bluuebunny/crossref_metadata_2025 huggingface dataset:
Temporal variability of mineral dust concentrations over West Africa: analyses of a pluriannual monitoring from the AMMA Sahelian Dust Transect
Authors: Marticorena B. , Chatenet B. , Rajot J. L. , Traoré S. , Coulibaly M. , Diallo A. , Koné I. , Maman A. , NDiaye T. , Zakou A.
Published: April 2010

Abstract. The Sahelian belt is known to be a region where the mineral dust content is among the highest observed on Earth. In the framework of the AMMA (African Monsoon Multidisciplinary Analysis) International Program, a transect of 3 ground based stations, the "Sahelian Dust Transect" (SDT), has been deployed in order to obtain quantitative information on the mineral dust content and its variability over the Sahel. The three stations, namely Banizoumbou (Niger), Cinzana (Mali) and M'Bour (Senegal) are aligned around 14° N along the east-west main pathway of the Saharan and Sahelian dust towards the Atlantic Ocean. We discuss data collected between January 2006 and December 2008 to investigate the main characteristics of the mineral dust concentration over West Africa and their connection with the dominant meteorological situations. The succession of the dry season during which the Sahel is under the influence of the dry Harmattan wind and the wet season induced by the entrance of the monsoon flow is clearly identified from the basic meteorological parameters (air temperature and moisture, wind direction). Atmospheric dust concentrations at the three stations exhibit a similar seasonal cycle, with a monthly maximum during the dry season and a minimum occurring during the rainy season, indicating that the general pattern of dust concentration is similar at regional scale. This seasonal cycle of the dust concentrations is not phased with the seasonal cycle of surface wind velocity suggesting that it is mainly controlled by Saharan dust transport. A decrease in the dust concentration is observed when moving from Niger to Senegal. However, local dust emissions induced by strong surface winds are responsible for the occurrence of extremely high daily concentrations observed at the beginning of the rainy season.
Snow-sourced bromine and its implications for polar tropospheric ozone
Authors:   Xin Yang, Pyle J. A. , Cox R. A. , Theys N. , Van Roozendael M.
Published: April 2010

Abstract. In the last two decades, significant depletion of boundary layer ozone (ozone depletion events, ODEs) has been observed in both Arctic and Antarctic spring. ODEs are attributed to catalytic destruction by bromine radicals (Br plus BrO), especially during bromine explosion events (BEs), when high concentrations of BrO periodically occur. However, neither the exact source of bromine nor the mechanism for sustaining the observed high BrO concentrations is completely understood. Here, by considering the production of sea salt aerosol from snow lying on sea ice during blowing snow events and the subsequent release of bromine, we successfully simulate the BEs using a global chemistry transport model. We find that heterogeneous reactions play an important role in sustaining a high fraction of the total inorganic bromine as BrO. Modeled tropospheric BrO columns generally compare well with the tropospheric BrO columns retrieved from the GOME satellite instrument (Global Ozone Monitoring Experiment). The additional blowing snow bromine source, identified here, reduces modeled high latitude lower tropospheric ozone amounts by up to an average 8% in polar spring.
A 6-year global cloud climatology from the Atmospheric InfraRed Sounder AIRS and a statistical analysis in synergy with CALIPSO and CloudSat
Authors: Stubenrauch C. J. , Cros S. , Guignard A. , Lamquin N.
Published: April 2010

Abstract. We present a six-year global climatology of cloud properties, obtained from observations of the Atmospheric Infrared Sounder (AIRS) onboard the NASA Aqua satellite. Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) combined with CloudSat observations, both missions launched as part of the A-Train in 2006, provide a unique opportunity to evaluate the retrieved AIRS cloud properties such as cloud amount and height as well as to explore the vertical structure of different cloud types. AIRS-LMD cloud detection agrees with CALIPSO about 85% over ocean and about 75% over land. Global cloud amount has been estimated as about 66% to 74%, depending on the weighting of not cloudy AIRS footprints by partial cloud cover (0 or 0.3). 40% of all clouds are high clouds, and about 44% of all clouds are single layer low-level clouds. The "radiative" cloud height determined by the AIRS-LMD retrieval corresponds well to the height of the maximum backscatter signal and of the "apparent middle" of the cloud. Whereas the real cloud thickness of high opaque clouds often fills the whole troposphere, their "apparent" cloud thickness (at which optical depth reaches about 5) is on average only 2.5 km. The real geometrical thickness of optically thin cirrus as identified by AIRS-LMD is identical to the "apparent" cloud thickness with an average of about 2.5 km in the tropics and midlatitudes. High clouds in the tropics have slightly more diffusive cloud tops than at higher latitudes. In general, the depth of the maximum backscatter signal increases nearly linearly with increasing "apparent" cloud thickness. For the same "apparent" cloud thickness optically thin cirrus show a maximum backscatter about 10% deeper inside the cloud than optically thicker clouds. We also show that only the geometrically thickest opaque clouds and (the probably surrounding anvil) cirrus penetrate the stratosphere in the tropics.
Composition of semi-volatile organic compounds in the urban atmosphere of Singapore: influence of biomass burning
Authors: He J. , Zielinska B. , Balasubramanian R.
Published: April 2010

Abstract. An intensive field study was conducted in the urban atmosphere of Singapore to investigate the composition of organic compounds in both gaseous and particulate phases during the period of August to early November 2006. 17 atmospheric samples were collected. These samples were subjected to accelerated solvent extraction with a mixture of dichloromethane and acetone and separated into functional group fractions for analyses by GC/MS. Over 180 organic compounds belonging to three major fractions (n-alkanes, polycyclic aromatic hydrocarbons (PAHs), and polar organic compounds (POCs)) were identified and quantified. The characteristics and abundance of the n-alkanes, PAHs, mono and dicarboxylic acids, methoxylated phenols and other POCs were determined. The composition of these organic compounds fluctuated temporally with most of them being relatively higher in October than those in other months of the sampling period. 3-D backward air mass trajectory analyses together with the carbon preference index (CPI), molecular diagnostic ratios and molecular markers were used to investigate the origin of organic species measured in this study. Based on these diagnostic tools, the increased abundance of atmospheric organic species during October could be attributed to the occurrence of regional smoke haze episodes due to biomass burning in Indonesia. Among the POCs investigated, phthalic acid and cis-pinonic acid showed a strong linear relationship with maximum daily ozone concentration, indicating secondary organic aerosols (SOA) to be an important contributor to ambient atmospheric organics over Singapore.
GOMOS ozone profile validation using ground-based and balloon sonde measurements
Authors: van Gijsel J. A. E. , Swart D. P. J. , Baray J.-L. , Bencherif H. , Claude H. , Fehr T. , Godin-Beekmann S. , Hansen G. H. , Keckhut P. , Leblanc T. , McDermid I. S. , Meijer Y. J. , Nakane H. , Quel E. J. , Stebel K. , Steinbrecht W. , Strawbridge K. B. , Tatarov B. I. , Wolfram E. A.
Published: April 2010

Abstract. The validation of ozone profiles retrieved by satellite instruments through comparison with data from ground-based instruments is important to monitor the evolution of the satellite instrument, to assist algorithm development and to allow multi-mission trend analyses. In this study we compare ozone profiles derived from GOMOS night-time observations with measurements from lidar, microwave radiometer and balloon sonde. Collocated pairs are analysed for dependence on several geophysical and instrument observational parameters. Validation results are presented for the operational ESA level 2 data (GOMOS version 5.00) obtained during nearly seven years of observations and a comparison for a smaller dataset from the previous processor (version 4.02) is also included. The profiles obtained from dark limb measurements when the provided processing flag is properly considered match the ground-based measurements within ±2% over the altitude range 20 to 40 km. Outside this range, the pairs start to deviate more and there is a latitudinal dependence: in the polar region (higher amount of straylight contamination), differences are start to occur lower in the mesosphere than in the tropics, whereas for the lower part of the stratosphere the opposite happens: the profiles in the tropics reach less far down as the signal reduces faster because of the higher altitude at which the maximum ozone concentration is found compared to the mid and polar latitudes. Also the bias is shifting from mostly negative in the polar region to more positive in the tropics Profiles measured under "twilight" conditions are often matching the ground-based measurements very well, but care has to be taken in all cases when dealing with "straylight" contaminated profiles. For the selection criteria applied here (data within 800 km, 3 degrees in equivalent latitude, 20 h (5 h above 50 km) and a relative ozone error in the GOMOS data of 20% or less), no dependence was found on stellar magnitude, star temperature, nor the azimuth angle of the line of sight. No evidence of a temporal trend was seen either in the bias or frequency of outliers, but a comparison applying less strict data selection criteria might show differently.
Particle size distributions from laboratory-scale biomass fires using fast response instruments
Authors: Hosseini S. , Qi L. , Cocker D. , Weise D. , Miller A. , Shrivastava M. , Miller W. , Mahalingam S. , Princevac M. , Jung H.
Published: April 2010

Abstract. Particle size distribution from biomass combustion is an important parameter as it affects air quality, climate modelling and health effects. To date particle size distributions reported from prior studies vary not only due to difference in fuels but also difference in experimental conditions. This study aims to report characteristics of particle size distribution in a well controlled repeatable lab scale biomass fires for southwestern US fuels. The combustion facility at the USDA Forest Service's Fire Science Laboratory (FSL), Missoula, MT provided repeatable combustion and dilution environment ideal for particle size distribution study. For a variety of fuels tested the major mode of particle size distribution was in the range of 29 to 52 nm, which was attributable to dilution of the fresh smoke. Comparing volume size distribution from Fast Mobility Particle Sizer (FMPS) and Aerodynamic Particle Sizer (APS) measurements, ~30% of particle volume was attributable to the particles ranging from 0.5 to 10 μm for PM10. Geometric mean diameter rapidly increased during flaming and gradually decreased during mixed and smoldering phase combustion. Most of fuels gave unimodal distribution during flaming phase and strong biomodal distribution during smoldering phase. The mode of combustion (flaming, mixed and smoldering) could be better distinguished using slopes in Modified Combustion Efficiency (MCE) vs. geometric mean diameter from each mode of combustion than only using MCE values.
Analysis of HCl and ClO time series in the upper stratosphere using satellite data sets
Authors: Jones A. , Urban J. , Murtagh D. P. , Sanchez C. , Walker K. A. , Livesay L. , Froidevaux L. , Santee M.
Published: April 2010

Abstract. Previous analyses of satellite and ground-based measurements of hydrogen chloride (HCl) and chlorine monoxide (ClO) have suggested that total inorganic chlorine in the upper stratosphere is on the decline. We create HCl and ClO time series using satellite data sets with the intension of extending them to beyond November 2008 so that an update can be made on the long term evolution of these two species. We use the HALogen Occultation Experiment (HALOE) and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) data for the HCl analysis, and the Odin Sub-Millimetre Radiometer (SMR) and the Aura Microwave Limb Sounder (Aura-MLS) measurements for the study of ClO. Altitudes between 35 and 45 km and three latitude bands between 60° S–60° N for HCl, and 20° S–20° N for ClO are studied. HCl shows values to be reducing from peak 1997 values by −4.4% in the tropics and between −6.4% to −6.7% per decade in the mid-latitudes. Trend values are significantly different from a zero trend at the 2 sigma level. ClO is decreasing in the tropics by −7.1% ± 7.8%/decade based on measurements made from December 2001. As both of these species contribute most to the chlorine budget at these altitudes then HCl and ClO should decrease at similar rates. The results found here confirm how effective the 1987 Montreal protocol objectives and its amendments have been in reducing the total amount of inorganic chlorine.
Sulfur dioxide emissions in China and sulfur trends in East Asia since 2000
Authors: Lu Z. , Streets D. G. , Zhang Q. , Wang S. , Carmichael G. R. , Cheng Y. F. , Wei C. , Chin M. , Diehl T. , Tan Q.
Published: April 2010

Abstract. With the rapid development of the economy, the sulfur dioxide (SO2) emission from China since 2000 is of increasing concern. In this study, we estimate the annual SO2 emission in China after 2000 using a technology-based methodology specifically for China. From 2000 to 2006, total SO2 emission in China increased by 53%, from 21.7 Tg to 33.2 Tg, at an annual growth rate of 7.3%. Emissions from power plants are the main sources of SO2 in China and they increased from 10.6 Tg to 18.6 Tg in the same period. Geographically, emission from north China increased by 85%, whereas that from the south increased by only 28%. The emission growth rate slowed around 2005, and emissions began to decrease after 2006 mainly due to the wide application of Flue-Gas Desulfurization (FGD) devices in power plants in response to a new policy of China's government. This paper shows that the trend of estimated SO2 emission in China is consistent with the trends of SO2 concentration and acid rain pH and frequency in China, as well as with the increasing trends of background SO2 and sulfate concentration in East Asia. A longitudinal gradient in the percentage change of urban SO2 concentration in Japan is found during 2000–2007, indicating that the decrease of urban SO2 is lower in areas close to the Asian continent. This implies that the transport of increasing SO2 from the Asian continent partially counteracts the local reduction of SO2 emission downwind. The Aerosol Optical Depth (AOD) products of Moderate Resolution Imaging Spectroradiometer (MODIS) are found to be highly correlated with the Surface Solar Radiation (SSR) measurements in East Asia. Using MODIS AOD data as a surrogate of SSR, we found that China and East Asia excluding Japan underwent a continuous dimming after 2000, which is in line with the dramatic increase in SO2 emission in East Asia. The trends of AOD from both satellite retrievals and model over East Asia are also consistent with the trend of SO2 emission in China, especially during the second half of the year, when sulfur contributes the largest fraction of AOD. The arrested growth in SO2 emissions since 2006 is also reflected in the decreasing trends of SO2 and SO42− concentrations, acid rain pH values and frequencies, and AOD over East Asia.
Radiative forcing associated with a springtime case of Bodélé and Sudan dust transport over West Africa
Authors: Lemaître C. , Flamant C. , Cuesta J. , Raut J.-C. , Chazette P. , Formenti P. , Pelon J.
Published: April 2010

Abstract. The radiative forcing due to mineral dust over West Africa is investigated using the radiative code STREAMER, as well as remote sensing and in situ observations gathered during the African Monsoon Multidisciplinary Analysis Special Observing Period (AMMA SOP). We focus on two days (13 and 14 June 2006) of an intense and long-lasting episode of dust being lifted in remote sources in Chad and Sudan and transported across West Africa in the African easterly jet region, during which airborne operations were conducted at the regional scale, from the southern fringes of the Sahara to the Gulf of Guinea. Profiles of heating rates are computed from airborne LEANDRE 2 and space-borne CALIOP lidar observations using two mineral dust model constrained by airborne in situ data and ground-based sunphotometer obtained during the campaign. Complementary space-borne observations (from MODIS) and in-situ observations such as dropsondes are also used to take into account a realistic infrared contribution of the water vapour. We investigate the variability of the heating rate on the vertical within a dust plume, as well as the contribution of longwave radiation to the heating rate and the radiative forcing of dust during the nighttime. The sensitivity of the so-derived heating rate is also analyzed for some key variables for which the associated uncertainties are quite large. During daytime, the warming associated with the presence of dust was found to be between 1.5 K day−1 and 4 K day−1, on average, depending on altitude and latitude. Strong warming (i.e. heating rates as high as 8 K day−1) was also observed locally in some limited part of the dust plumes. Obviously, during nighttime much smaller values of heating/cooling are retrieved (less than ±1 K day−1) but large enough to modify the low tropospheric equilibrium. Furthermore, cooling is observed as the result of the longwave forcing in the dust layer, while warming is observed below the dust layer, in the monsoon layer.
Technical Note: Evaluation of the WRF-Chem "aerosol chemical to aerosol optical properties" module using data from the MILAGRO campaign
Authors: Barnard J. C. , Fast J. D. , Paredes-Miranda G. , Arnott W. P. , Laskin A.
Published: April 2010

Abstract. A comparison between observed aerosol optical properties from the MILAGRO field campaign, which took place in the Mexico City Metropolitan Area (MCMA) during March 2006, and values simulated by the Weather Research and Forecasting (WRF-Chem) model, reveals large differences. To help identify the source of the discrepancies, data from the MILAGRO campaign are used to evaluate the "aerosol chemical to aerosol optical properties" module implemented in the full chemistry version of the WRF-Chem model. The evaluation uses measurements of aerosol size distributions and chemical properties obtained at the MILAGRO T1 site. These observations are fed to the module, which makes predictions of various aerosol optical properties, including the scattering coefficient, Bscat; the absorption coefficient, Babs; and the single-scattering albedo, ϖ0; all as a function of time. Values simulated by the module are compared with independent measurements obtained from a photoacoustic spectrometer (PAS) at a wavelength of 870 nm. Because of line losses and other factors, only "fine mode" aerosols with aerodynamic diameters less than 2.5 μm are considered here. Over a 10-day period, the simulations of hour-by-hour variations of Bscat are not satisfactory, but simulations of Babs and ϖ0 are considerably better. When averaged over the 10-day period, the computed and observed optical properties agree within the uncertainty limits of the measurements and simulations. Specifically, the observed and calculated values are, respectively: (1) Bscat, 34.1±5.1 Mm−1 versus 30.4±3.4 Mm−1; (2) Babs, 9.7±1.0 Mm−1 versus 11.7±1.2 Mm−1; and (3) ϖ0, 0.78±0.05 and 0.74±0.03. The discrepancies in values of ϖ0 simulated by the full WRF-Chem model thus cannot be attributed to the "aerosol chemistry to optics" module. The discrepancy is more likely due, in part, to poor characterization of emissions near the T1 site, particularly black carbon emissions.
The relative importance of various source regions on East Asian surface ozone
Authors: Nagashima T. , Ohara T. , Sudo K. , Akimoto H.
Published: April 2010

Abstract. The Source-Receptor (S-R) relationship for surface O3 in East Asia is estimated for recent years in this study utilizing the tagged tracer method with a global chemical transport model. The estimation shows the importance of intra-continental transport of O3 inside East Asia as well as the transport of O3 from distant source regions. The model well simulated the absolute concentration and seasonal variation of surface O3 in the East Asian region, and demonstrated significant seasonal difference in the origin of surface O3. More than half of surface O3 is attributable to the O3 transported from distant sources outside of East Asia in the cold season (October to March). In the warm season (April to September), most of the surface O3 is attributed to O3 created within East Asia in most areas of East Asia. The contribution of domestically-created O3 accounts for 20% of surface O3 in Japan and the Korean Peninsula, 40% in North China Plain and around 50% in the southern part of China in spring, which increase greatly in summer. The contribution of China and the Korean Peninsula to Japan are estimated at about 10% and 5%, respectively. A large contribution (20%) of China to the Korean Peninsula is also demonstrated. In the northern and southern part of China, large contribution of over 10% from East Siberia and the Indochina Peninsula are identified, respectively. The contribution of intercontinental transport increases with latitude; it is 21% in Northeast China and 13% in Japan and the Korean Peninsula in spring. As for one-hourly mean surface O3, domestically-created O3 is the main contributor in most areas of East Asia, except for the low O3 class (<30 ppbv), and accounts for more than 50% in very high O3 class (>90 ppbv). The mean relative contribution of China to central Japan was about 10% in every class, but that from the Korean Peninsula is important in all expect the low O3 class. Substantial impact of foreign sources on the exceedance of Japan's AAQS is identified in the high O3 class (60–90 ppbv) in spring.
Forecasts and assimilation experiments of the Antarctic Ozone Hole 2008
Authors: Flemming J. , Inness A. , Jones L. , Eskes H. J. , Huijnen V. , Schultz M. G. , Stein O. , Cariolle D. , Kinnison D. , Brasseur G.
Published: April 2010

Abstract. The 2008 Antarctic ozone hole was one of the largest and most long-lived in recent years. Predictions of the ozone hole were made in near-real time (NRT) and hindcast mode with the Integrated Forecast System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). The forecasts were carried out both with and without assimilation of satellite observations from multiple instruments to provide more realistic initial conditions. Three different chemistry schemes were applied for the description of stratospheric ozone chemistry: (i) a linearization of the ozone chemistry, (ii) the stratospheric chemical mechanism of the MOZART-3 chemical transport model (CTM) and (iii) the relaxation to a climatology as implemented in the TM5 CTM. The IFS uses the latter two schemes by means of a two-way coupled system. Without assimilation, the forecasts showed model-specific shortcomings in predicting start time, extent and duration of the ozone hole. The assimilation of satellite observations from the Microwave Limb Sounder (MLS), the Ozone Monitoring Instrument (OMI), the Solar Backscattering Ultraviolet radiometer (SBUV-2) and the SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY) led to a significant improvement of the forecasts when compared with total columns and vertical profiles from ozone sondes. The combined assimilation of observations from multiple instruments helped to overcome limitations of the ultraviolet (UV) sensors at low solar elevation over Antarctica. The assimilation of data from MLS was crucial to obtain a good agreement with the observed ozone profiles both in the polar stratosphere and troposphere. The ozone analyses by the three model configurations were very similar despite the different underlying chemistry schemes. During the chemically instigated development of the ozone hole, differences quickly developed in the initialized forecasts by the different schemes. The predictions of the ozone-hole closure, which is driven mainly by dynamical processes, benefited from the initialization over a much longer forecast length. The characteristics of the individual chemistry schemes became apparent after the third forecast day but the forecasts were still close to the respective analyses. The initialization with ozone analyses was beneficial up to at least 15 days.
Updraft and downdraft characterization with Doppler lidar: cloud-free versus cumuli-topped mixed-layer
Authors: Ansmann A. , Fruntke J. , Engelmann R.
Published: April 2010

Abstract. For the first time, a comprehensive, height-resolved Doppler lidar study of updrafts and downdrafts in the mixing layer is presented. The Doppler lidar measurements were performed at Leipzig, Germany, in the summer half year of 2006. The conditional sampling method is applied to the measured vertical velocities to identify, count, and analyze significant updraft and downdraft events. Three cases of boundary layer evolution with and without fair weather cumuli formation are discussed. Updrafts occur with an average frequency of 1–2 per unit length zi (boundary layer depth zi), downdrafts 20%–30% more frequently. In the case with cumuli formation, the draft occurrence frequency is enhanced by about 50% at cloud level or near cloud base. The counted updraft events cover 30%–34%, downdrafts 53%–57% of the velocity time series during the main period of convective activity. By considering all drafts with horizontal extent >36 m in the analysis, the updraft mean horizontal extent ranges from 200–350 m and is about 0.15zi in all three cases. Downdrafts are a factor of 1.3–1.5 larger. The average value of the updraft mean vertical velocities is 0.5–0.7 m/s or 0.4w∗ (convective velocity scale w∗), and the negative downdraft mean vertical velocities are weaker by roughly 10%–20%. The analysis of the relationship between the size (horizontal extent) of the updrafts and downdrafts and their mean vertical velocity reveals a pronounced increase of the average vertical velocity in updrafts from 0.4–0.5 m/s for small thermals (100–200 m) to about 1.5 m/s for large updrafts (>600 m) in the case with fair weather cumuli. At cloudless conditions, the updraft velocities were found to be 20% smaller for the large thermals.
Observational constraints on the global atmospheric budget of ethanol
Authors: Naik V. , Fiore A. M. , Horowitz L. W. , Singh H. B. , Wiedinmyer C. , Guenther A. , de Gouw J. A. , Millet D. B. , Goldan P. D. , Kuster W. C. , Goldstein A.
Published: April 2010

Abstract. Energy security and climate change concerns have led to the promotion of biomass-derived ethanol, an oxygenated volatile organic compound (OVOC), as a substitute for fossil fuels. Although ethanol is ubiquitous in the troposphere, our knowledge of its current atmospheric budget and distribution is limited. Here, for the first time we use a global chemical transport model in conjunction with atmospheric observations to place constraints on the ethanol budget, noting that additional measurements of ethanol (and its precursors) are still needed to enhance confidence in our estimated budget. Global sources of ethanol in the model include 5.0 Tg yr−1 from industrial sources and biofuels, 9.2 Tg yr−1 from terrestrial plants, ~0.5 Tg yr−1 from biomass burning, and 0.05 Tg yr−1 from atmospheric reactions of the ethyl peroxide radical (C2H5O2) with itself and with the methyl peroxide radical (CH3O2). The resulting atmospheric lifetime of ethanol in the model is 2.8 days. Gas-phase oxidation by hydroxyl radical (OH) is the primary global sink of ethanol in the model (65%), followed by dry deposition to land (25%), and wet deposition (10%). Over continental areas, ethanol concentrations predominantly reflect direct anthropogenic and biogenic emission sources. Uncertainty in the biogenic ethanol emissions estimated at a factor of three may contribute to the 50% model underestimate of observations in the North American boundary layer. Furthermore, current levels of ethanol measured in remote atmospheres are an order of magnitude larger than those explained by surface sources or by in-situ atmospheric production from observed precursor hydrocarbons in the model, suggesting a major gap in understanding. Stronger constraints on the budget and distribution of ethanol and other VOCs are a critical step towards assessing the impacts of increasing use of ethanol as a fuel.
New particle formation events measured on board the ATR-42 aircraft during the EUCAARI campaign
Authors: Crumeyrolle S. , Manninen H. E. , Sellegri K. , Roberts G. , Gomes L. , Kulmala M. , Weigel R. , Laj P. , Schwarzenboeck A.
Published: April 2010

Abstract. Aerosol properties were studied during an intensive airborne measurement campaign that took place at Rotterdam in Netherlands in May 2008 within the framework of the European Aerosol Cloud Climate and Air Quality Interactions project (EUCAARI). The objective of this study is to illustrate seven events of new particle formation (NPF) observed with two Condensation Particle Counters (CPCs) operated on board the ATR-42 research aircraft in airsectors around Rotterdam, and to provide information on the spatial extent of the new particle formation phenomenon based on 1-s resolution measurements of ultra-fine particle (in the size range 3–10 nm diameter, denoted N3–10 hereafter) concentrations. The results show that particle production occurred under the influence of different air mass origins, at different day times and over the North Sea as well as over the continent. The number concentration of freshly nucleated particles (N3–10) varied between 5000 and 100 000 cm−3 within the boundary layer (BL). Furthermore the vertical extension for all nucleation events observed on the ATR-42 never exceeded the upper limit of the BL. The horizontal extent of N3–10 could not be delimited due to inflexible flight plans which could not be modified to accommodate real-time results. However, the NPF events were observed over geographically large areas; typically the horizontal extension was about 100 km and larger.
Impact of model resolution on chemical ozone formation in Mexico City; application of the WRF-Chem model
Authors: Tie X. , Brasseur G. , Ying Z.
Published: April 2010

Abstract. The resolution of regional chemical/dynamical models has important effects on the calculation of distributions of air pollutants in large cities. In this study, the sensitivity of air pollutants and photochemical O3 production to different model resolutions is studied by using a regional chemical/dynamical model (version 3 of Weather Research and Forecasting Chemical model – WRF-Chemv3) in Mexico City. The model results with 3, 6, 12, and 24 km resolutions are compared to the surface measurements for CO, NOx, and O3. The study shows that the model resolutions with 3 and 6 km have reasonable simulations of surface CO, NOx, and O3 concentrations and diurnal variations. The model results intend to underestimate the measurements when the resolution is reduced to 12 km. The calculated surface CO, NOx, and O3 concentrations significantly underestimate the measured values at 24 km resolution. This study suggests that 12 km is a threshold resolution for the O3 and O3 precursor calculations for using a regional chemical/dynamical model in Mexico City. There are three major factors related to the effects of model resolution on the calculations of O3 and O3 precursors, including; (1) the calculated meteorological conditions with different model resolutions, (2) the emission spatial distribution of ozone precursors, and (3) the non-linearly O3 photochemical productions with different resolutions. Model studies suggest that model resolution (resulting in different meteorological condition and transport process) have larger impacts than emission inventory resolutions for the calculations of O3 and O3 precursors. The model calculations show that with coarse resolution of emission inventory (24 km) and fine meteorological condition resolution (6 km), the calculated CO and O3 are considerably improved compared to the calculation with coarse resolution for both emission inventory and meteorological condition (24 km), suggesting that the impacts of resolution on meteorological condition and transport process are largest for the calculations of O3 and O3 precursors. The emission resolution has important effects on the calculation, but the effects are smaller than the model resolution. This study also suggests that the changes of O3 precursors at different resolutions lead to important impacts on O3 chemical formation due to the non-linear relationship between O3 formation and O3 precursors. Finally, this study suggests that with the balance between the model performance and required computation time, the 6 km resolution is an optimal resolution for the calculation of O3 and O3 precursors in Mexico City.
Global ozone monitoring by occultation of stars: an overview of GOMOS measurements on ENVISAT
Authors: Bertaux J. L. , Kyrölä E. , Fussen D. , Hauchecorne A. , Dalaudier F. , Sofieva V. , Tamminen J. , Vanhellemont F. , Fanton d'Andon O. , Barrot G. , Mangin A. , Blanot L. , Lebrun J. C. , Pérot K. , Fehr T. , Saavedra L. , Fraisse R.
Published: April 2010

Abstract. GOMOS on ENVISAT (launched in February, 2002) is the first space instrument dedicated to the study of the atmosphere of the Earth by the technique of stellar occultations (Global Ozone Monitoring by Occultation of Stars). From a polar orbit, it allows to have a good latitude coverage. Because it is self-calibrated, it is particularly well adapted to the long time trend monitoring of stratospheric species. With 4 spectrometers the wavelength coverage of 248 nm to 942 nm allows to monitor ozone, H2O, NO2, NO3, air, aerosols, and O2. Two additional fast photometers (1 kHz sampling rate) allow for the correction of scintillations, as well as the study of the structure of air density irregularities, resulting from gravity waves and turbulence. A high vertical resolution profile of the temperature may also be obtained from the time delay between the red and the blue photometer. Noctilucent clouds (Polar Mesospheric Clouds, PMC), are routinely observed in both polar summers, and global observations of OCLO and sodium are achieved. The instrument configuration, dictated by the scientific objectives rationale and technical constraints, are described, together with the typical operations along one orbit, and statistics over 5 years of operation. Typical atmospheric transmission spectra are presented, and some retrieval difficulties are discussed, in particular for O2 and H2O. An overview of a number of scientific results is presented, already published or found in more details as companion papers in the same ACP GOMOS special issue. This paper is particularly intended to provide the incentive for GOMOS data exploitation, available to the whole scientific community in the ESA data archive, and to help the GOMOS data users to better understand the instrument, its capabilities and the quality of its measurements, for an optimized scientific return.
Modelling of the photooxidation of toluene: conceptual ideas for validating detailed mechanisms
Authors: Wagner V. , Jenkin M. E. , Saunders S. M. , Stanton J. , Wirtz K. , Pilling M. J.
Published: April 2010

Abstract. Toluene photooxidation is chosen as an example to examine how simulations of smog-chamber experiments can be used to unravel shortcomings in detailed mechanisms and to provide information on complex reaction systems that will be crucial for the design of future validation experiments. The mechanism used in this study is extracted from the Master Chemical Mechanism Version 3 (MCM v3) and has been updated with new modules for cresol and g-dicarbonyl chemistry. Model simulations are carried out for a toluene-NOx experiment undertaken at the European Photoreactor (EUPHORE). The comparison of the simulation with the experimental data reveals two fundamental shortcomings in the mechanism: OH production is too low by about 80%, and the ozone concentration at the end of the experiment is over-predicted by 55%. The radical budget was analysed to identify the key intermediates governing the radical transformation in the toluene system. Ring-opening products, particularly conjugated g-dicarbonyls, were identified as dominant radical sources in the early stages of the experiment. The analysis of the time evolution of radical production points to a missing OH source that peaks when the system reaches highest reactivity. First generation products are also of major importance for the ozone production in the system. The analysis of the radical budget suggests two options to explain the concurrent under-prediction of OH and over-prediction of ozone in the model: 1) missing oxidation processes that produce or regenerate OH without or with little NO to NO2 conversion or 2) NO3 chemistry that sequesters reactive nitrogen oxides into stable nitrogen compounds and at the same time produces peroxy radicals. Sensitivity analysis was employed to identify significant contributors to ozone production and it is shown how this technique, in combination with ozone isopleth plots, can be used for the design of validation experiments.
A discussion on the determination of atmospheric OH and its trends
Authors: Jöckel P. , Brenninkmeijer C. A. M. , Crutzen P. J.
Published: April 2010

Abstract. The global hydroxyl radical distribution largely determines the oxidation efficiency of the atmosphere and, together with their sources and atmospheric transport, the distributions and lifetimes of most trace gases. Because of the great importance of several of these gases for climate, ozone budget and OH itself, it is of fundamental importance to acquire knowledge about atmospheric OH and possible trends in its concentrations. In the past, average concentrations of OH and trends were largely derived using industrially produced CH3CCl3 as a chemical tracer. The analyses have given valuable, but also rather uncertain results. In this paper we describe an idealized computer aided tracer experiment which has as one of its goals to derive tracer concentration weighted, global average , where he temporal and spatial OH distribution is prescribed and k is the reaction rate coefficient of OH with a hitherto never produced (Gedanken) tracer, which is injected at a number of surface sites in the atmosphere in well known amounts over a given time period. Using a three-dimensional (3D) time-dependent chemistry/transport model  can be accurately determined from the calculated 3-D tracer distribution. It is next explored how well  can be retrieved solely from tracer measurements at a limited number of surface sites. The results from this analysis are encouraging enough to actually think about the feasibility to carry out a global dedicated tracer experiment to derive  and its temporal trends. However, before that, we propose to test the methods which are used to derive , so far largely using CH3CCl3, with an idealized tracer experiment, in which a global model is used to calculate the "Gedanken"  tracer distribution, representing the real 3-D world, from which we next derive , using only the tracer information from a limited set of surface sites. We propose here that research groups which are, or will be, involved in global average OH studies to participate in such an inter-comparison of methods, organized and over-seen by a committee appointed by the International Global Atmospheric Chemistry (IGAC) program.
Experimental investigation of homogeneous freezing of sulphuric acid particles in the aerosol chamber AIDA
Authors: Möhler O. , Stetzer O. , Schaefers S. , Linke C. , Schnaiter M. , Tiede R. , Saathoff H. , Krämer M. , Mangold A. , Budz P. , Zink P. , Schreiner J. , Mauersberger K. , Haag W. , Kärcher B. , Schurath U.
Published: April 2010

Abstract. The homogeneous freezing of supercooled H2SO4/H2O solution droplets was investigated in the aerosol chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere) of Forschungszentrum Karlsruhe. 24 freezing experiments were performed at temperatures between 194 and 235 K with aerosol particles in the diameter range 0.05 to 1 µm. Individual experiments started at homogeneous temperatures and ice saturation ratios between 0.85 and 0.95. Cloud cooling rates up to -2.8 K/min were simulated dynamically in the chamber by expansion cooling using a mechanical pump. Depending on the cooling rate and starting temperature, freezing threshold relative humidities were exceeded after expansion time periods between about 1 and 10 min. The onset of ice formation was measured with three independent methods showing good agreement among each other. Ice saturation ratios measured at the onset of ice formation increased from about 1.4 at 231 K to about 1.75 at 189 K. The experimental data set including thermodynamic parameters as well as physical and chemical aerosol analysis provides a good basis for microphysical model applications.
Global chemical weather forecasts for field campaign planning: predictions and observations of large-scale features during MINOS, CONTRACE, and INDOEX
Authors: Lawrence M. G. , Rasch P. J. , von Kuhlmann R. , Williams J. , Fischer H. , de Reus M. , Lelieveld J. , Crutzen P. J. , Schultz M. , Stier P. , Huntrieser H. , Heland J. , Stohl A. , Forster C. , Elbern H. , Jakobs H. , Dickerson R. R.
Published: April 2010

Abstract. The first global tropospheric forecasts of O3 and its precursors have been used in the daily flight planning of field measurement campaigns. The 3-D chemistry-transport model MATCH-MPIC is driven by meteorological data from a weather center (NCEP) to produce daily 3-day forecasts of the global distributions of O3 and related gases, as well as regional CO tracers. This paper describes the forecast system and its use in three field campaigns, MINOS, CONTRACE and INDOEX. An overview is given of the forecasts by MATCH-MPIC and by three other chemical weather forecast models (EURAD, ECHAM, and FLEXPART), focusing on O3 and CO. Total CO and regional CO tracers were found to be the most valuable gases for flight planning, due to their relatively well-defined anthropogenic source regions and lifetimes of one to a few months. CO was in good agreement with the observations on nearly all the flights (generally  r > 0.7, RMS  < 20%). In every case in which the chemical weather forecasts were primarily responsible for the flight plans, the targeted features were observed. Two forecasted phenomena are discussed in detail: outflow from Asia observed in the Mediterranean upper troposphere during MINOS, and outflow from North America observed in the middle troposphere over northern Europe during CONTRACE. It is shown that although such pollution plumes occur repeatedly during the months around the campaigns, their frequency is sufficiently low (~10--30% of the time) that global chemical weather forecasts are important for enabling them to be observed during limited-duration field campaigns. The MATCH-MPIC chemical weather forecasts, including an interface for making customized figures from the output, are available for community use via  http://www.mpch-mainz.mpg.de/~lawrence/forecasts.html .
The Hohenpeissenberg aerosol formation experiment (HAFEX): a long-term study including size-resolved aerosol, H2SO4, OH, and monoterpenes measurements
Authors: Birmili W. , Berresheim H. , Plass-Dülmer C. , Elste T. , Gilge S. , Wiedensohler A. , Uhrner U.
Published: April 2010

Abstract. Ambient aerosol size distributions (> 3 nm) and OH, H2SO4, and terpene concentrations were measured from April 1998 to August 2000 at a rural continental site in southern Germany. New particle formation (NPF) events were detected on 18% of all days, typically during midday hours under sunny and dry conditions. Surprisingly, most NPF events occurred during spring and winter, whereas the concentrations of aerosol precursors (H2SO4, monoterpenes) clearly peaked in summer. The number of newly formed particles correlated significantly with solar irradiance and ambient levels of H2SO4 and anti-correlated, especially in the cold season, with relative humidity and the condensational sink provided by pre-existing particles. The particle formation rates were experimentally estimated to be on order of 1 cm-3 s-1. Binary homogeneous H2SO4-H2O nucleation rates calculated from measured H2SO4 were substantially lower than this, even if assuming particle formation under the thermodynamic conditions on top of the boundary layer. The nucleation mode particle growth rates derived from the evolution of the size distribution were 2.6 nm h-1 on average, with a fraction of 0.7 nm h-1 attributed to the co-condensation of H2SO4/H2O/NH3. Turn-over rate calculations of measured monoterpenes and aromatic hydrocarbons suggest that especially the oxidation products of monoterpenes may contribute to the observed particle growth, although no indications were found that the reaction products of organic compounds would generally control the occurrence of NPF events.
Wavelength dependence of isotope fractionation in N2O photolysis
Authors: Kaiser J. , Röckmann T. , Brenninkmeijer C. A. M. , Crutzen P. J.
Published: April 2010

Abstract. In previous reports on isotopic fractionation in the ultraviolet photolysis of nitrous oxide (N2O) only enrichments of heavy isotopes in the remaining N2O fraction have been found. However, most direct photolysis experiments have been performed at wavelengths far from the absorption maximum at 182 nm. Here we present high-precision measurements of the 15N and 18O fractionation constants (e) in photolysis at 185 nm. Small, but statistically robust depletions of heavy isotopes for the terminal atoms in the linear N2O molecule are found. This means that the absorption cross sections s(15N14N16O) and s(14N218O) are larger than s(14N216O) at this specific wavelength. In contrast, the central N atom becomes enriched in 15N. The corresponding fractionation constants (±1 standard deviation) are 15e1 = s(15N14N16O)/s(14N216O) - 1 = (3.7 ± 0.2)%o 18e = s(14N218O)/s(14N216O) - 1 = (4.5 ± 0.2)%o  and   15e2 = s(\\chem{14N15N16O)/s(14N216O) - 1 = (-18.6 ± 0.5) %o To our knowledge, this is the first documented case of such a heavy isotope depletion in the photolysis of N2O which supports theoretical models and pioneering vacuum ultraviolet spectroscopic measurements of 15N substituted N2O species that predict fluctuations of e around zero in this spectral region (Selwyn and Johnston, 1981). Such a variability in isotopic fractionation could have consequences for atmospheric models of N2O isotopes since actinic flux varies also strongly over narrow wavelength regions between 175 and 200 nm due to the Schumann-Runge bands of oxygen. However, the spacing between maxima and minima of the fractionation constants and of the actinic flux differ by two orders of magnitude in the wavelength  domain. The wavelength dependence of fractionation constants in N2O photolysis can thus be approximated by a linear fit with negligible consequences on the actual value of the spectrally averaged fractionation constant. In order to establish this linear fit, additional measurements at wavelengths other than 185 nm were made using broadband light sources, namely D2, Hg/Xe and Sb lamps. The latter lamp was used in conjunction with various interference filters to shift the peak photolysis rate to longer wavelengths. From these experiments and existing data in the literature, a comprehensive picture of the wavelength dependence of N2O photolysis is created.