bluuebunny/crossref_metadata_2025 huggingface dataset:
A numerical evaluation of global oceanic emissions of α-pinene and isoprene
Authors: Luo G. , Yu F.
Published: April 2010

Abstract. A numerical evaluation of global oceanic emissions of α-pinene and isoprene based on both "bottom-up" and "top-down" methods is presented. As far as we know, this is the first quantification of global oceanic emission of α-pinene. We infer that the global "bottom-up" oceanic emissions of α-pinene and isoprene are 0.013 Tg C yr−1 and 0.32 Tg C yr−1, respectively. By constraining global chemistry model simulations with the shipboard measurement of Organics over the Ocean Modifying Particles in both Hemispheres summer cruise, we derived the global "top-down" oceanic α-pinene source of 35.1 Tg C yr−1 and isoprene source of 2.5 Tg C yr−1. The global oceanic α-pinene source and its impact on organic aerosol formation is significant based on "top-down" method, but is negligible based on "bottom-up" approach. Our research highlights the importance to carry out further research (especially measurements) to resolve the large offset in the derived oceanic organic emission based on two different approaches.
Emissions and ambient distributions of Biogenic Volatile Organic Compounds (BVOC) in a Ponderosa pine ecosystem: interpretation of PTR-MS mass spectra
Authors: Kim S. , Karl T. , Guenther A. , Tyndall G. , Orlando J. , Harley P. , Rasmussen R. , Apel E.
Published: April 2010

Abstract. Two proton-transfer-reaction mass spectrometry systems were deployed at the Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics and Nitrogen-Southern Rocky Mountain 2008 field campaign (BEACHON-SRM08; July to September 2008) at the Manitou Forest observatory in a Ponderosa pine woodland near Woodland Park, Colorado USA to simultaneously measure BVOC emissions and ambient distributions of their oxidation products. Here, we present mass spectral analysis in a wide range of masses (m/z=40+ to 210+) to assess our understanding of BVOC emissions and their photochemical process inside of the forest canopy. The biogenic terpenoids, 2-methyl-3-butene-2-ol (MBO, 50.2%) and several monoterpenes (MT, 33.5%) were identified as the dominant BVOC emissions from a transmission corrected mass spectrum, averaged over the daytime (11 am to 3 p.m., local time) of three days. To assess contributions of oxidation products of local BVOC, we calculate a oxidation product spectrum with the OH- and ozone-initiated oxidation product distribution mass spectra of two major BVOC at the ecosystem (MBO and β-pinene) that were observed from laboratory oxidation experiments. A majority (~73%) of the total signal could be explained by known compounds. The remainder are attributed to oxidation products of BVOC, emitted from nearby ecosystems and transported to the site, and oxidation products of unidentified BVOC emitted from the Ponderosa pine ecosystem.
Origin and transport of Mediterranean moisture and air
Authors: Schicker I. , Radanovics S. , Seibert P.
Published: April 2010

Abstract. Considering the Mediterranean as a region of high evaporation and low precipitation, evaluations of the moisture and precipitation sources and sinks in the Mediterranean basin have been carried out within the frame of the CIRCE project. In addition, residence time and stagnation/ventilation have been analysed to investigate transport to and from the Mediterranean basin and in the basin itself. A Lagrangian moisture diagnosis method calculating budgets of evaporation minus precipitation was applied to a 5.5 year (October 1999–April 2005) trajectory data set and evaluated for eight Mediterranean regions of interest. The Mediterranean basin has been identified as a major source of moisture and precipitation to the surrounding land area and to the basin itself. Regions of stagnation have been identified through the analysis of the average 24-hour and 5-day displacements for the four seasons, and the Po basin was identified as being strongly affected by stagnation. Evaluation of the transport to and from the basin shows that the Mediterranean is a crossroad of airstreams where air enters mainly from the northwest and continues in two separate airstreams: one turns towards southwest, passing over North Africa into the trade wind zone while the other one continues northeastwards through Central Asia.
Spatio-temporal aerosol optical characteristics over the Arabian Sea during the pre monsoon season
Authors: Kaskaoutis D. G. , Kalapureddy M. C. R. , Devara P. C. S. , Kosmopoulos P. G. , Nastos P. T. , Krishna Moorthy K. , Kambezidis H. D.
Published: April 2010

Abstract. Ship-borne Aerosol Optical Depth (AOD) measurements obtained by a sunphotometer have been used to retrieve the Ångström wavelength exponent (α). These measurements were obtained in the Arabian Sea (AS), where the surrounded arid-region influence is expected, during the pre-monsoon season. Spectral variation of α is typically considered in this study. It is found that the polynomial fit becomes more accurate in the wavelength band 340–1020 nm rather than the 340–870 nm. The coarse-mode (positive curvature in the ln τα vs ln λ) aerosols are mainly depicted in the Northern part of the AS closely associated with the nearby arid areas and fine-mode aerosols are mainly observed over the far and coastal AS regions. In the study period the mean AOD at 500 nm is 0.246±0.114 and the α340−1020 is 0.904±0.186. The α340−870 exhibits similar values (0.924±0.179), while significant differences revealed for the constant terms of the polynomial fit (a1 and (a2) proportionally to the wavelength band used for their determination. Observed day-to-day variability in the aerosol load and optical properties are direct consequence of the local winds and air-mass trajectories along with the position of the ship.
Size dependence of volume and surface nucleation rates for homogeneous freezing of supercooled water droplets
Authors: Kuhn T. , Earle M. E. , Khalizov A. F. , Sloan J. J.
Published: April 2010

Abstract. We investigated the relative roles of volume and surface nucleation in the freezing of water droplets. Nucleation experiments were carried out in a cryogenic laminar aerosol flow tube using supercooled liquid water aerosols with radii between about 1 and 3 μ m. Temperature- and size-dependent values of volume- and surface-based homogeneous nucleation rate between 234.8 and 236.2 K are derived with help of a microphysical model from aerosol compositions and size distributions based on infrared extinction measurements in the aerosol flow tube. The results show that the contribution from nucleation at the droplet surface increases with decreasing droplet radius and dominates over nucleation in the bulk droplet volume for droplets with radii smaller than approximately 5 μm. This is interpreted in terms of a lowered free energy of ice germ formation in the surface-based process and has implications for the parameterization of homogeneous ice nucleation in numerical models.
A daytime climatological distribution of high opaque ice cloud classes over the Indian summer monsoon region observed from 25-year AVHRR data
Authors: Devasthale A. , Grassl H.
Published: April 2010

Abstract. A daytime climatological spatio-temporal distribution of high opaque ice cloud (HOIC) classes over the Indian subcontinent (0–40° N, 60° E–100° E) is presented using 25-year data from the Advanced Very High Resolution Radiometers (AVHRRs) for the summer monsoon months. The HOICs are important for regional radiative balance, precipitation and troposphere-stratosphere exchange. In this study, HOICs are sub-divided into three classes based on their cloud top brightness temperatures (BT). Class I represents very deep convection (BT<220 K). Class II represents deep convection (220 K≤BT<233 K) and Class III background convection (233 K≤BT<253 K). Apart from presenting finest spatial resolution (0.1×0.1 degrees) and long-term climatology of such cloud classes from AVHRRs to date, this study for the first time illustrates on 1) how these three cloud classes are climatologically distributed during monsoon months, and 2) how their distribution changes during active and break monsoon conditions. It is also investigated that how many overshooting convective clouds reach the tropopause layer during individual monsoon months. It is seen that Class I and Class II clouds dominate the Indian subcontinent during monsoon. The movement of monsoon over continent is very well reflected in these cloud classes. During monsoon breaks strong suppression of convective activity is observed over the Arabian Sea and the western coast of India. On the other hand, the presence of such convective activity is crucial for active monsoon conditions and all-India rainfall. It is found that a significant fraction of HOICs (3–5%) reach the tropopause layer over the Bay of Bengal during June and over the north and northeast India during July and August. Many cases are observed when clouds penetrate the tropopause layer and reach the lower stratosphere. Such cases mostly occur during June compared to the other months.
IASI measurements of tropospheric ozone over Chinese megacities: Beijing, Shanghai, and Hong Kong
Authors: Dufour G. , Eremenko M. , Orphal J. , Flaud J.-M.
Published: April 2010

Abstract. IASI observations of tropospheric ozone over Beijing, Shanghai and Hong Kong during one year have been analysed, demonstrating the capability of space-borne infrared nadir measurements to probe both seasonal and daily variations of lower tropospheric ozone around megacities on the regional scale. The monthly variations of lower tropospheric ozone retrieved from IASI show the influence of the Asian summer monsoon that brings clean air masses from the Pacific during summer. They exhibit indeed a sharp ozone maximum in late spring and early summer (May–June) followed by a summer minimum. The time periods and the intensities of the maxima and of the decreases are latitude-dependent: they are more pronounced in Hong Kong and Shanghai than in Beijing. Moreover, IASI provides the opportunity to follow the spatial variations of ozone over the surroundings of each megacity as well as its daily variability. We show indeed that the large lower tropospheric ozone amounts observed with IASI are consistent with the highest population density distribution in each region, thus suggesting the anthropogenic origin of the large ozone amounts observed. Finally an analysis of the mean daily ozone profiles over each region for selected periods with high ozone events shows that the high ozone amounts observed during winter are likely related to descents of ozone-rich air from the stratosphere whereas in spring and summer the tropospheric ozone is likely enhanced by photochemical production in polluted areas and/or in fire plumes.
Atmospheric total gaseous mercury (TGM) concentrations and wet and dry deposition of mercury at a high-altitude mountain peak in south China
Authors: Fu X. W. , Feng X. , Dong Z. Q. , Yin R. S. , Wang J. X. , Yang Z. R. , Zhang H.
Published: April 2010

Abstract. China is regarded as the largest contributor of mercury (Hg) to the global atmospheric Hg budget. However, concentration levels and depositions of atmospheric Hg in China are poorly known. Continuous measurements of atmospheric total gaseous mercury (TGM) were carried out from May 2008 to May 2009 at the summit of Mt. Leigong in south China. Wet and dry deposition fluxes of Hg were also calculated following collection of precipitation, throughfall and litterfall. Atmospheric TGM concentrations averaged 2.80±1.51 ng m−3, which was highly elevated compared to global background values but much lower than semi-rural and industrial/urban areas in China, indicating great emissions of Hg in central, south and southwest China. Seasonal and diurnal variations of TGM were observed, which reflected variations in source intensity, deposition processes and meteorological factors. Wet deposition of Hg was quite low, while its dry deposition of Hg (litterfall + throughfall-direct wet deposition) constituted a major portion of total deposition (~88% for total mercury (THg) and 84% for methyl mercury (MeHg)). This highlights the importance of vegetation to Hg atmospheric cycling. In a remote forest ecosystem of China, dry deposition of TGM, especially gaseous elemental mercury (GEM), was very important for the depletion of atmospheric Hg. Elevated TGM level in ambient air may accelerate the foliar uptake of Hg through air which may partly explain the elevated Hg dry deposition fluxes observed in Mt. Leigong.
Using aircraft measurements to determine the refractive index of Saharan dust during the DODO experiments
Authors: McConnell C. L. , Formenti P. , Highwood E. J. , Harrison M. A. J.
Published: April 2010

Abstract. Much uncertainty in the value of the imaginary part of the refractive index of mineral dust contributes to uncertainty in the radiative effect of mineral dust in the atmosphere. A synthesis of optical, chemical and physical in-situ aircraft measurements from the DODO experiments during February and August 2006 are used to calculate the refractive index mineral dust encountered over West Africa. Radiative transfer modeling and measurements of broadband shortwave irradiance at a range of altitudes are used to test and validate these calculations for a specific dust event on 23 August 2006 over Mauritania. Two techniques are used to determine the refractive index: firstly a method combining measurements of scattering, absorption, size distributions and Mie code simulations, and secondly a method using composition measured on filter samples to apportion the content of externally mixed quartz, calcite and iron oxide-clay aggregates, where the iron oxide is represented by either hematite or goethite and clay by either illite or kaolinite. The imaginary part of the refractive index at 550 nm (ni550) is found to range between 0.0001i to 0.0046i, and where filter samples are available, agreement between methods is found depending on mineral combination assumed. The refractive indices are also found to agree well with AERONET data where comparisons are possible. ni550 is found to vary with dust source, which is investigated with the NAME model for each case. The relationship between both size distribution and ni550 on the accumulation mode single scattering albedo at 550nm (ω0550) are examined and size distribution is found to have no correlation to ω0550, while ni550 shows a strong linear relationship with ω0550. Radiative transfer modeling indicates that Mie-derived values of ni550 and the goethite-kaolinite combination resulted in the best agreement with irradiance measurements, for the particular dust event examined. The radiative effect of the dust is found to be very sensitive to the mineral combination (and hence refractive index) assumed.
Finding the missing stratospheric Bry: a global modeling study of CHBr3 and CH2Br2
Authors: Liang Q. , Stolarski R. S. , Kawa S. R. , Nielsen J. E. , Rodriguez J. M. , Douglass A. R. , Blake D. R. , Atlas E. L. , Ott L.
Published: April 2010

Abstract. Recent in situ and satellite measurements suggest a contribution of ~5 pptv to stratospheric inorganic bromine from short-lived bromocarbons. We conduct a modeling study of the two most important short-lived bromocarbons, bromoform (CHBr3) and dibromomethane (CH2Br2), with the Goddard Earth Observing System Chemistry Climate Model (GEOS CCM) to account for this missing stratospheric bromine. We derive a "top-down" emission estimate of CHBr3 and CH2Br2 using airborne measurements in the Pacific and North American troposphere and lower stratosphere (LS) obtained during previous NASA aircraft campaigns. Our emission estimate suggests that to reproduce the observed concentrations in the free troposphere, a global oceanic emission of 425 Gg Br yr−1 for CHBr3 and 57 Gg Br yr−1 for CH2Br2 is needed, with 60% of emissions from open ocean and 40% from coastal regions. Although our simple emission scheme assumes no seasonal variations, the model reproduces the observed seasonal variations of the short-lived bromocarbons with high concentrations in winter and low concentrations in summer. This indicates that the seasonality of short-lived bromocarbons is largely due to seasonality in their chemical loss and transport. The inclusion of CHBr3 and CH2Br2 contributes ~5 pptv bromine throughout the stratosphere. Both the source gases and inorganic bromine produced from the source gas degradation (BryVSLS) in the troposphere are transported into the stratosphere, and are equally important. Inorganic bromine accounts for half (2.5 pptv) of the bromine from the inclusion of CHBr3 and CH2Br2 near the tropical tropopause and its contribution rapidly increases to ~100% as altitude increases. More than 85% of the wet scavenging of BryVSLS occurs in large-scale precipitation below 500 hPa and BryVSLS in the stratosphere is not sensitive to convection.
Inter-comparison of four different carbon monoxide measurements techniques and evaluation of the long-term carbon monoxide time series of Jungfraujoch
Authors: Zellweger C. , Hüglin C. , Klausen J. , Steinbacher M. , Vollmer M. K. , Buchmann B.
Published: April 2010

Abstract. The 12-year record (1996–2007) of continuous carbon monoxide (CO) measurements of the high-alpine site Jungfraujoch (JFJ), Switzerland, was investigated with a focus on trend analysis. To date this is one of the longest time series of continuous CO measurements in the free troposphere over Central Europe. A significant negative trend was observed at JFJ with a decrease of 21.4±0.3% in the investigated period, or an average annual decrease of 2.65±0.04 ppb/yr (1.78%/yr). These results were compared with emission inventory data reported to the Long-range Transboundary Air Pollution (LRTAP) Convention. It could be shown that long range transport significantly influences the CO levels observed at JFJ, with air masses of non-European origin contributing to at least a third to observed mixing ratios. Such trend analysis and inter-comparison with emission inventories are only possible with data of known quality. To this end, the Non-dispersive Infrared Absorption (NDIR) technique used for CO measurements at JFJ was inter-compared over two months using three additional analytical techniques, namely Vacuum UV Resonance Fluorescence (VURF), gas chromatographic separation with a mercuric oxide reduction detector (GC/HgO), and gas chromatographic separation followed by reduction on a nickel catalyst and analysis by a flame ionization detector (GC/FID). The agreement among all techniques was better than 2% for one-hourly averages which confirmed the suitability of the NDIR method for CO measurements even at remote sites.
Size-resolved aerosol water-soluble ionic compositions in the summer of Beijing: implication of regional secondary formation
Authors: Guo S. , Hu M. , Wang Z. B. , Slanina J. , Zhao Y. L.
Published: April 2010

Abstract. To characterize aerosol pollution in Beijing, size-resolved aerosols were collected by MOUDIs during CAREBEIJING-2006 field campaign at Peking University (urban site) and Yufa (upwind rural site). Fine particle concentrations (PM1.8 by MOUDI) were 99.8±77.4 μg/m3 and 78.2±58.4 μg/m3, with PM1.8/PM10 ratios of 0.64±0.08 and 0.76±0.08 at PKU and Yufa, respectively, and secondary compounds accounted for more than 50% in fine particles. PMF model was used to resolve the particle modes. Three modes were resolved at Yufa, representing condensation, droplet and coarse mode. However, one more droplet mode with bigger size was resolved, which was considered probably from regional transport. Condensation mode accounted for 10%–60% of the total mass at both sites, indicating it must be taken into account in summer. The formation of sulfate was mainly attributed to in-cloud or aerosol droplet process (PKU 80%, Yufa 70%) and gas condensation process (PKU 14%, Yufa 22%). According to the thermodynamic instability of NH4NO3, size distributions of nitrate were classified as three categories by RH. The existence of Ca(NO3)2 in droplet mode indicated the reaction of HNO3 with crustal particles was also important in fine particles. Linear regression gave a rough estimation that 69% of the PM10 and 87% of the PM1.8 at PKU were regional contributions. Sulfate, ammonium and oxalate were formed regionally, with the regional contributions of 90%, 87% and 95% to PM1.8. Nitrate formation was local dominant. In summary regional secondary formation led to aerosol pollution in the summer of Beijing.
Pressure dependent deuterium fractionation in the formation of molecular hydrogen in formaldehyde photolysis
Authors: Nilsson E. J. K. , Andersen V. F. , Skov H. , Johnson M. S.
Published: April 2010

Abstract. The pressure dependence of the relative photolysis rates of HCHO and HCDO has been investigated using a new photochemical reactor at the University of Copenhagen. The relative photolysis rate of HCHO vs. HCDO under UVA lamp irradiation was measured at total pressures of 50, 200, 400, 600 and 1030 mbar. The relative dissociation rate kHCHO/kHCDO was found to depend strongly on pressure, varying from 1.1±0.1 at 50 mbar to 1.75±0.10 at 1030 mbar. The products of formaldehyde photodissociation are either H2+CO (molecular channel) or HCO+H (radical channel). The partitioning between the channels has been estimated using available values for the absorption cross section and quantum yield. As a result of the change in pressure with altitude the isotope effect for production of molecular hydrogen is found to change from a value of kH/kD=1.8±0.2 at the surface to unity at 50 km. The relative importance of the two product channels changes with altitude as a result of changes in both pressure and actinic flux. The study concludes that the δD of photochemical hydrogen produced in situ will increase substantially with altitude.
Chemical evolution of volatile organic compounds in the outflow of the Mexico City Metropolitan area
Authors: Apel E. C. , Emmons L. K. , Karl T. , Flocke F. , Hills A. J. , Madronich S. , Lee-Taylor J. , Fried A. , Weibring P. , Walega J. , Richter D. , Tie X. , Mauldin L. , Campos T. , Sive B. , Kleinman L. , Springston S. , Zaveri R. , Ortega J. , Voss P. , Blake D. , Baker A. , Warneke C. , Welsh-Bon D. , de Gouw J. , Zheng J. , Zhang R. , Rudolph J. , Junkermann W. , Riemer D. D.
Published: April 2010

Abstract. The volatile organic compound (VOC) distribution in the Mexico City Metropolitan Area (MCMA) and its evolution as it is uplifted and transported out of the MCMA basin was studied during the 2006 MILAGRO/MIRAGE-Mex field campaign. The results show that in the morning hours in the city center, the VOC distribution is dominated by non-methane hydrocarbons (NMHCs) but with a substantial contribution from oxygenated volatile organic compounds (OVOCs), predominantly from primary emissions. Alkanes account for a large part of the NMHC distribution in terms of mixing ratios. In terms of reactivity, NMHCs also dominate overall, especially in the morning hours. However, in the afternoon, as the boundary layer lifts and air is mixed and aged within the basin, the distribution changes as secondary products are formed. The WRF-Chem (Weather Research and Forecasting with Chemistry) model and MOZART (Model for Ozone and Related chemical Tracers) were able to reproduce the general features of the daytime cycle of the VOC OH reactivity distribution showing that NMHCs dominate the distribution except in the afternoon hours and that the VOC OH reactivity peaks in the early morning due to high morning emissions from the city into a shallow boundary layer. The WRF-Chem and MOZART models showed higher reactivity than the experimental data during the nighttime cycle, perhaps indicating problems with the modeled nighttime boundary layer height. In addition, a plume was studied in which air was advected out of the MCMA and intercepted downwind with the DOE G1 on 18~March and the NCAR C130 one day later on 19~March. A number of identical species measured aboard each aircraft gave insight into the chemical evolution of the plume as it aged and was transported as far as 1000 km downwind. Ozone and many OVOCs were photochemically produced in the plume. The WRF-Chem and MOZART models were used to examine the spatial and temporal extent of the 19~March plume and to help interpret the OH reactivity in the downwind plume. The model results generally showed good agreement with experimental results for the total VOC OH reactivity downwind and gave insight into the distributions of VOC chemical classes downwind. A box model with detailed gas phase chemistry (NCAR Master Mechanism), initialized with concentrations observed at one of the ground sites in the MCMA, was used to examine the expected evolution of specific VOCs over a 1–2~day period. The models clearly supported the experimental evidence for NMHC oxidation leading to the formation of OVOCs downwind, which then become the primary fuel for ozone production far away from the MCMA.
Global atmospheric budget of acetaldehyde: 3-D model analysis and constraints from in-situ and satellite observations
Authors: Millet D. B. , Guenther A. , Siegel D. A. , Nelson N. B. , Singh H. B. , de Gouw J. A. , Warneke C. , Williams J. , Eerdekens G. , Sinha V. , Karl T. , Flocke F. , Apel E. , Riemer D. D. , Palmer P. I. , Barkley M.
Published: April 2010

Abstract. We construct a global atmospheric budget for acetaldehyde using a 3-D model of atmospheric chemistry (GEOS-Chem), and use an ensemble of observations to evaluate present understanding of its sources and sinks. Hydrocarbon oxidation provides the largest acetaldehyde source in the model (130 Tg a−1), with alkanes, alkenes, ethanol, and isoprene the main precursors. We use an updated chemical mechanism for GEOS-Chem, and photochemical acetaldehyde yields are consistent with the Master Chemical Mechanism. We apply SeaWiFS satellite observations to define the global distribution of light absorption due to marine dissolved organic matter (DOM), and estimate the corresponding sea-to-air acetaldehyde flux based on measured photoproduction rates from DOM. The resulting net ocean emission is 58 Tg a−1, the second largest global source of acetaldehyde. Quantitative model evaluation over the ocean is complicated by known measurement artifacts in clean air. Simulated concentrations in surface air over the ocean generally agree well with aircraft measurements, though the model tends to overestimate the vertical gradient. PAN:NOx ratios are well-simulated in the marine boundary layer, providing some support for the modeled ocean source. A key uncertainty is the acetaldehyde turnover time in the ocean mixed layer. We introduce the Model of Emissions of Gases and Aerosols from Nature (MEGANv2.1) for acetaldehyde and ethanol and use it to quantify their net flux from living terrestrial plants. Including emissions from decaying plants the total direct acetaldehyde source from the land biosphere is 22 Tg a−1. Other terrestrial acetaldehyde sources include biomass burning (3 Tg a−1) and anthropogenic emissions (2 Tg a−1). Simulated concentrations in the continental boundary layer are generally unbiased and capture the spatial gradients seen in observations over North America, Europe, and tropical South America. However, the model underestimates acetaldehyde levels in urban outflow, suggesting a missing source in polluted air. Ubiquitous high measured concentrations in the free troposphere are not captured by the model, and based on present understanding are not consistent with concurrent measurements of PAN and NOx. We find no compelling evidence for a widespread missing acetaldehyde source in the free troposphere. We estimate the current US source of ethanol and acetaldehyde (primary+secondary) at 1.3 Tg a−1 and 7.0 Tg a−1, approximately 60% and 400% of the corresponding increases expected for a national transition from gasoline to ethanol fuel.
Relationship between wind speed and aerosol optical depth over remote ocean
Authors: Huang H. , Thomas G. E. , Grainger R. G.
Published: April 2010

Abstract. The effect of wind speed on aerosol optical depth (AOD) at 550 nm over remote ocean regions is investigated. Remote ocean regions are defined by the combination of AOD from satellite observation and wind direction from ECMWF. According to our definition, many oceanic regions cannot be taken as remote ocean regions due to long-range transportation of aerosols from continents. Highly correlated linear relationships are found in remote ocean regions with a wind speed range of 4–20 ms−1. The enhancement of AOD at high wind speed is explained as the increase of sea salt aerosol production.
Tropospheric photooxidation of CF3CH2CHO and CF3(CH2)2CHO initiated by Cl atoms and OH radicals
Authors: Antiñolo M. , Jiménez E. , Notario A. , Martínez E. , Albaladejo J.
Published: April 2010

Abstract. The absolute rate coefficients for the tropospheric reactions of chlorine (Cl) atoms and hydroxyl (OH) radicals with CF3CH2CHO and CF3(CH2)2CHO were measured as a function of temperature (263–371 K) and pressure (50–215 Torr of He) by pulsed UV laser photolysis techniques. Vacuum UV resonance fluorescence was employed to detect and monitor the time evolution of Cl atoms. Laser induced fluorescence was used in this work as a detection of OH radicals as a function of reaction time. No pressure dependence of the bimolecular rate coefficients, kCl and kOH, was found at all temperatures. At room temperature kCl and kOH were (in 10−11 cm3 molecule−1 s−1): kCl(CF3CH2CHO) = (1.55±0.53); kCl(CF3(CH2)2CHO) = (3.39±1.38); kOH(CF3CH2CHO) = (0.259±0.050); kOH(CF3(CH2)2CHO) = (1.28±0.24). A slightly negative temperature dependence of kCl was observed for CF3CH2CHO and CF3(CH2)2CHO, and kOH(CF3CH2CHO). In contrast, kOH(CF3(CH2)2CHO) did not exhibit a temperature dependence in the studied ranged. Arrhenius expressions for these reactions were: kCl(CF3CH2CHO) =(4.4±1.0) × 10−11 exp{−(316±68)/T} cm3 molecule−1 s−1, kCl(CF3(CH2)2CHO) = (2.9±0.7) × 10−10 exp{−625±80)/T} cm3 molecule−1 s−1, kOH(CF3CH2CHO) = (7.8±2.2) × 10−12 exp{−(314±90)/T} cm3 molecule−1 s−1. The atmospheric impact of the homogeneous removal by OH radicals and Cl atoms of these fluorinated aldehydes is discussed in terms of the global atmospheric lifetimes, taking into account different degradation pathways. The calculated lifetimes show that atmospheric oxidation of CF3(CH2)xCHO are globally dominated by OH radicals, however reactions initiated by Cl atoms can act as a source of free radicals at dawn in the troposphere.
An investigation of the origins of reactive gaseous mercury in the Mediterranean marine boundary layer
Authors: Sprovieri F. , Hedgecock I. M. , Pirrone N.
Published: April 2010

Abstract. Atmospheric mercury concentrations were measured during two oceanographic cruise campaigns covering the Adriatic Sea, the first during the autumn in 2004 and the second in the summer of 2005. The inclement weather during the autumn campaign meant that no clear in-situ production of oxidised gas phase mercury was seen, and that events where high values of HgII(g) and/or Hg associated with particulates (HgP) were observed, were the result of plumes from anthropogenic emission sources. During the summer campaign however, the by now rather familiar diurnal variation of HgII(g) concentration, with maxima around midday, was observed. Again there were events when high HgII(g) and particulates (HgP) concentrations were seen which did not fit with the pattern of daily in-situ HgII(g) production, which were traceable, with the help of back trajectory calculations, to anthropogenic emission sources. All the emission plumes encountered could be traced back to ports, not all of which are associated with major industrial installations. It therefore seems likely in theses cases that the emissions are either due to shipping or to port activities. Box modelling studies of the summer 2005 campaign show that although the in-situ production of HgII(g) occurs in the MBL, the exact chemical mechanism responsible is difficult to determine. However given the high O3 concentrations encountered during this campaign it seems clear that if Hg0 does react with O3, it does not produce gas phase HgII, and the reaction between Hg0 and OH if it occurs, does not contribute appreciably to HgII(g) production.
Validation of Ozone Monitoring Instrument (OMI) ozone profiles and stratospheric ozone columns with Microwave Limb Sounder (MLS) measurements
Authors: Liu X. , Bhartia P. K. , Chance K. , Froidevaux L. , Spurr R. J. D. , Kurosu T. P.
Published: April 2010

Abstract. We validate OMI ozone profiles between 0.22–215 hPa and stratospheric ozone columns down to 215 hPa (SOC215) against v2.2 MLS data from 2006. The validation demonstrates convincingly that SOC can be derived accurately from OMI data alone, with errors comparable to or smaller than those from current MLS retrievals, and it demonstrates implicitly that tropospheric ozone column can be retrieved accurately from OMI or similar nadir-viewing ultraviolet measurements alone. The global mean biases are within 2.5% above 100 hPa and 5–10% below 100 hPa; the standard deviations (1σ) are 3.5–5% between 1–50 hPa, 6–9% above 1 hPa and 8–15% below; 50 hPa. OMI shows some latitude and solar zenith angle dependent biases, but the mean biases are mostly within 5% and the standard deviations are mostly within 2–5% except for low altitudes and high latitudes. The excellent agreement with MLS data shows that OMI retrievals can be used to augment the validation of MLS and other stratospheric ozone measurements made with even higher vertical resolution than that for OMI. OMI SOC215 shows a small bias of −0.6% with a standard deviation of 2.8%. When compared as a function of latitude and solar zenith angle, the mean biases are within 2% and the standard deviations range from 2.1 to 3.4%. Assuming 2% precision for MLS SOC215, we deduce that the upper limits of random-noise and smoothing errors for OMI SOC215 range from 0.6% in the southern tropics to 2.8% at northern middle latitudes.
Tracer concentration profiles measured in central London as part of the REPARTEE campaign
Authors: Martin D. , Petersson K. F. , White I. R. , Henshaw S. J. , Nickless G. , Lovelock A. , Barlow J. F. , Dunbar T. , Wood C. R. , Shallcross D. E.
Published: April 2010

Abstract. There have been relatively few tracer experiments carried out that have looked at vertical plume spread in urban areas. In this paper we present results from cyclic perfluorocarbon tracer experiments carried out in 2006 and 2007 in central London centred on the BT Tower as part of the REPARTEE (Regent's Park and Tower Environmental Experiment) campaign. The height of the tower gives a unique opportunity to study dispersion over a large vertical gradient. These gradients are then compared with classical Gaussian profiles of the relevant stability classes over a range of distances as well as interpretation of data with reference to both anemometry and LIDAR measurements made. Data are then compared with an operational model and contrasted with data taken in central London as part of the DAPPLE campaign looking at dosage compared with non-dimensionalised distance from source. Such analysis illustrates the feasibility of the use of these empirical correlations over these prescribed distances in central London.
Total aerosol effect: radiative forcing or radiative flux perturbation?
Authors: Lohmann U. , Rotstayn L. , Storelvmo T. , Jones A. , Menon S. , Quaas J. , Ekman A. , Koch D. , Ruedy R.
Published: April 2010

Abstract. Uncertainties in aerosol radiative forcings, especially those associated with clouds, contribute to a large extent to uncertainties in the total anthropogenic forcing. The interaction of aerosols with clouds and radiation introduces feedbacks which can affect the rate of rain formation. In former assessments of aerosol radiative forcings, these effects have not been quantified. Also, with global aerosol-climate models simulating interactively aerosols and cloud microphysical properties, a quantification of the aerosol forcings in the traditional way is difficult to properly define. Here we argue that fast feedbacks should be included because they act quickly compared with the time scale of global warming. We show that for different forcing agents (aerosols and greenhouse gases) the radiative forcings as traditionally defined agree rather well with estimates from a method, here referred to as radiative flux perturbations (RFP), that takes these fast feedbacks and interactions into account. Based on our results, we recommend RFP as a valid option to compare different forcing agents, and to compare the effects of particular forcing agents in different models.
The sensitivity of stratocumulus-capped mixed layers to cloud droplet concentration: do LES and mixed-layer models agree?
Authors: Uchida J. , Bretherton C. S. , Blossey P. N.
Published: April 2010

Abstract. The sensitivity of a stratocumulus-capped mixed layer to a change in cloud droplet concentration is evaluated with a large-eddy simulation (LES) and a mixed layer model (MLM), to see if the two model types agree on the strength of the second aerosol indirect effect. Good agreement can be obtained if the MLM entrainment closure explicitly reduces entrainment efficiency proportional to the rate of cloud droplet sedimentation at cloud top for cases in which the LES-simulated boundary layer remains well mixed, with a single peak in the vertical profile of vertical velocity variance. To achieve this agreement, the MLM entrainment closure and the drizzle parameterization must be modified from their observationally-based defaults. This is because the LES advection scheme and microphysical parameterization significantly bias the entrainment rate and precipitation profile compared to observational best guesses. Before this modification, the MLM simulates more liquid water path and much more drizzle at a given droplet concentration than the LES and is more sensitive to droplet concentration, even undergoing a drizzle-induced boundary layer collapse at low droplet concentrations. After this modification, both models predict a similar decrease of cloud liquid water path as droplet concentration increases, cancelling 30–50% of the Twomey effect for our case. The agreement breaks down at the lowest simulated droplet concentrations, for which the boundary layer in the LES is not well mixed. Our results highlight issues with both types of model. Potential LES biases due to inadequate resolution, subgrid mixing and microphysics must be carefully considered when trying to make a quantitative inference of the second indirect effect from an LES of a stratocumulus-topped boundary layer. On the other hand, even slight internal decoupling of the boundary layer invalidates MLM-predicted sensitivity to droplet concentrations.
Marine boundary layer over the subtropical southeast Pacific during VOCALS-REx – Part 2: Synoptic variability
Authors: Rahn D. A. , Garreaud R. D.
Published: April 2010

Abstract. In the second part of this work we study the day-to-day variability of the marine atmospheric boundary layer (MBL) over the subtropical southeast Pacific using primarily results from a numerical simulation that covered the whole VOCALS-REx period (October–November 2008). In situ and satellite-derived observations of the MBL height in the offshore region indicate rapid, significant variations (from 500 m to 1700 m a.s.l. over a few days) during October. These MBL changes are connected with the passage of midlatitude troughs that altered the large-scale environment over the VOCALS-REx region. In contrast, the synoptic forcing and MBL changes were less prominent during November. Modelled and observed MBL depth at Point Omega (20° S, 85° W) compare quite well during October (but the simulation is on average 200 m lower) while in November the simulation does not perform as well. Each term in the prognostic local MBL height equation (horizontal MBL height advection, large scale vertical velocity at MBL top, and entrainment velocity) is calculated explicitly from the simulation except the entrainment velocity which is calculated as the residual of the other terms in the equation. While the vertical velocity and residual terms are opposing and generally have the largest magnitude on average, it is the variability in the advection that explains most of the large changes in the MBL depth. Examination of several cases during VOCALS-REx suggests that the advective term is in turn largely controlled by changes in wind direction, driven by midlatitude activity, acting on a MBL that generally slopes down toward the coast. In one phase, the subtropical anticyclone is reinforced and extends toward the Chilean coast, leading to easterly wind that advects low MBL heights from the coast as far as Point Omega. The opposite phase occurs after the passage of an extratropical cyclone over southern Chile, leading to southwesterly wind that advects a deeper MBL towards subtropical latitudes.
Nanoparticles in boreal forest and coastal environment: a comparison of observations and implications of the nucleation mechanism
Authors: Lehtipalo K. , Kulmala M. , Sipilä M. , Petäjä T. , Vana M. , Ceburnis D. , Dupuy R. , O'Dowd C.
Published: April 2010

Abstract. The detailed mechanism of secondary new particle formation in the atmosphere is still under debate. It is proposed that particle formation happens via activation of 1–2 nm atmospheric neutral molecular clusters and/or large molecules. Since traditional instrumentation does not reach these sizes, the hypothesis has not yet been verified. By directly measuring particle size distributions down to mobility diameters of about 1.3 nm with a pulse-height CPC we provide evidence of the nucleation mechanism in coastal environment (Mace Head, Ireland) and in boreal forest (Hyytiälä, Finland). In both places neutral sub-3 nm condensation nuclei (nano-CN) were continuously present, even when no new particle formation was detected. In Mace Head, however, the concentration of the nano-CN was far too low to account for the particle formation rates during particle bursts. Thus the results imply that on coastal sites new particle formation initiates, as proposed earlier, via homogenous nucleation from biogenic iodine vapors. In contrary, activation of pre-existing nano-CN remains a possible explanation in the boreal forest, but the observed concentrations are not the limiting factor for the particle formation events.
Process analysis and sensitivity study of regional ozone formation over the Pearl River Delta, China, during the PRIDE-PRD2004 campaign using the CMAQ model
Authors: Wang X. , Zhang Y. , Hu Y. , Zhou W. , Lu K. , Zhong L. , Zeng L. , Shao M. , Hu M. , Russell A. G.
Published: April 2010

Abstract. In this study, the Community Multiscale Air Quality (CMAQ) modeling system is used to simulate the ozone (O3) episodes during the Program of Regional Integrated Experiments of Air Quality over the Pearl River Delta, China, in October 2004 (PRIDE-PRD2004). The simulation suggests that O3 pollution is a regional phenomenon in the PRD. Elevated O3 levels often occurred in the southwestern inland PRD, Pearl River estuary (PRE), and southern coastal areas during the 1-month field campaign. Three evolution patterns of simulated surface O3 are summarized based on different near-ground flow conditions. More than 75% of days featured interaction between weak synoptic forcing and local sea-land circulations. Integrated process rate (IPR) analysis shows that photochemical production is the dominant contributor to O3 enhancement from 09:00 to 15:00 LST (local standard time) in the atmospheric boundary layer over most areas with elevated O3 occurrence in the mid-afternoon. The simulated ozone production efficiency is 2–8 O3 molecules per NOx molecule oxidized in areas with high O3 chemical production. Precursors of O3 originating from different source regions in the central PRD are mixed during transport to downwind rural areas during nighttime and early morning, where they then contribute to the daytime O3 photochemical production. Such close interactions among precursor emissions, transports, and O3 photochemical production result in the regional O3 pollution over the PRD. Sensitivity studies suggest that O3 formation is volatile organic compound-limited in the central inland PRD, PRE, and surrounding coastal areas with less chemical aging (NOx/NOy>0.6), but is NOx-limited in the rural southwestern PRD with photochemically aged air (NOx/NOy<0.3).
Hygroscopic properties of Amazonian biomass burning and European background HULIS and investigation of their effects on surface tension with two models linking H-TDMA to CCNC data
Authors: Fors E. O. , Rissler J. , Massling A. , Svenningsson B. , Andreae M. O. , Dusek U. , Frank G. P. , Hoffer A. , Bilde M. , Kiss G. , Janitsek S. , Henning S. , Facchini M. C. , Decesari S. , Swietlicki E.
Published: April 2010

Abstract. HUmic-LIke Substances (HULIS) have been identified as major contributors to the organic carbon in atmospheric aerosol. The term HULIS is used to describe the organic material found in aerosol particles which resembles the humic organic material in river and sea water and in soils. In this study two sets of filter samples from atmospheric aerosols were collected at different sites. One sample was collected at the K-puszta rural site in Hungary, about 80 km SE of Budapest, and a second set of samples was collected at a site in Rondônia, Amazonia, Brazil, during the LBA-SMOCC biomass burning season experiment. HULIS were extracted from the samples, and their hygroscopic properties were studied using a Hygroscopicity Tandem Differential Mobility Analyzer (H-TDMA) at relative humidity (RH) <100%, and a cloud condensation nucleus counter (CCNC) at RH >100%. The H-TDMA measurements were carried out at a dry diameter of 100 nm and for RH ranging from 30 to 98%. At 90% RH the HULIS samples showed diameter growth factors between 1.04 and 1.07, reaching values of 1.4 at 98% RH. The cloud nucleating properties of the two sets of aerosol samples were analyzed using two types of thermal static cloud condensation nucleus counters (CCNC). Two different parameterization models were used to investigate the potential effect of HULIS surface activity, both yielding similar results. For the K-puszta winter HULIS sample, the surface tension at the point of activation was estimated to be lowered by between 34% (47.7 mN/m) and 31% (50.3 mN/m) for dry sizes between 50 and 120 nm in comparison to pure water. A moderate lowering was also observed for the entire water soluble aerosol sample, including both organic and inorganic compounds, where the surface tension was decreased by between 2% (71.2 mN/m) and 13% (63.3 mN/m).