bluuebunny/crossref_metadata_2025 huggingface dataset:
Vertical transport rates and concentrations of OH and Cl radicals in the Tropical Tropopause Layer from Observations of CO2 and halocarbons: implications for distributions of long- and short-lived chemical species
Authors: Park S. , Atlas E. L. , Jiménez R. , Daube B. C. , Gottlieb E. W. , Nan J. , Jones D. B. A. , Pfister L. , Conway T. J. , Bui T. P. , Gao R.-S. , Wofsy S. C.
Published: April 2010

Abstract. Rates for large-scale vertical transport of air in the Tropical Tropopause Layer (TTL) were determined using high-resolution, in situ observations of CO2 concentrations in the tropical upper troposphere and lower stratosphere during the NASA Tropical Composition, Cloud and Climate Coupling (TC4) campaign in August 2007. Upward movement of trace gases in the deep tropics was notably slower in TC4 than during the Costa Rica AURA Validation Experiment (CR-AVE), in January 2006. Transport rates in the TTL were combined with in situ measurements of chlorinated and brominated organic compounds from whole air samples to determine chemical loss rates for reactive chemical species, providing empirical vertical profiles for 24-h mean concentrations of hydroxyl radicals (OH) and chlorine atoms in the TTL. The analysis shows that important short-lived species such as CHCl3, CH2Cl2, and CH2Br2 have longer chemical lifetimes than the time for transit of the TTL, implying that these species, which are not included in most models, could readily reach the stratosphere and make significant contributions of chlorine and/or bromine to stratospheric loading.
A global climatology of the mesospheric sodium layer from GOMOS data during the 2002–2008 period
Authors: Fussen D. , Vanhellemont F. , Tétard C. , Mateshvili N. , Dekemper E. , Loodts N. , Bingen C. , Kyrölä E. , Tamminen J. , Sofieva V. , Hauchecorne A. , Dalaudier F. , Bertaux J.-L. , Barrot G. , Blanot L. , Fanton d'Andon O. , Fehr T. , Saavedra L. , Yuan T. , She C.-Y.
Published: April 2010

Abstract. This paper presents a climatology of the mesospheric sodium layer built from the processing of 7 years of GOMOS data. With respect to preliminary results already published for the year 2003, a more careful analysis was applied to the averaging of occultations inside the climatological bins (10° in latitude-1 month). Also, the slant path absorption lines of the Na doublet around 589 nm shows evidence of partial saturation that was responsible for an underestimation of the Na concentration in our previous results. The sodium climatology has been validated with respect to the Fort Collins lidar measurements and, to a lesser extent, to the OSIRIS 2003–2004 data. Despite the important natural sodium variability, we have shown that the Na vertical column has a marked semi-annual oscillation at low latitudes that merges into an annual oscillation in the polar regions, a spatial distribution pattern that was unreported so far. The sodium layer seems to be clearly influenced by the mesospheric global circulation and the altitude of the layer shows clear signs of subsidence during polar winter. The climatology has been parameterized by time-latitude robust fits to allow for easy use. Taking into account the non-linearity of the transmittance due to partial saturation, an experimental approach is proposed to derive mesospheric temperatures from limb remote sounding measurements.
Mineral dust effects on clouds and rainfall in the West African Sahel
Authors: Klüser L. , Holzer-Popp T.
Published: April 2010

Abstract. Aerosol cloud interactions are known to be of great importance to many parts of the climate system. Five years of observations from three different satellites (NASA EOS Aqua, Meteosat Second Generation and the Tropical Rainfall Measuring Mission) are used to statistically analyse the influence of mineral dust aerosol, separated from other aerosol species, on monsoon season cloudiness and precipitation in the West African Sahel domain. The aerosol-cloud-interactions were analysed separately by season and flow (air mass) in order to exclude spurious correlations with meteorological conditions. As expected from theory and previous case studies a reduction of precipitation due to reduced droplet sizes and suppression of convective activity under the influence of dust aerosol is evident from the analysis of this multiple year dataset. These results thus support the theory of a positive desertification feedback loop of mineral dust aerosol from a large-scale dataset.
Size distributions of dicarboxylic acids, ketoacids, α-dicarbonyls, sugars, WSOC, OC, EC and inorganic ions in atmospheric particles over Northern Japan: implication for long-range transport of Siberian biomass burning and East Asian polluted aerosols
Authors: Agarwal S. , Aggarwal S. G. , Okuzawa K. , Kawamura K.
Published: April 2010

Abstract. To better understand the size-segregated chemical composition of aged organic aerosols in the western Pacific rim, day- and night-time aerosol samples were collected in Sapporo, Japan during summer 2005 using Andersen impactor sampler with 5 size bins: <1.1, 1.1–2.0, 2.0–3.3, 3.3–7.0, >7.0 μm. Samples were analyzed for the molecular compositions of dicarboxylic acids, ketoacids, α-dicarbonyls, and sugars, together with water-soluble organic carbon (WSOC), organic carbon (OC), elemental carbon (EC) and inorganic ions. Based on the analyses of backward trajectory and chemical tracers, we found that during campaign, the air masses were arrived from Siberia (biomass burning source region) on 8–9 August, China (anthropogenic source region) on 9–10 August and from the East China Sea/Sea of Japan (a mixed source receptor region) on 10–11 August. Most of the diacids, ketoacids, dicarbonyls, levoglucosan, WSOC, and inorganic ions, i.e., SO42−, NH42+ and K+ were enriched in fine particles (PM1.1) whereas Ca2+, Mg2+ and Cl− peaked in coarse sizes (>1.1 μm). Interestingly, OC, most sugar compounds and NO4
Comparison of UV irradiances from Aura/Ozone Monitoring Instrument (OMI) with Brewer measurements at El Arenosillo (Spain) – Part 1: Analysis of parameter influence
Authors: Antón M. , Cachorro V. E. , Vilaplana J. M. , Toledano C. , Krotkov N. A. , Arola A. , Serrano A. , de la Morena B.
Published: April 2010

Abstract. The main objective of this study is to compare the erythemal UV irradiance (UVER) and spectral UV irradiances (at 305, 310 and 324 nm) from Ozone Monitoring Instrument (OMI) onboard NASA EOS/Aura polar sun-synchronous satellite (launched in July 2004, local equator crossing time 01:45 p.m.) with ground-based measurements from the Brewer spectroradiometer #150 located at El Arenosillo (South of Spain). The analyzed period comprises more than four years, from October 2004 to December 2008. The effects of several factors (clouds, aerosols, ozone and the solar elevation) on OMI-Brewer comparisons were analyzed. The proxies used for each factor were: OMI Lambertian Equivalent Reflectivity (LER) at 360 nm (clouds), the Aerosol Optical Depth (AOD) at 440 nm measured from the ground-based Cimel sun-photometer (http://aeronet.gsfc.nasa.gov), OMI total column ozone, and solar elevation at OMI overpass time. The comparison for all sky conditions reveals positive biases (OMI higher than Brewer) 12.3% for UVER, 14.2% for UV irradiance at 305 nm, 10.6% for 310 nm and 8.7% for 324 nm. The OMI-Brewer Root Mean Square Error (RMSE) is reduced when cloudy cases are removed from the analysis, (e.g., RMSE ~20% for all sky conditions and RMSE smaller than 10% for cloud-free conditions). However, the biases remain and even become more significant for the cloud-free cases with respect to all sky conditions. The mentioned overestimation is clearly documented as due to aerosol extinction influence. The differences OMI-Brewer typically decrease with increasing the Solar Zenith Angle (SZA). The seasonal dependence of the OMI-Brewer difference for cloud-free conditions is driven by aerosol climatology. To account for the aerosol effect, a first evaluation in order to compare with previous TOMS results (Anton et al., 2007) was performed. This comparison shows that the OMI bias is between +14% and +19% for UVER and spectral UV irradiances for moderately-high aerosol load (AOD>0.25). The OMI bias is decreased by a factor of 2 (the typical bias varies from +8% to +12%) under cloud-free and low aerosol load conditions (AOD<0.1). More detailed analysis of absorbing aerosols influence on OMI bias at our station is presented in a companion paper (Cachorro et al., 2010).
Isotope modeling of nitric acid formation in the atmosphere using ISO-RACM: testing the importance of NO oxidation, heterogeneous reactions, and trace gas chemistry
Authors: Michalski G. , Xu F.
Published: April 2010

Abstract. Here we present ISO-RACM, an isotope mass balance model that utilizes the Regional Atmospheric Chemistry Mechanism to predict Δ17O values in atmospheric nitrate. A large number of simulations were carried out that varied atmospheric parameters that are important in altering the magnitude and range of Δ17O values generated in photochemically produce nitrate. These parameters included temperature, relative humidity, actinic flux, aerosol surface area and chemical speciation, and three different N2O5 uptake parameterizations. Trace gas mixing ratios were also varied including CH4, CO, NOx, O3, volatile organic compounds and biogenic organic compounds. The model predicts that there are seasonal, latitudinal and diurnal variations in Δ17O values due to changes in actinic flux with lower values corresponding to higher actinic fluxes. There was also a minor positive correlation between higher Δ17O values and increased temperature. There were distinct differences in Δ17O depending on which N2O5 parameterization was used, mostly the result of changing relative humidity being a factor in two of the parameterization schemes. Changing CO and CH4 mixing ratios had negligible impact on Δ17O values but significant variation in magnitude and range were predicted with NOx, O3, and organic loading. High NOx and O3 generated high Δ17O with a narrow (10 ‰) range, while high organics led to low Δ17O values and a wider range of possible values. Implications for using Δ17O to evaluate NOx-NOy chemistry and aerosol formation processes are discussed, as is needed future research.
Dicarboxylic acids, metals and isotopic compositions of C and N in atmospheric aerosols from inland China: implications for dust and coal burning emission and secondary aerosol formation
Authors: Wang G. , Xie M. , Hu S. , Tachibana E. , Kawamura K.
Published: April 2010

Abstract. Dicarboxylic acids (C2-C10), metals, elemental carbon (EC), organic carbon (OC), and stable isotopic compositions of total carbon (TC) and total nitrogen (TN) were determined for PM10 samples collected at three urban and one suburban sites of Baoji, an inland city of China, during winter and spring 2008. Oxalic acid (C2) was the dominant diacid, followed by succinic (C4) and malonic (C3) acids. Total diacids in the urban and suburban areas are 1546±203 and 1728±495 ng m−3 during winter and 1236±335 and 1028±193 ng m−3 during spring. EC in the urban and the suburban atmospheres are 17±3.8 and 8.0±2.1 μg m−3 during winter and 20±5.9 and 7.1±2.7 μg m−3 during spring whereas OC at the urban and suburban sites are 74±14 and 51±7.9 μg m−3 in winter and 51±20 and 23±6.1 μg m−3 in spring. Secondary organic carbon (SOC) accounted for 38±16% of OC in winter and 28±18% of OC in spring, suggesting an enhanced photochemical production of secondary organic aerosols in winter under an inversion layer development. Total metal elements in winter and spring are 34±10 and 61±27 μg m−3 in the urban air and 18±7 and 32±23 μg m−3 in the suburban air. A linear correlation (r2>0.8 in winter and r2>0.6 in spring) was found between primary organic carbon (POC) and Ca2+/Fe, together with a strong dependence of pH value on water-soluble inorganic carbon, suggesting fugitive dust as a major source of the airborne particles. Polycyclic aromatic hydrocarbons (PAHs), sulfate, and Pb in the samples well correlated each other (r2>0.6) in winter samples, suggesting an importance of emissions from coal burning for house heating. Stable carbon isotope compositions of TC (δ13C) became higher with an increase in the concentration ratios of C2/OC due to aerosol aging. In contrast, nitrogen isotope compositions of TN (δ15N) became lower with an increases in the mass ratios of NH4+/PM10 and NO3-/PM10 due to an enhanced adsorption and/or condensation of NH3 and HNO3 from gas phase onto solid phase.
Characteristics of size distributions at urban and rural locations in New York
Authors: Bae M.-S. , Schwab J. J. , Hogrefe O. , Frank B. P. , Lala G. G. , Demerjian K. L.
Published: April 2010

Abstract. Paired nano- and long-tube Scanning Mobility Particle Sizer (SMPS) systems were operated for four different intensive field campaigns in New York State. Two of these campaigns were at Queens College in New York City, during the summer of 2001 and the winter of 2004. The other field campaigns were at rural sites in New York State. The data with the computed diffusion loss corrections for the sampling lines and the SMPS instruments were examined and the combined SMPS data sets for each campaign were obtained. The diffusion corrections significantly affect total number concentrations, and in New York City, affect the mode structure of the size distributions. The relationship between merged and integrated SMPS total number concentrations with the diffusion loss corrections and the CPC number concentrations yield statistically significant increases (closer to 1) in the slope and correlation coefficient compared to the uncorrected values. The measurements are compared to PM2.5 mass concentrations and ion balance indications of aerosol acidity. Periods of low observed PM2.5 mass, high number concentration, and low median diameter due to small fresh particles are associated with primary emissions for the urban sites; and with particle nucleation and growth for the rural sites. The observations of high PM2.5 mass, lower number concentrations, and higher median diameter are mainly due to an enhancement of coagulation and/or condensation processes in relatively aged air. There are statistically different values for the condensation sink (CS) between urban and rural areas. While there is good association (r2>0.5) between the condensation sink (CS) in the range of 8.35–283.9 nm and PM2.5 mass in the urban areas, there is no discernable association in the rural areas. The average (±standard deviation) of CS lies in the range 6.5(±3.3)×10−3–2.4(±0.9)×10−2.
Observations of OH and HO2 radicals over West Africa
Authors: Commane R. , Floquet C. F. A. , Ingham T. , Stone D. , Evans M. J. , Heard D. E.
Published: April 2010

Abstract. The hydroxyl radical (OH) plays a key role in the oxidation of trace gases in the troposphere. However, observations of OH and the closely related hydroperoxy radical (HO2) have been sparse, especially in the tropics. Based on a low-pressure laser-induced fluorescence technique (FAGE – Fluorescence Assay by Gas Expansion), an instrument has been developed to measure OH and HO2 aboard the Facility for Airborne Atmospheric Measurement (FAAM) BAe-146 research aircraft. The instrument is described and the calibration method is discussed. During the African Monsoon Multidisciplinary Analyses (AMMA) campaign, observations of OH and HO2 (HOx) were made in the boundary layer and free troposphere over West Africa on 13 flights during July and August 2006. Mixing ratios of both OH and HO2 were found to be highly variable but followed a diurnal cycle, with a median HO2/OH ratio of 95. Daytime OH observations were compared with the primary production rate of OH from ozone photolysis in the presence of water vapour. Daytime HO2 observations were generally reproduced by a simple steady-state HOx calculation, where HOx was assumed to be formed from the primary production of OH and lost through HO2 self-reaction. Deviations between the observations and this simple model were found to be grouped into a number of specific cases: (a) in the presence of high levels of isoprene in the boundary layer, (b) within a biomass burning plume and (c) within cloud. In the forested boundary layer, HO2 was underestimated at altitudes below 500 m but overestimated between 500 m and 2 km. In the biomass burning plume, OH and HO2 were both significantly reduced compared to calculations. HO2 was sampled in and around cloud, with significant short-lived reductions of HO2 observed. HO2 observations were better reproduced by a steady state calculation with heterogeneous loss of HO2 onto cloud droplets included. Up to 9 pptv of HO2 was observed at night, increasing early in the morning. Potential sources of high altitude HO2 at night are also discussed.
Geoengineering by stratospheric SO2 injection: results from the Met Office HadGEM2 climate model and comparison with the Goddard Institute for Space Studies ModelE
Authors: Jones A. , Haywood J. , Boucher O. , Kravitz B. , Robock A.
Published: April 2010

Abstract. We examine the response of the Met Office Hadley Centre's HadGEM2-AO climate model to simulated geoengineering by continuous injection of SO2 into the lower stratosphere, and compare the results with those from the Goddard Institute for Space Studies ModelE. The HadGEM2 simulations suggest that the SO2 injection rate considered here (5 Tg[SO2] yr−1) could defer the amount of global warming predicted under the Intergovernmental Panel on Climate Change's A1B scenario by approximately 30–35 years, although both models indicate rapid warming if geoengineering is not sustained. We find a broadly similar geographic distribution of the response to geoengineering in both models in terms of near-surface air temperature and mean June–August precipitation. The simulations also suggest that significant changes in regional climate would be experienced even if geoengineering was successful in maintaining global-mean temperature near current values.
Why anisotropic turbulence matters: another reply
Authors: Lovejoy S. , Schertzer D. , Tuck A. F.
Published: April 2010

Abstract. We continue the debate on anisotropic but scaling turbulence and its effect on aircraft measurements of turbulence (cf. Lindborg et al., 2010a, b); hereafter LTNCG1, LTNCG2). We revisit the repeatedly presented back-of-the-envelope calculation and discuss wind statistics on real isobars. We then discuss theoretical and empirical evidence that a k−5/3 horizontal wind spectrum could extend out to planetary scales.
The influence of biomass burning on tropospheric composition over the tropical Atlantic Ocean and Equatorial Africa during the West African monsoon in 2006
Authors: Williams J. E. , Scheele M. P. , van Velthoven P. F. J. , Thouret V. , Saunois M. , Reeves C. E. , Cammas J.-P.
Published: April 2010

Abstract. We have performed simulations using a 3-D global chemistry-transport model (TM4_AMMA) to investigate the effect that continental transport of biomass burning plumes have on regional air quality over Equatorial Africa during the West African Monsoon (WAM) period in 2006. By performing a number of sensitivity studies we show that biomass burning emissions from southern Africa (0–40° S) have a strong influence on the composition of the tropical troposphere around Equatorial Africa and the outflow regions towards the west, especially between 10° S–10° N. By altering both the temporal distribution and the injection heights used for introducing the biomass burning emissions we show that changes in temporal distribution are much more important in determining the daily variability of trace gas species over the southern Atlantic than boundary layer processes. When adopting the GFEDv2 emission inventory the maximum concentrations in CO and O3 occur between 0–5° S, which coincides with the position of the southern African Easterly Jet. By comparing co-located model output with in-situ measurements made during the AMMA measurement campaign we show that the model fails to capture the tropospheric profile of CO in the burning region, as well as the "extreme" concentrations of both CO and O3 seen around 600–700 hPa above Equatorial Africa. Trajectory analysis show that the 6-hourly ECMWF meteorological fields do not allow transport of biomass burning plumes from southern Africa directly into the mid-troposphere around ~6° N. Similar trajectory simulations repeated using an updated meteorological dataset, which assimilates additional measurement data for the African region, shows markedly different origins for pollution events and reveals that the performance of the CTM is heavily constrained by the ECMWF operational analysis data which drives the model.
Longwave indirect effect of mineral dusts on ice clouds
Authors: Min Q. , Li R.
Published: April 2010

Abstract. In addition to microphysical changes in clouds, changes in nucleation processes of ice cloud due to aerosols would result in substantial changes in cloud top distribution as mildly supercooled clouds are glaciated through heterogonous nucleation processes. Measurements from multiple sensors on multiple observing platforms over the Atlantic Ocean show that the cloud effective temperature increases with mineral dust loading with a slope of +3.06 °C per unit AOD. The macrophysical changes in ice cloud top distributions as a consequence of mineral dust-cloud interaction exert a strong cooling effect (up to 16 w m−2) of thermal infrared radiation on cloud systems. Induced changes of ice particle size by mineral dusts influence cloud emissivity and play a minor role in modulating the outgoing longwave radiation for optically thin ice clouds. Such a strong cooling forcing of thermal infrared radiation would have significant impacts on cloud systems and subsequently on climate.
Impact of biomass burning on surface water quality in Southeast Asia through atmospheric deposition: eutrophication modeling
Authors: Sundarambal P. , Tkalich P. , Balasubramanian R.
Published: April 2010

Abstract. A numerical modeling approach is proposed for the assessment of the nutrient loading of coastal waters from atmospheric sources. The 3-D eutrophication model NEUTRO was enhanced to simulate the spatial distribution and temporal variations of nutrients, planktons and dissolved oxygen due to atmospheric nutrient loadings. It was found that nutrient loading from the atmospheric wet and dry deposition was remarkable during hazy days, the contribution being between 2 and 8 times that of non-hazy days; the smoke haze was due to biomass burning in the Southeast Asian region as discussed in a companion paper on field observations. Atmospheric nutrient loads during hazy days can lead to anthropogenic eutrophication and chemical contamination. The importance of regional smoke haze events in relation to non-hazy days to atmospheric nutrient deposition in terms of their biological responses in the coastal water of the Singapore region was investigated. The percentage increases of surface water nutrients due to atmospheric deposition during non-hazy and hazy days from seawater baseline were estimated. Model computations showed that atmospheric fluxes might account for up to 17–88% of total mass of nitrate nitrogen in the water column during hazy days and 4 to 24% during non-hazy days, which might be a relatively significant contribution into regional eutrophication. The results obtained from the modeling study could be used for a better understanding of the energy flow through the marine food web, exploring various possible scenarios concerning the atmospheric deposition of nutrients onto the coastal zone and studying their impacts on water quality.
An overview of the MILAGRO 2006 campaign: Mexico City emissions and their transport and transformation
Authors: Molina L. T. , Madronich S. , Gaffney J. S. , Apel E. , de Foy B. , Fast J. , Ferrare R. , Herndon S. , Jimenez J. L. , Lamb B. , Osornio-Vargas A. R. , Russell P. , Schauer J. J. , Stevens P. S. , Zavala M.
Published: April 2010

Abstract. MILAGRO (Megacity Initiative: Local And Global Research Observations) is an international collaborative project to examine the behavior and the export of atmospheric emissions from a megacity. The Mexico City Metropolitan Area (MCMA) – one of the world's largest megacities and North America's most populous city – was selected as the case study to characterize the sources, concentrations, transport, and transformation processes of the pollutants emitted to the MCMA atmosphere and to evaluate the regional and global impacts of these emissions. The findings of this study are relevant to the evolution and impacts of pollution from many other megacities. The measurement phase consisted of a month-long series of carefully coordinated observations of the chemistry and physics of the atmosphere in and near Mexico City during March 2006, using a wide range of instruments at ground sites, on aircraft and satellites, and enlisting over 450 scientists from 150 institutions in 30 countries. Three ground supersites were set up to examine the evolution of the primary emitted gases and fine particles. Additional platforms in or near Mexico City included mobile vans containing scientific laboratories and mobile and stationary upward-looking lidars. Seven instrumented research aircraft provided information about the atmosphere over a large region and at various altitudes. Satellite-based instruments peered down into the atmosphere, providing even larger geographical coverage. The overall campaign was complemented by meteorological forecasting and numerical simulations, satellite observations and surface networks. Together, these research observations have provided the most comprehensive characterization of the MCMA's urban and regional atmospheric composition that will take years to analyze and evaluate fully. In this paper we review over 120 papers resulting from the MILAGRO/INTEX-B Campaign that have been published or submitted, as well as relevant papers from the earlier MCMA-2003 campaign, with the aim of providing a road map for the scientific community interested in understanding the emissions from a megacity such as the MCMA and their impacts on air quality and climate. This paper describes the measurements performed during MILAGRO and the results obtained on MCMA's atmospheric meteorology and dynamics, emissions of gases and fine particles, sources and concentrations of volatile organic compounds, urban and regional photochemistry, ambient particulate matter, aerosol radiative properties, urban plume characterization, and health studies. A summary of key findings from the field study is presented.
Moisture effects on carbon and nitrogen emission from burning of wildland biomass
Authors: Chen L.-W. A. , Verburg P. , Shackelford A. , Zhu D. , Susfalk R. , Chow J. C. , Watson J. G.
Published: April 2010

Abstract. Carbon (C) and nitrogen (N) released from biomass burning have multiple effects on the Earth's biogeochemical cycle, climate change, and ecosystem. These effects depend on the relative abundances of C and N species emitted, which vary with fuel type and combustion conditions. This study systematically investigates the emission characteristics under different fuel moisture contents, through controlled burning experiments with biomass and soil collected from a typical alpine forest. Fuel moisture in general lowers combustion efficiency, shortens flaming phase, and introduces prolonged smoldering before ignition. It increases emission factors of incompletely oxidized C and N species, such as carbon monoxide (CO) and ammonia (NH3). Substantial particulate carbon and nitrogen (up to 4 times C in CO and 75% of N in NH3) were measured mainly from the pre-flame smoldering of fuels with high moisture contents; this process emits particles larger than soot agglomerates commonly observed in flaming smoke. Hydrogen (H)/C ratio and optical properties of particulate carbon from the high-moisture fuels show their resemblance to plant cellulous and brown carbon, respectively. These findings have implications for modeling biomass burning emission and impacts.
Vertical structure of Antarctic tropospheric ozone depletion events: characteristics and broader implications
Authors: Jones A. E. , Anderson P. S. , Wolff E. W. , Roscoe H. K. , Marshall G. J. , Richter A. , Brough N. , Colwell S. R.
Published: April 2010

Abstract. The majority of tropospheric ozone depletion event (ODE) studies have focussed on time-series measurements, with comparatively few studies of the vertical component. Those that exist have almost exclusively used free-flying balloon-borne ozonesondes and almost all have been conducted in the Arctic. Here we use measurements from two separate Antarctic field experiments to examine the vertical profile of ozone during Antarctic ODEs. We use tethersonde data to probe details in the lowest few hundred meters and find considerable structure in the profiles associated with complex atmospheric layering. The profiles were all measured at wind speeds less than 7 ms−1, and on each occasion the lowest inversion height lay between 10 m and 40 m. We also use data from a free-flying ozonesonde study to select events where ozone depletion was recorded at altitudes >1 km above ground level. Using ERA-40 meteorological charts, we find that on every occasion the high altitude depletion was preceded by an atmospheric low pressure system. An examination of limited published ozonesonde data from other Antarctic stations shows this to be a consistent feature. Given the link between BrO and ODEs, we also examine ground-based and satellite BrO measurements, and find a strong association between enhanced BrO and atmospheric low pressure systems. The results suggest that, in Antarctica, such depressions are responsible for driving high altitude ODEs and for generating the large-scale BrO clouds observed from satellites. In the Arctic, the prevailing meteorology differs from that in Antarctica, but we show that major low pressure systems in the Arctic, when they occur, can also generate BrO clouds. Such depressions thus appear to be fundamental when considering the broader influence of ODEs, particularly in Antarctica, such as halogen export and the radiative influence of ozone-depleted air masses.
Precipitation and cloud cellular structures in marine stratocumulus over the southeast pacific: model simulations
Authors: Wang H. , Feingold G. , Wood R. , Kazil J.
Published: April 2010

Abstract. Microphysical and meteorological controls on the formation of open and closed cellular structures in the Southeast Pacific are explored using model simulations based on aircraft observations during the VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx). The effectiveness of factors such as boundary-layer moisture and temperature perturbations, surface heat and moisture fluxes, large-scale vertical motion and solar heating in promoting drizzle and open cell formation for prescribed aerosol number concentrations is explored. For the case considered, drizzle and subsequent open cell formation over a broad region are more sensitive to the observed boundary-layer moisture and temperature perturbations (=0.9 g kg−1; −1 K) than to a five-fold decrease in aerosol number concentrations (150 vs. 30 mg−1). When embedding the perturbations in closed cells, local drizzle and pockets of open cells (POCs) formation respond faster to the aerosol reduction than to the moisture increase, but the latter generate stronger and more persistent drizzle. The local negative perturbation in temperature drives a mesoscale circulation that prevents local drizzle formation but promotes it in a remote area where lower-level horizontal transport of moisture is blocked and converges to enhance liquid water path. This represents a potential mechanism for POC formation in the Southeast Pacific stratocumulus region whereby the circulation is triggered by strong precipitation in adjacent broad regions of open cells. A simulation that attempts to mimic the influence of a coastally induced upsidence wave results in an increase in cloud water but this alone is insufficient to initiate drizzle. An increase of surface sensible heat flux is also effective in triggering local drizzle and POC formation. Both open and closed cells simulated with observed initial conditions exhibit distinct diurnal variations in cloud properties. A stratocumulus deck that breaks up due solely to solar heating can recover at night. Precipitation in the open-cell cases depletes the aerosol to the extent that cloud formation is significantly suppressed within one diurnal cycle. A replenishment rate of cloud condensation nuclei of 0.72 mg−1 h−1 is sufficient to maintain clouds and prevent the boundary layer from collapsing the following day, suggesting that some local and/or remote aerosol sources are necessary for POCs to be able to last for days.
Aerosol mass and black carbon concentrations, two year-round observations at NCO-P (5079 m, Southern Himalayas)
Authors: Marinoni A. , Cristofanelli P. , Laj P. , Duchi R. , Calzolari F. , Decesari S. , Sellegri K. , Vuillermoz E. , Verza G. P. , Villani P. , Bonasoni P.
Published: April 2010

Abstract. Aerosol mass and the absorbing fraction are important variables, needed to constrain the role of atmospheric particles in the Earth radiation budget, both directly and indirectly through CCN activation. In particular, their monitoring in remote areas and mountain sites is essential for determining source regions, elucidating the mechanisms of long range transport of anthropogenic pollutants, and validating regional and global models. Since March 2006, aerosol mass and black carbon concentration have been monitored at the Nepal Climate Observatory-Pyramid, a permanent high-altitude research station located in the Khumbu valley at 5079 m a.s.l. below Mt. Everest. The first two-year averages of PM1 and PM1-10 mass were 1.94 μg m−3 and 1.88 μg m−3, with standard deviations of 3.90 μg m−3 and 4.45 μg m−3, respectively, while the black carbon concentration average is 160.5 ng m−3, with a standard deviation of 296.1 ng m−3. Both aerosol mass and black carbon show well defined annual cycles, with a maximum during the pre-monsoon season and a minimum during the monsoon. They also display a typical diurnal cycle during all the seasons, with the lowest particle concentration recorded during the night, and a considerable increase during the afternoon, revealing the major role played by thermal winds in influencing the behaviour of atmospheric compounds over the high Himalayas. The aerosol concentration is subject to high variability: in fact, as well as frequent "background conditions" (55% of the time) when BC concentrations are mainly below 100 ng m−3, concentrations up to 5 μg m−3 are reached during some episodes (a few days every year) in the pre-monsoon seasons. The variability of PM and BC is the result of both short-term changes due to thermal wind development in the valley, and long-range transport/synoptic circulation. At NCO-P, higher concentrations of PM1 and BC are mostly associated with regional circulation and westerly air masses from the Middle East, while the strongest contributions of mineral dust arrive from the Middle East and regional circulation, with a special contribution from North Africa and South-West Arabian Peninsula in post-monsoon and winter season.
Peroxy radical partitioning during the AMMA radical intercomparison exercise
Authors: Andrés-Hernández M. D. , Stone D. , Brookes D. M. , Commane R. , Reeves C. E. , Huntrieser H. , Heard D. E. , Monks P. S. , Burrows J. P. , Schlager H. , Kartal D. , Evans M. J. , Floquet C. F. A. , Ingham T. , Methven J. , Parker A. E.
Published: April 2010

Abstract. Peroxy radicals were measured onboard two scientific aircrafts during the AMMA (African Monsoon Multidisciplinary Analysis) campaign in summer 2006. This paper reports results from the flight on 16 August 2006 during which measurements of HO2 by laser induced fluorescence spectroscopy at low pressure (LIF-FAGE) and total peroxy radicals (RO2*=HO2+ΣRO2, R= organic chain) by two similar instruments based on the peroxy radical chemical amplification (PerCA) technique were subject of a blind intercomparison. The German DLR-Falcon and the British FAAM-BAe-146 flew wing tip to wing tip for about 30 min making concurrent measurements on 2 horizontal level runs at 697 and 485 hPa over the same geographical area in Burkina Faso. A full set of supporting measurements comprising photolysis frequencies, and relevant trace gases like CO, NO, NO2, NOy, O3 and a wider range of VOCs were collected simultaneously. Results are discussed on the basis of the characteristics and limitations of the different instruments used. Generally, no data bias are identified and the RO2* data available agree quite reasonably within the instrumental errors. The [RO2*]/[HO2] ratios, which vary between 1:1 and 3:1, as well as the peroxy radical variability, concur with variations in photolysis rates and in other potential radical precursors. Model results provide additional information about dominant radical formation and loss processes.
Temperature effect on physical and chemical properties of secondary organic aerosol from m-xylene photooxidation
Authors: Qi L. , Nakao S. , Tang P. , Cocker III D. R.
Published: April 2010

Abstract. The chemical and physical differences of secondary organic aerosol (SOA) formed at select isothermal temperatures (278 K, 300 K, and 313 K) are explored with respect to density, particle volatility, particle hygroscopicity, and elemental chemical composition. A transition point in SOA density, volatility, hygroscopicity and elemental composition is observed near 290–292 K as SOA within an environmental chamber is heated from 278 K to 313 K, indicating the presence of a thermally labile compound. No such transition points are observed for SOA produced at 313 K or 300 K and subsequently cooled to 278 K. The SOA formed at the lowest temperatures (278 K) is more than double the SOA formed at 313 K. SOA formed at 278 K is less hydrophilic and oxygenated while more volatile and dense than SOA formed at 300 K or 313 K. The properties of SOA formed at 300 K and 313 K when reduced to 278 K did not match the properties of SOA initially formed at 278 K. This study demonstrates that it is insufficient to utilize the enthalpy of vaporization when predicting SOA temperature dependence.
Empirical predictions of CCN from aerosol optical properties at four remote sites
Authors: Jefferson A.
Published: April 2010

Abstract. This study presents an empirical method to predict the CCN concentration as a function of percent supersaturation. The aerosol optical properties of backscatter fraction and single scatter albedo function as proxies for the aerosol size and composition in a power law relationship to CCN. This method is tested at four sites with aged aerosol: SGP (Oklahoma, USA), FKB (Black Forest, Germany), HFE (Hefei, China) and GRW (Graciosa, Azores). Each site represents a different aerosol type and thus demonstrates the method robustness and limitations. Good agreement was found between the calculated and measured CCN with slopes between 0.81 and 1.03 and correlation coefficients (r2 values) between 0.59 and 0.67. The fit quality declined at low CCN concentrations in a region with higher data uncertainty.
Functional group composition of ambient and source organic aerosols determined by tandem mass spectrometry
Authors: Dron J. , El Haddad I. , Temime-Roussel B. , Jaffrezo J.-L. , Wortham H. , Marchand N.
Published: April 2010

Abstract. The functional group composition of various organic aerosols (OA) is being investigated using a recently developed analytical approach based on atmospheric pressure chemical ionisation-tandem mass spectrometry (APCI-MS/MS). The determinations of the three functional groups' contents are performed quantitatively by neutral loss (carboxylic and carbonyl groups) and precursor ion (nitro groups) scanning modes of a tandem mass spectrometer. Major organic aerosol sources are studied: vehicular emission and wood combustion for primary aerosol sources; and a secondary organic aerosol (SOA) produced through photo-oxidation of o-xylene. The results reveal significant differences in the functional group contents of these source aerosols. The laboratory generated SOA is dominated by carbonyls while carboxylics are preponderate in the wood combustion particles. On the other hand, vehicular emissions are characterised by a strong nitro content. The total amount of the three functional groups accounted for 1.7% (vehicular) to 13.5% (o-xylene photo-oxidation) of the organic carbon. The diagnostic functional group ratios are then used to tentatively differentiate sources of particles collected in an urban background environment located in an Alpine valley (Chamonix, France) during a strong winter pollution event. The three functional groups under study account for a total functionalisation rate of 2.2 to 3.8% of the organic carbon in this ambient aerosol, which is also dominated by carboxylic moieties. In this particular case study of a deep alpine valley during winter, we show that the nitro- and carbonyl-to-carboxylic diagnostic ratios can be a useful tool to distinguish the sources. In these conditions, the total OA concentrations are highly dominated by wood combustion OA. This result is confirmed by an organic markers source apportionment approach which assesses a wood burning organic carbon contribution of about 60%. Finally, examples of functional group mass spectra of all aerosols under study are presented, and additional perspectives offered by the mass spectra in terms of the OA characterisation are discussed.
Contributions to stratospheric ozone changes from ozone depleting substances and greenhouse gases
Authors: Plummer D. A. , Scinocca J. F. , Shepherd T. G. , Reader M. C. , Jonsson A. I.
Published: April 2010

Abstract. A state-of-the-art chemistry climate model coupled to a three-dimensional ocean model is used to produce three experiments, all seamlessly covering the period 1950–2100, forced by different combinations of long-lived Greenhouse Gases (GHGs) and Ozone Depleting Substances (ODSs). The experiments are designed to investigate the mechanisms by which GHGs and ODSs affect the evolution of ozone, including changes in the Brewer-Dobson circulation of the stratosphere and cooling of the upper stratosphere by CO2. Separating the effects of GHGs and ODSs on ozone, we find the decrease in upper stratospheric ozone from ODSs up to the year 2000 is approximately 30% larger than the actual decrease in ozone due to the offsetting effects of cooling by increased CO2. Over the 21st century, as ODSs decrease, continued cooling from CO2 is projected to account for more than 50% of the projected increase in upper stratospheric ozone. Changes below 20 hPa show a redistribution of ozone from tropical to extra-tropical latitudes with an increase in the Brewer-Dobson circulation, while globally averaged the amount of ozone below 20 hPa decreases over the 21st century. Further analysis by linear regression shows that changes associated with GHGs do not appreciably alter the recovery of stratospheric ozone from the effects of ODSs; over much of the stratosphere ozone recovery follows the decline of halogen concentrations within statistical uncertainty, though the lower polar stratosphere of the Southern Hemisphere is an exception with ozone concentrations recovering more slowly than indicated by the halogen concentrations. These results also reveal the degree to which climate change, and stratospheric CO2 cooling in particular, mutes the chemical effects of N2O on ozone in the standard future scenario used for the WMO Ozone Assessment. Increases in the residual circulation of the atmosphere and chemical effects from CO2 cooling more than halve the increase in reactive nitrogen in the mid to upper stratosphere that results from the specified increase in N2O between 1950 and 2100.
Impacts of absorbing biomass burning aerosol on the climate of southern Africa: a Geophysical Fluid Dynamics Laboratory GCM sensitivity study
Authors: Randles C. A. , Ramaswamy V.
Published: April 2010

Abstract. Tropospheric aerosols emitted from biomass burning reduce solar radiation at the surface and locally heat the atmosphere. Equilibrium simulations using an atmospheric general circulation model (GFDL AGCM) indicate that strong atmospheric absorption from these particles can cool the surface and increase upward motion and low-level convergence over southern Africa during the dry season. These changes increase sea level pressure over land in the biomass burning region and spin-up the hydrologic cycle by increasing clouds, atmospheric water vapor, and, to a lesser extent, precipitation. Cloud increases serve to reinforce the surface radiative cooling tendency of the aerosol. Conversely, if the climate over southern Africa were hypothetically forced by high loadings of scattering aerosol, then the change in the low-level circulation and increased subsidence would serve to decrease clouds, precipitation, and atmospheric water vapor. Warming from cloud decreases mitigates surface cooling associated with scattering-only aerosols.
Accommodation coefficient of HOBr on deliquescent sodium bromide aerosol particles
Authors: Wachsmuth M. , Gäggeler H. W. , von Glasow R. , Ammann M.
Published: April 2010

Abstract. Uptake of HOBr on sea salt aerosol, sea salt brine or ice is believed to be a key process providing a source of photolabile bromine (Br2) and sustaining ozone depletion cycles in the arctic troposphere. In the present study, uptake of HOBr on sodium bromide (NaBr) aerosol particles was investigated at an extremely low HOBr concentration of 300 cm-3 using the short-lived radioactive isotopes 83-86Br. Under these conditions, at maximum one HOBr molecule was taken up per particle. The rate of uptake was clearly limited by the mass accommodation coefficient, which was calculated to be 0.6±0.2. This value is a factor of 10 larger than estimates used in earlier models. The atmospheric implications are discussed using the box model "MOCCA'', showing that the increase of the accommodation coefficient of HOBr by a factor of 10 only slightly affects net ozone loss, but significantly increases chlorine release.