hourly in Xi'an, an inland megacity of China near the Loess Plateau, during a dust storm event of 2013 (9 March 18:00?12 March 10:00 LT), along with a size-resolved aerosol sampling and an online measurement of PM2.5. The TSP and size-resolved samples were determined for elemental carbon (EC), organic carbon (OC), water-soluble organic carbon (WSOC) and nitrogen (WSON), inorganic ions and elements to investigate chemistry evolution of dust particles. Hourly concentrations of Cl, NO3, SO42−, Na+ and Ca2+ in the TSP samples reached up to 34, 12, 180, 72 and 28 ?g m?3, respectively, when dust peak arrived over Xi'an. Chemical compositions of the TSP samples showed that during the whole observation period NH4+ and NO3 were linearly correlated with each other (r2=0.76) with a molar ratio of 1 : 1, while SO42− and Cl were well correlated with Na+, Ca2+, Mg2+ and K+ (r2 > 0.85). Size distributions of NH4+ and NO3 presented a same pattern, which dominated in the coarse mode (> 2.1 ?m) during the event and predominated in the fine mode (< 2.1 ?m) during the non-event. SO42− and Cl also dominated in the coarse mode during the event hours, but both exhibited two equivalent peaks in both the fine and the coarse modes during the non-event, due to the fine-mode accumulations of secondarily produced SO42− and biomass-burning-emitted Cl- and the coarse-mode enrichments of urban soil-derived SO42− and Cl. Linear fit regression analysis further indicated that SO42− and Cl in the dust samples possibly exist as Na2SO4, CaSO4 and NaCl, which directly originated from Gobi desert surface soil, while NH4+ and NO3 in the dust samples exist as NH4NO3. We propose a mechanism to explain these observations in which aqueous phase of dust particle surface is formed via uptake of water vapor by hygroscopic salts such as Na2SO4 and NaCl, followed by heterogeneous formation of nitrate on the liquid phase and subsequent absorption of ammonia. Our data indicate that 54 ± 20% and 60 ± 23% of NH4+ and NO3 during the dust period were secondarily produced via this pathway, with the remaining derived from the Gobi desert and Loess Plateau, while SO42− in the event almost entirely originated from the desert regions. Such cases are different from those in the East Asian continental outflow region, where during Asia dust storm events SO42− is secondarily produced and concentrates in sub-micrometer particles as (NH4)2SO4 and/or NH4HSO4. To the best of our knowledge, the current work for the first time revealed an infant state of the East Asian dust ageing process in the regions near the source, which is helpful for researchers to understand the panorama of East Asian dust ageing process from the desert area to the downwind region.

Posted on 5 November 2014 | 12:00 am


On direct passive microwave remote sensing of sea spray aerosol production

On direct passive microwave remote sensing of sea spray aerosol production

Atmospheric Chemistry and Physics, 14, 11611-11631, 2014

Author(s): I. B. Savelyev, M. D. Anguelova, G. M. Frick, D. J. Dowgiallo, P. A. Hwang, P. F. Caffrey, and J. P. Bobak

This study addresses and attempts to mitigate persistent uncertainty and scatter among existing approaches for determining the rate of sea spray aerosol production by breaking waves in the open ocean. The new approach proposed here utilizes passive microwave emissions from the ocean surface, which are known to be sensitive to surface roughness and foam. Direct, simultaneous, and collocated measurements of the aerosol production and microwave emissions were collected aboard the FLoating Instrument Platform (FLIP) in deep water ~ 150 km off the coast of California over a period of ~ 4 days. Vertical profiles of coarse-mode aerosol (0.25–23.5 ?m) concentrations were measured with a forward-scattering spectrometer and converted to surface flux using dry deposition and vertical gradient methods. Back-trajectory analysis of eastern North Pacific meteorology verified the clean marine origin of the sampled air mass over at least 5 days prior to measurements. Vertical and horizontal polarization surface brightness temperature were measured with a microwave radiometer at 10.7 GHz frequency. Data analysis revealed a strong sensitivity of the brightness temperature polarization difference to the rate of aerosol production. An existing model of microwave emission from the ocean surface was used to determine the empirical relationship and to attribute its underlying physical basis to microwave emissions from surface roughness and foam within active and passive phases of breaking waves. A possibility of and initial steps towards satellite retrievals of the sea spray aerosol production are briefly discussed in concluding remarks.

Posted on 5 November 2014 | 12:00 am


Composition of 15–85 nm particles in marine air

Composition of 15–85 nm particles in marine air

Atmospheric Chemistry and Physics, 14, 11557-11569, 2014

Author(s): M. J. Lawler, J. Whitehead, C. O'Dowd, C. Monahan, G. McFiggans, and J. N. Smith

The chemical composition of 15–85 nm diameter particles was measured at Mace Head, Ireland, during May 2011 using the TDCIMS (thermal desorption chemical ionization mass spectrometer). Measurable levels of chloride, sodium, and sulfate were present in essentially all collected samples of these particles at this coastal Atlantic site. Acetaldehyde and benzoic acid were also frequently detected. Concomitant particle hygroscopicity observations usually showed a sea-salt mode and a lower hygroscopicity mode with growth factors near to that of ammonium sulfate. There were many periods lasting from hours to about 2 days during which the 10–60 nm particle number increased dramatically in polar oceanic air. These periods were correlated with the presence of benzoic acid in the particles and an increase in the number of lower hygroscopicity mode particles. Very small (< 10 nm) particles were also present, suggesting that new particle formation contributed to these nanoparticle enhancement events.

Posted on 5 November 2014 | 12:00 am


Sensitivity of free tropospheric carbon monoxide to atmospheric weather states and their persistency: an observational assessment over the Nordic countries

Sensitivity of free tropospheric carbon monoxide to atmospheric weather states and their persistency: an observational assessment over the Nordic countries

Atmospheric Chemistry and Physics, 14, 11545-11555, 2014

Author(s): M. A. Thomas and A. Devasthale

Among various factors that influence the long-range transport of pollutants in the free troposphere (FT), the prevailing atmospheric weather states probably play the most important role in governing characteristics and efficacy of such transport. The weather states, such as a particular wind pattern, cyclonic or anticyclonic conditions, and their degree of persistency determine the spatio-temporal distribution and the final fate of the pollutants. This is especially true in the case of Nordic countries, where baroclinic disturbances and associated weather fronts primarily regulate local meteorology, in contrast to the lower latitudes where a convective paradigm plays a similarly important role. Furthermore, the long-range transport of pollutants in the FT has significant contribution to the total column burden over the Nordic countries. However, there is insufficient knowledge on the large-scale co-variability of pollutants in the FT and atmospheric weather states based solely on observational data over this region. The present study attempts to quantify and understand this statistical co-variability while providing relevant meteorological background.

To that end, we select eight weather states that predominantly occur over the Nordic countries and three periods of their persistency (3 days, 5 days, and 7 days), thus providing in total 24 cases to investigate sensitivity of free tropospheric carbon monoxide, an ideal tracer for studying pollutant transport, to these selected weather states. The eight states include four dominant wind directions (namely, NW, NE, SE and SW), cyclonic and anticyclonic conditions, and the enhanced positive and negative phases of the North Atlantic Oscillation (NAO). For our sensitivity analysis, we use recently released Version 6 retrievals of CO at 500 hPa from the Atmospheric Infrared Sounder (AIRS) onboard Aqua satellite covering the 11-year period from September 2002 through August 2013 and winds from the ECMWF's ERA-Interim project to classify weather states for the same 11-year period.

We show that, among the various weather states studied here, southeasterly winds lead to highest observed CO anomalies (up to +8%) over the Nordic countries while transporting pollution from the central and eastern parts of Europe. The second (up to +4%) and third highest (up to +2.5%) CO anomalies are observed when winds are northwesterly (facilitating inter-continental transport from polluted North American regions) and during the enhanced positive phase of the NAO respectively. Higher than normal CO anomalies are observed during anticyclonic conditions (up to +1%) compared to cyclonic conditions. The cleanest conditions are observed when winds are northeasterly and during the enhanced negative phases of the NAO, when relatively clean Arctic air masses are transported over the Nordic regions in the both cases. In the case of nearly all weather states, the CO anomalies consistently continue to increase or decrease as the degree of persistency of a weather state is increased. The results of this sensitivity study further provide an observational basis for the process-oriented evaluation of chemistry transport models, especially with regard to the representation of large-scale coupling of chemistry and local weather states and its role in the long-range transport of pollutants in such models.

Posted on 3 November 2014 | 12:00 am


Denitrification by large NAT particles: the impact of reduced settling velocities and hints on particle characteristics

Denitrification by large NAT particles: the impact of reduced settling velocities and hints on particle characteristics

Atmospheric Chemistry and Physics, 14, 11525-11544, 2014

Author(s): W. Woiwode, J.-U. Grooß, H. Oelhaf, S. Molleker, S. Borrmann, A. Ebersoldt, W. Frey, T. Gulde, S. Khaykin, G. Maucher, C. Piesch, and J. Orphal

Vertical redistribution of HNO3 through large HNO3-containing particles associated with polar stratospheric clouds (PSCs) plays an important role in the chemistry of the Arctic winter stratosphere. During the RECONCILE (Reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and its climate interactions) campaign, apparently very large NAT (nitric acid trihydrate) particles were observed by the airborne in situ probe FSSP-100 (Molleker et al., 2014). Our analysis shows that the FSSP-100 observations associated with the flight on 25 January 2010 cannot easily be explained assuming compact spherical NAT particles due to much too short growing time at temperatures below the existence temperature of NAT (TNAT). State-of-the-art simulations using CLaMS (Chemical Lagrangian Model of the Stratosphere; Grooß et al., 2014) suggest considerably smaller particles. We consider the hypothesis that the simulation reproduces the NAT particle masses in a realistic way, but that real NAT particles may have larger apparent sizes compared to compact spherical particles, e.g. due to non-compact morphology or aspheric shape. Our study focuses on the consequence that such particles would have reduced settling velocities compared to compact spheres, altering the vertical redistribution of HNO3. Utilising CLaMS simulations, we investigate the impact of reduced settling velocities of NAT particles on vertical HNO3 redistribution and compare the results with observations of gas-phase HNO3 by the airborne Fourier transform spectrometer MIPAS-STR associated with two RECONCILE flights. The MIPAS-STR observations confirm conditions consistent with denitrification by NAT particles for the flight on 25 January 2010 and show good agreement with the simulations within the limitations of the comparison. Best agreement is found if settling velocities between 100 and 50% relative to compact spherical particles are considered (slight preference for the 70% scenario). In contrast, relative settling velocities of 30% result in too weak vertical HNO3 redistribution. Sensitivity simulations considering temperature biases of ±1 K and multiplying the simulated nucleation rates by factors of 0.5 and 2.0 affect the comparisons to a similar extent, but result in no effective improvement compared to the reference scenario. Our results show that an accurate knowledge of the settling velocities of NAT particles is important for quantitative simulations of vertical HNO3 redistribution.

Posted on 31 October 2014 | 12:00 am


AERONET-based models of smoke-dominated aerosol near source regions and transported over oceans, and implications for satellite retrievals of aerosol optical depth

AERONET-based models of smoke-dominated aerosol near source regions and transported over oceans, and implications for satellite retrievals of aerosol optical depth

Atmospheric Chemistry and Physics, 14, 11493-11523, 2014

Author(s): A. M. Sayer, N. C. Hsu, T. F. Eck, A. Smirnov, and B. N. Holben

Smoke aerosols from biomass burning are an important component of the global aerosol system. Analysis of Aerosol Robotic Network (AERONET) retrievals of aerosol microphysical/optical parameters at 10 sites reveals variety between biomass burning aerosols in different global source regions, in terms of aerosol particle size and single scatter albedo (SSA). Case studies of smoke observed at coastal/island AERONET sites also mostly lie within the range of variability at the near-source sites. Differences between sites tend to be larger than variability at an individual site, although optical properties for some sites in different regions can be quite similar. Across the sites, typical midvisible SSA ranges from ~ 0.95–0.97 (sites dominated by boreal forest or peat burning, typically with larger fine-mode particle radius and spread) to ~ 0.88–0.9 (sites most influenced by grass, shrub, or crop burning, typically smaller fine-mode particle radius and spread). The tropical forest site Alta Floresta (Brazil) is closer to this second category, although with intermediate SSA ~ 0.92. The strongest absorption is seen in southern African savannah at Mongu (Zambia), with average midvisible SSA ~ 0.85. Sites with stronger absorption also tend to have stronger spectral gradients in SSA, becoming more absorbing at longer wavelengths. Microphysical/optical models are presented in detail so as to facilitate their use in radiative transfer calculations, including extension to UV (ultraviolet) wavelengths, and lidar ratios. One intended application is to serve as candidate optical models for use in satellite aerosol optical depth (AOD) retrieval algorithms. The models presently adopted by these algorithms over ocean often have insufficient absorption (i.e. too high SSA) to represent these biomass burning aerosols. The underestimates in satellite-retrieved AOD in smoke outflow regions, which have important consequences for applications of these satellite data sets, are consistent with the level of underestimated absorption.

Posted on 31 October 2014 | 12:00 am


Simulating black carbon and dust and their radiative forcing in seasonal snow: a case study over North China with field campaign measurements

Simulating black carbon and dust and their radiative forcing in seasonal snow: a case study over North China with field campaign measurements

Atmospheric Chemistry and Physics, 14, 11475-11491, 2014

Author(s): C. Zhao, Z. Hu, Y. Qian, L. Ruby Leung, J. Huang, M. Huang, J. Jin, M. G. Flanner, R. Zhang, H. Wang, H. Yan, Z. Lu, and D. G. Streets

A state-of-the-art regional model, the Weather Research and Forecasting (WRF) model (Skamarock et al., 2008) coupled with a chemistry component (Chem) (Grell et al., 2005), is coupled with the snow, ice, and aerosol radiative (SNICAR) model that includes the most sophisticated representation of snow metamorphism processes available for climate study. The coupled model is used to simulate black carbon (BC) and dust concentrations and their radiative forcing in seasonal snow over North China in January–February of 2010, with extensive field measurements used to evaluate the model performance. In general, the model simulated spatial variability of BC and dust mass concentrations in the top snow layer (hereafter BCS and DSTS, respectively) are consistent with observations. The model generally moderately underestimates BCS in the clean regions but significantly overestimates BCS in some polluted regions. Most model results fall within the uncertainty ranges of observations. The simulated BCS and DSTS are highest with > 5000 ng g?1 and up to 5 mg g?1, respectively, over the source regions and reduce to < 50 ng g?1 and < 1 ?g g?1, respectively, in the remote regions. BCS and DSTS introduce a similar magnitude of radiative warming (~ 10 W m?2) in the snowpack, which is comparable to the magnitude of surface radiative cooling due to BC and dust in the atmosphere. This study represents an effort in using a regional modeling framework to simulate BC and dust and their direct radiative forcing in snowpack. Although a variety of observational data sets have been used to attribute model biases, some uncertainties in the results remain, which highlights the need for more observations, particularly concurrent measurements of atmospheric and snow aerosols and the deposition fluxes of aerosols, in future campaigns.

Posted on 30 October 2014 | 12:00 am


A 2-year record of atmospheric mercury species at a background Southern Hemisphere station on Amsterdam Island

A 2-year record of atmospheric mercury species at a background Southern Hemisphere station on Amsterdam Island

Atmospheric Chemistry and Physics, 14, 11461-11473, 2014

Author(s): H. Angot, M. Barret, O. Magand, M. Ramonet, and A. Dommergue

Although essential to fully understand the cycling of mercury at the global scale, mercury species records in the Southern Hemisphere are scarce. Under the framework of the Global Mercury Observation System (GMOS) project, a monitoring station has been set up on Amsterdam Island (37°48´ S, 77°34´ E) in the remote southern Indian Ocean. For the first time in the Southern Hemisphere, a 2-year record of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particle-bound mercury (PBM) is presented. GEM concentrations were remarkably steady (1.03 ± 0.08 ng m?3) while RGM and PBM concentrations were very low and exhibited a strong variability (mean: 0.34 pg m?3, range: < detection limit–4.07 pg m?3; and mean: 0.67 pg m?3, range: < detection limit–12.67 pg m?3, respectively). Despite the remoteness of the island, wind sector analysis, air mass back trajectories and the observation of radonic storms highlighted a long-range contribution from the southern African continent to the GEM and PBM budgets from July to September during the biomass burning season. Low concentrations of GEM were associated with southerly polar and marine air masses from the remote southern Indian Ocean. This unique data set provides new baseline GEM concentrations in the Southern Hemisphere midlatitudes while mercury speciation along with upcoming wet deposition data will help to improve our understanding of the mercury cycle in the marine boundary layer.

Posted on 30 October 2014 | 12:00 am


Increase in elemental carbon values between 1970 and 2004 observed in a 300-year ice core from Holtedahlfonna (Svalbard)

Increase in elemental carbon values between 1970 and 2004 observed in a 300-year ice core from Holtedahlfonna (Svalbard)

Atmospheric Chemistry and Physics, 14, 11447-11460, 2014

Author(s): M. M. Ruppel, I. Isaksson, J. Ström, E. Beaudon, J. Svensson, C. A. Pedersen, and A. Korhola

Black carbon (BC) is a light-absorbing particle that warms the atmosphere–Earth system. The climate effects of BC are amplified in the Arctic, where its deposition on light surfaces decreases the albedo and causes earlier melt of snow and ice. Despite its suggested significant role in Arctic climate warming, there is little information on BC concentrations and deposition in the past. Here we present results on BC (here operationally defined as elemental carbon (EC)) concentrations and deposition on a Svalbard glacier between 1700 and 2004. The inner part of a 125 m deep ice core from Holtedahlfonna glacier (79°8' N, 13°16' E, 1150 m a.s.l.) was melted, filtered through a quartz fibre filter and analysed for EC using a thermal–optical method. The EC values started to increase after 1850 and peaked around 1910, similar to ice core records from Greenland. Strikingly, the EC values again increase rapidly between 1970 and 2004 after a temporary low point around 1970, reaching unprecedented values in the 1990s. This rise is not seen in Greenland ice cores, and it seems to contradict atmospheric BC measurements indicating generally decreasing atmospheric BC concentrations since 1989 in the Arctic. For example, changes in scavenging efficiencies, post-depositional processes and differences in the vertical distribution of BC in the atmosphere are discussed for the differences between the Svalbard and Greenland ice core records, as well as the ice core and atmospheric measurements in Svalbard. In addition, the divergent BC trends between Greenland and Svalbard ice cores may be caused by differences in the analytical methods used, including the operational definitions of quantified particles, and detection efficiencies of different-sized BC particles. Regardless of the cause of the increasing EC values between 1970 and 2004, the results have significant implications for the past radiative energy balance at the coring site.

Posted on 30 October 2014 | 12:00 am


Impact of the Asian monsoon anticyclone on the variability of mid-to-upper tropospheric methane above the Mediterranean Basin

Impact of the Asian monsoon anticyclone on the variability of mid-to-upper tropospheric methane above the Mediterranean Basin

Atmospheric Chemistry and Physics, 14, 11427-11446, 2014

Author(s): P. Ricaud, B. Si?, L. El Amraoui, J.-L. Attié, R. Zbinden, P. Huszar, S. Szopa, J. Parmentier, N. Jaidan, M. Michou, R. Abida, F. Carminati, D. Hauglustaine, T. August, J. Warner, R. Imasu, N. Saitoh, and V.-H. Peuch

The space and time variabilities of methane (CH4) total column and upper tropospheric mixing ratios are analysed above the Mediterranean Basin (MB) as part of the Chemical and Aerosol Mediterranean Experiment (ChArMEx) programme. Since the analysis of the mid-to-upper tropospheric CH4 distribution from spaceborne sensors and model outputs is challenging, we have adopted a climatological approach and have used a wide variety of data sets. We have combined spaceborne measurements from the Thermal And Near infrared Sensor for carbon Observations – Fourier Transform Spectrometer (TANSO-FTS) instrument on the Greenhouse gases Observing SATellite (GOSAT) satellite, the Atmospheric InfraRed Spectrometer (AIRS) on the AURA platform and the Infrared Atmospheric Sounder Interferometer (IASI) instrument aboard the MetOp-A platform with model results from the Chemical Transport Model (CTM) MOCAGE, and the Chemical Climate Models (CCMs) CNRM-AOCCM and LMDz-OR-INCA (according to different emission scenarios). In order to minimize systematic errors in the spaceborne measurements, we have only considered maritime pixels over the MB. The period of interest spans from 2008 to 2011 considering satellite and MOCAGE data and, regarding the CCMs, from 2001 to 2010. Although CH4 is a long-lived tracer with lifetime of ~12 years and is supposed to be well mixed in the troposphere, an east–west gradient in CH4 is observed and modelled in the mid-to-upper troposphere with a maximum in the Western MB in all seasons except in summer when CH4 accumulates above the Eastern MB. The peak-to-peak amplitude of the east–west seasonal variation in CH4 above the MB in the upper troposphere (300 hPa) is weak but almost twice as great in the satellite measurements (~25 ppbv) as in the model data (~15 ppbv). The maximum of CH4 in summer above the eastern MB can be explained by a series of dynamical processes only occurring in summer. The Asian monsoon traps and uplifts high amounts of CH4 to the upper troposphere where they build up. The Asian Monsoon Anticyclone redistributes these elevated CH4 amounts towards North Africa and the Middle East to finally reach and descend in the eastern MB. In the lower troposphere, the CH4 variability is mainly driven by the local sources of emission in the vicinity of the MB.

Posted on 29 October 2014 | 12:00 am


The effect of low solubility organic acids on the hygroscopicity of sodium halide aerosols

The effect of low solubility organic acids on the hygroscopicity of sodium halide aerosols

Atmospheric Chemistry and Physics, 14, 11409-11425, 2014

Author(s): L. Miñambres, E. Méndez, M. N. Sánchez, F. Castaño, and F. J. Basterretxea

In order to accurately assess the influence of fatty acids on the hygroscopic and other physicochemical properties of sea salt aerosols, hexanoic, octanoic or lauric acid together with sodium halide salts (NaCl, NaBr and NaI) have been chosen to be investigated in this study. The hygroscopic properties of sodium halide sub-micrometre particles covered with organic acids have been examined by Fourier-transform infrared spectroscopy in an aerosol flow cell. Covered particles were generated by flowing atomized sodium halide particles (either dry or aqueous) through a heated oven containing the gaseous acid. The obtained results indicate that gaseous organic acids easily nucleate onto dry and aqueous sodium halide particles. On the other hand, scanning electron microscopy (SEM) images indicate that lauric acid coating on NaCl particles makes them to aggregate in small clusters. The hygroscopic behaviour of covered sodium halide particles in deliquescence mode shows different features with the exchange of the halide ion, whereas the organic surfactant has little effect in NaBr particles, NaCl and NaI covered particles experience appreciable shifts in their deliquescence relative humidities, with different trends observed for each of the acids studied. In efflorescence mode, the overall effect of the organic covering is to retard the loss of water in the particles. It has been observed that the presence of gaseous water in heterogeneously nucleated particles tends to displace the cover of hexanoic acid to energetically stabilize the system.

Posted on 29 October 2014 | 12:00 am


Constraining mass–diameter relations from hydrometeor images and cloud radar reflectivities in tropical continental and oceanic convective anvils

Constraining mass–diameter relations from hydrometeor images and cloud radar reflectivities in tropical continental and oceanic convective anvils

Atmospheric Chemistry and Physics, 14, 11367-11392, 2014

Author(s): E. Fontaine, A. Schwarzenboeck, J. Delanoë, W. Wobrock, D. Leroy, R. Dupuy, C. Gourbeyre, and A. Protat

In this study the density of ice hydrometeors in tropical clouds is derived from a combined analysis of particle images from 2-D-array probes and associated reflectivities measured with a Doppler cloud radar on the same research aircraft. Usually, the mass–diameter m(D) relationship is formulated as a power law with two unknown coefficients (pre-factor, exponent) that need to be constrained from complementary information on hydrometeors, where absolute ice density measurement methods do not apply. Here, at first an extended theoretical study of numerous hydrometeor shapes simulated in 3-D and arbitrarily projected on a 2-D plan allowed to constrain the exponent ?of the m(D) relationship from the exponent σ of the surface–diameterS(D)relationship, which is likewise written as a power law. Since S(D) always can be determined for real data from 2-D optical array probes or other particle imagers, the evolution of the m(D) exponent can be calculated. After that, the pre-factor ? of m(D) is constrained from theoretical simulations of the radar reflectivities matching the measured reflectivities along the aircraft trajectory.

The study was performed as part of the Megha-Tropiques satellite project, where two types of mesoscale convective systems (MCS) were investigated: (i) above the African continent and (ii) above the Indian Ocean. For the two data sets, two parameterizations are derived to calculate the vertical variability of m(D) coefficients ? and ? as a function of the temperature. Originally calculated (with T-matrix) and also subsequently parameterized m(D) relationships from this study are compared to other methods (from literature) of calculating m(D) in tropical convection. The significant benefit of using variable m(D) relations instead of a single m(D) relationship is demonstrated from the impact of all these m(D) relations on Z-CWC (Condensed Water Content) and Z-CWC-T-fitted parameterizations.

Posted on 29 October 2014 | 12:00 am


New emission factors for Australian vegetation fires measured using open-path Fourier transform infrared spectroscopy – Part 2: Australian tropical savanna fires

New emission factors for Australian vegetation fires measured using open-path Fourier transform infrared spectroscopy – Part 2: Australian tropical savanna fires

Atmospheric Chemistry and Physics, 14, 11335-11352, 2014

Author(s): T. E. L. Smith, C. Paton-Walsh, C. P. Meyer, G. D. Cook, S. W. Maier, J. Russell-Smith, M. J. Wooster, and C. P. Yates

Savanna fires contribute approximately 40–50% of total global annual biomass burning carbon emissions. Recent comparisons of emission factors from different savanna regions have highlighted the need for a regional approach to emission factor development, and better assessment of the drivers of the temporal and spatial variation in emission factors. This paper describes the results of open-path Fourier transform infrared (OP-FTIR) spectroscopic field measurements at 21 fires occurring in the tropical savannas of the Northern~Territory, Australia, within different vegetation assemblages and at different stages of the dry season. Spectra of infrared light passing through a long (22–70 m) open-path through ground-level smoke released from these fires were collected using an infrared lamp and a field-portable FTIR system. The IR spectra were used to retrieve the mole fractions of 14 different gases present within the smoke, and these measurements used to calculate the emission ratios and emission factors of the various gases emitted by the burning. Only a handful of previous emission factor measures are available specifically for the tropical savannas of Australia and here we present the first reported emission factors for methanol, acetic acid, and formic acid for this biome. Given the relatively large sample size, it was possible to study the potential causes of the within-biome variation of the derived emission factors. We find that the emission factors vary substantially between different savanna vegetation assemblages; with a majority of this variation being mirrored by variations in the modified combustion efficiency (MCE) of different vegetation classes. We conclude that a significant majority of the variation in the emission factor for trace gases can be explained by MCE, irrespective of vegetation class, as illustrated by variations in the calculated methane emission factor for different vegetation classes using data sub-set by different combustion efficiencies. Therefore, the selection of emission factors for emissions modelling purposes need not necessarily require detailed fuel type information, if data on MCE (e.g. from future spaceborne total column measurements) or a correlated variable were available.

From measurements at 21 fires, we recommend the following emission factors for Australian tropical savanna fires (in grams of gas emitted per kilogram of dry fuel burned), which are our mean measured values: 1674 ± 56 g kg?1 of carbon dioxide; 87 ± 33 g kg?1 of carbon monoxide; 2.1 ± 1.2 g kg?1 of methane; 0.11 ± 0.04 g kg?1 of acetylene; 0.49 ± 0.22 g kg?1 of ethylene; 0.08 ± 0.05 g kg?1 of ethane; 1.57 ± 0.44 g kg?1 of formaldehyde; 1.06 ± 0.87 g kg?1 of methanol; 1.54 ± 0.64 g kg?1 of acetic acid; 0.16 ± 0.07 g kg?1 of formic acid; 0.53 ± 0.31 g kg?1 of hydrogen cyanide; and 0.70 ± 0.36 g kg?1 of ammonia. In a companion paper, similar techniques are used to characterise the emissions from Australian temperate forest fires.

Posted on 29 October 2014 | 12:00 am


Impacts of new particle formation on aerosol cloud condensation nuclei (CCN) activity in Shanghai: case study

Impacts of new particle formation on aerosol cloud condensation nuclei (CCN) activity in Shanghai: case study

Atmospheric Chemistry and Physics, 14, 11353-11365, 2014

Author(s): C. Leng, Q. Zhang, J. Tao, H. Zhang, D. Zhang, C. Xu, X. Li, L. Kong, T. Cheng, R. Zhang, X. Yang, J. Chen, L. Qiao, S. Lou, H. Wang, and C. Chen

New particle formation (NPF) events and their impacts on cloud condensation nuclei (CCN) were investigated using continuous measurements collected in urban Shanghai from 1 to 30 April 2012. During the campaign, NPF occurred in 8 out of the 30 days and enhanced CCN number concentration (NCCN) by a factor of 1.2–1.8, depending on supersaturation (SS). The NPF event on 3 April 2012 was chosen as an example to investigate the NPF influence on CCN activity. In this NPF event, secondary aerosols were produced continuously and increased PM2.5 mass concentration at a rate of 4.33 μg cm−3 h−1, and the growth rate (GR) and formation rate (FR) were on average 5 nm h?1 and 0.36 cm?3 s?1, respectively. The newly formed particles grew quickly from nucleation mode (10–20 nm) into CCN size range. NCCN increased rapidly at SS of 0.4–1.0% but weakly at SS of 0.2%. Correspondingly, aerosol CCN activities (fractions of activated aerosol particles in total aerosols, NCCN/NCN) were significantly enhanced from 0.24–0.60 to 0.30–0.91 at SS of 0.2–1.0% due to the NPF. On the basis of the κ-Köhler theory, aerosol size distributions and chemical composition measured simultaneously were used to predict NCCN. There was a good agreement between the predicted and measured NCCN (R2=0.96, Npredicted/Nmeasured=1.04). This study reveals that NPF exerts large impacts on aerosol particle abundance and size spectra; thus, it significantly promotes NCCN and aerosol CCN activity in this urban environment. The GR of NPF is the key factor controlling the newly formed particles to become CCN at all SS levels, whereas the FR is an effective factor only under high SS (e.g., 1.0%) conditions.

Posted on 29 October 2014 | 12:00 am


Airborne observations of IEPOX-derived isoprene SOA in the Amazon during SAMBBA

Airborne observations of IEPOX-derived isoprene SOA in the Amazon during SAMBBA

Atmospheric Chemistry and Physics, 14, 11393-11407, 2014

Author(s): J. D. Allan, W. T. Morgan, E. Darbyshire, M. J. Flynn, P. I. Williams, D. E. Oram, P. Artaxo, J. Brito, J. D. Lee, and H. Coe

Isoprene is a potentially highly significant but currently poorly quantified source of secondary organic aerosols (SOA). This is especially important in the tropics, where large rainforests act as significant sources of isoprene. Methylfuran, produced through thermal decomposition during analysis, has recently been suggested as a marker for isoprene SOA formation through the isoprene epoxydiol (IEPOX) route, which mostly occurs under low NOx conditions. This is manifested as a peak at m/z=82 in Aerodyne Aerosol Mass Spectrometer (AMS) data. Here we present a study of this marker measured during five flights over the Amazon rainforest on board the UK Facility for Airborne Atmospheric Measurement (FAAM) BAe-146 research aircraft during the South American Biomass Burning Analysis (SAMBBA) campaign. Cases where this marker is and is not present are contrasted and linked to the presence of acidic seed particles, lower NOx concentrations and higher humidities. There are also data to suggest a role of organic nitrogen in the particulate composition. Furthermore, an inspection of the vertical trends of the marker indicates that concentrations are highest at the top of the boundary layer (possibly due to semivolatile repartitioning) and that upwards through the free troposphere, the mass spectral profile evolves towards that of low volatility oxygenated aerosol. These observations offer insights into the behaviour of IEPOX-derived SOA formation above the Amazon rainforest and the suitability of methylfuran as a marker for this process.

Posted on 29 October 2014 | 12:00 am


New emission factors for Australian vegetation fires measured using open-path Fourier transform infrared spectroscopy – Part 1: Methods and Australian temperate forest fires

New emission factors for Australian vegetation fires measured using open-path Fourier transform infrared spectroscopy – Part 1: Methods and Australian temperate forest fires

Atmospheric Chemistry and Physics, 14, 11313-11333, 2014

Author(s): C. Paton-Walsh, T. E. L. Smith, E. L. Young, D. W. T. Griffith, and É.-A. Guérette

Biomass burning releases trace gases and aerosol particles that significantly affect the composition and chemistry of the atmosphere. Australia contributes approximately 8% of gross global carbon emissions from biomass burning, yet there are few previous measurements of emissions from Australian forest fires available in the literature. This paper describes the results of field measurements of trace gases emitted during hazard reduction burns in Australian temperate forests using open-path Fourier transform infrared spectroscopy. In a companion paper, similar techniques are used to characterise the emissions from hazard reduction burns in the savanna regions of the Northern Territory. Details of the experimental methods are explained, including both the measurement set-up and the analysis techniques employed. The advantages and disadvantages of different ways to estimate whole-fire emission factors are discussed and a measurement uncertainty budget is developed.

Emission factors for Australian temperate forest fires are measured locally for the first time for many trace gases. Where ecosystem-relevant data are required, we recommend the following emission factors for Australian temperate forest fires (in grams of gas emitted per kilogram of dry fuel burned) which are our mean measured values: 1620 ± 160 g kg?1 of carbon dioxide; 120 ± 20 g kg?1 of carbon monoxide; 3.6 ± 1.1 g kg?1 of methane; 1.3 ± 0.3 g kg?1 of ethylene; 1.7 ± 0.4 g kg?1 of formaldehyde; 2.4 ± 1.2 g kg?1 of methanol; 3.8 ± 1.3 g kg?1 of acetic acid; 0.4 ± 0.2 g kg?1 of formic acid; 1.6 ± 0.6 g kg?1 of ammonia; 0.15 ± 0.09 g kg?1 of nitrous oxide and 0.5 ± 0.2 g kg?1 of ethane.

Posted on 29 October 2014 | 12:00 am





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Chronological list of recent articles on Chemistry, Atmospheric Chemistry, Atmospheric Chemistry and Physics.
Update:
28.09.2013


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