bluuebunny/crossref_metadata_2025 huggingface dataset:
Tropospheric NO2 columns: a comparison between model and retrieved data from GOME measurements
Authors: Lauer A. , Dameris M. , Richter A. , Burrows J. P.
Published: April 2010

Abstract. Tropospheric NO2 plays a variety of significant roles in atmospheric chemistry. In the troposphere it is one of the most significant precursors of photochemical ozone (O3) production and nitric acid (HNO3). In this study tropospheric NO2 columns were calculated by the fully coupled chemistry-climate model ECHAM4.L39(DLR)/CHEM. These have been compared with tropospheric NO2 columns, retrieved using the tropospheric excess method from measurements by the Global Ozone Monitoring Experiment (GOME) of up-welling earthshine irradiance and the extraterrestrial radiance. GOME is part of the core payload of the second European Research Satellite (ERS-2). For this study the first five years of GOME measurements have been used. The period of five years of observational data is sufficient to enable a comparison based on climatological averages and with global coverage, focussing on the geographical distribution of the tropospheric NO2, for the first time. A new approach of analysing regional differences (i.e. on continental scales) by calculating individual averages for different environments provides more detailed information about specific NOx sources and of their seasonal variations. The results obtained enable the validity of the model NO2 source distribution and the assumptions used to separate tropospheric and stratospheric parts of the NO2 column amount from the satellite measurements to be investigated.
A multi-decadal history of biomass burning plume heights identified using aerosol index measurements
Authors: Guan H. , Esswein R. , Lopez J. , Bergstrom R. , Warnock A. , Follette-Cook M. , Fromm M. , Iraci L.
Published: April 2010

Abstract. We have quantified the relationship between Aerosol Index (AI) measurements and plume height for young biomass burning plumes using coincident OMI and CALIPSO measurements. This linear relationship allows the determination of high-altitude plumes wherever AI data are available, and it provides a data set for validating global fire plume injection heights in chemistry transport models. We find that all plumes detected from June 2006 to February 2009 with an AI value ≥9 are located at altitudes higher than 5 km. Older high-altitude plumes have lower AI values than young plumes at similar altitudes. We have examined available AI data from the OMI and TOMS instruments (1978–2009) and find that large AI plumes occur more frequently over North America than over Australia or Russia/Northeast Asia. According to the derived relationship, during this time interval, 181 plumes reached altitudes above 8 km. One hundred and thirty-two had injection heights ≥8 km but below 12 km, and 49 were lofted to 12 km or higher, including 14 plumes injected above 16 km.
Retrieval of atmospheric parameters from GOMOS data
Authors: Kyrölä E. , Tamminen J. , Sofieva V. , Bertaux J. L. , Hauchecorne A. , Dalaudier F. , Fussen D. , Vanhellemont F. , Fanton d'Andon O. , Barrot G. , Guirlet M. , Mangin A. , Blanot L. , Fehr T. , Saavedra de Miguel L. , Fraisse R.
Published: April 2010

Abstract. The Global Ozone Monitoring by Occultation of Stars (GOMOS) instrument on board the European Space Agency's ENVISAT satellite measures attenuation of stellar light in occultation geometry. Daytime measurements also record scattered solar light from the atmosphere. The wavelength regions are the ultraviolet-visible band 248–690 nm and two infrared bands at 755–774 nm and at 926–954 nm. From UV-Visible and IR spectra the vertical profiles of O3, NO2, NO3, H2O, O2 and aerosols can be retrieved. In addition there are two 1 kHz photometers at blue 473–527 nm and red 646–698 nm. Photometer data are used to correct spectrometer measurements for scintillations and to retrieve high resolution temperature profiles as well as gravity wave and turbulence parameters. Measurements cover altitude region 5–150 km. Atmospherically valid data are obtained in 15–100 km. In this paper we present an overview of the GOMOS retrieval algorithms for stellar occultation measurements. The low signal-to-noise ratio and the refractive effects due to the point source nature of stars have been important drivers in the development of GOMOS retrieval algorithms. We present first the Level 1b algorithms that are used to correct instrument related disturbances in the spectrometer and photometer measurements The Level 2 algorithms deal with the retrieval of vertical profiles of atmospheric gaseous constituents, aerosols and high resolution temperature. We divide the presentation into correction for refractive effects, high resolution temperature retrieval and spectral/vertical inversion. The paper also includes discussion about the GOMOS algorithm development, expected improvements, access to GOMOS data and alternative retrieval approaches.
Role of aldehyde chemistry and NOx concentrations in secondary organic aerosol formation
Authors: Chan A. W. H. , Chan M. N. , Surratt J. D. , Chhabra P. S. , Loza C. L. , Crounse J. D. , Yee L. D. , Flagan R. C. , Wennberg P. O. , Seinfeld J. H.
Published: April 2010

Abstract. Aldehydes are an important class of products from atmospheric oxidation of hydrocarbons. Isoprene (2-methyl-1,3-butadiene), the most abundantly emitted atmospheric non-methane hydrocarbon, produces a significant amount of secondary organic aerosol (SOA) via methacrolein (a C4-unsaturated aldehyde) under urban high-NOx conditions. Previously, we have identified peroxy methacryloyl nitrate (MPAN) as the important intermediate to isoprene and methacrolein SOA in this NOx regime. Here we show that as a result of this chemistry, NO2 enhances SOA formation from methacrolein and two other α, β-unsaturated aldehydes, specifically acrolein and crotonaldehyde, a NOx effect on SOA formation previously unrecognized. Oligoesters of dihydroxycarboxylic acids and hydroxynitrooxycarboxylic acids are observed to increase with increasing NO2/NO ratio, and previous characterizations are confirmed by both online and offline high-resolution mass spectrometry techniques. Molecular structure also determines the amount of SOA formation, as the SOA mass yields are the highest for aldehydes that are α, β-unsaturated and contain an additional methyl group on the α-carbon. Aerosol formation from 2-methyl-3-buten-2-ol (MBO232) is insignificant, even under high-NO2 conditions, as PAN (peroxy acyl nitrate, RC(O)OONO2) formation is structurally unfavorable. At atmospherically relevant NO2/NO ratios, the SOA yields from isoprene high-NOxphotooxidation are 3 times greater than previously measured at lower NO2/NO ratios. At sufficiently high NO2 concentrations, in systems of α, β-unsaturated aldehydes, SOA formation from subsequent oxidation of products from acyl peroxyl radicals+NO2 can exceed that from RO2+HO2 reactions under the same inorganic seed conditions, making RO2+NO2 an important channel for SOA formation.
Source apportionment of PM2.5 in Cork Harbour, Ireland using a combination of single particle mass spectrometry and quantitative semi-continuous measurements
Authors: Healy R. M. , Hellebust S. , Kourtchev I. , Allanic A. , O'Connor I. P. , Bell J. M. , Sodeau J. R. , Wenger J. C.
Published: April 2010

Abstract. An aerosol time-of-flight mass spectrometer (ATOFMS) was co-located with a suite of semi-continuous instrumentation for the quantitative measurement of elemental carbon (EC), organic carbon (OC), sulfate, particle number and PM2.5 mass at a site in Cork Harbour, Ireland for three weeks in August 2008. Off-line analysis of polar organic markers was also performed for the same period. The data collected was used to identify and apportion local and regional sources of PM2.5. Over 550 000 ATOFMS particle mass spectra were generated and classified using the K-means algorithm. The vast majority of particles ionised by the ATOFMS were attributed to local sources, although one class of carbonaceous particles detected is attributed to North American or Canadian anthropogenic sources. The temporality of the ambient ATOFMS particle classes was subsequently used in conjunction with the semi-continuous measurements to apportion PM2.5 mass using positive matrix factorisation. Six factors were obtained, corresponding to vehicular traffic, marine, long-range transport, power generation, domestic solid fuel combustion and shipping traffic. The estimated contribution of each factor to the measured PM2.5 mass was 23%, 14%, 13%, 11%, 5% and 1.5%, respectively. Shipping was found to contribute 18% of the measured particle number (20–600 nm mobility diameter), and thus may have implications for human health considering the size and composition of ship exhaust particles.
Intercomparison of aerosol-cloud-precipitation interactions in stratiform orographic mixed-phase clouds
Authors: Muhlbauer A. , Hashino T. , Xue L. , Teller A. , Lohmann U. , Rasmussen R. M. , Geresdi I. , Pan Z.
Published: April 2010

Abstract. Anthropogenic aerosols serve as a source of both cloud condensation nuclei (CCN) and ice nuclei (IN) and affect microphysical properties of clouds. Increasing aerosol number concentrations is hypothesized to retard the cloud droplet collision/coalescence and the riming in mixed-phase clouds, thereby decreasing orographic precipitation. This study presents results from a model intercomparison of 2-D simulations of aerosol-cloud-precipitation interactions in stratiform orographic mixed-phase clouds. The sensitivity of orographic precipitation to changes in the aerosol number concentrations is analyzed and compared for various dynamical and thermodynamical situations. Furthermore, the sensitivities of microphysical processes such as collision/coalescence, aggregation and riming to changes in the aerosol number concentrations are evaluated and compared. The participating models are the Consortium for Small-Scale Modeling's (COSMO) model with bulk-microphysics, the Weather Research and Forecasting (WRF) model with bin-microphysics and the University of Wisconsin modeling system (UWNMS) with a spectral ice-habit prediction microphysics scheme. All models are operated on a cloud-resolving scale with 2 km horizontal grid spacing. The results of the model intercomparison suggest that the sensitivity of orographic precipitation to aerosol modifications varies greatly from case to case and from model to model. Neither a precipitation decrease nor a precipitation increase is found robustly in all simulations. Qualitative robust results can only be found for a subset of the simulations but even then quantitative agreement is scarce. Estimates of the second indirect aerosol effect on orographic precipitation are found to range from –19% to 0% depending on the simulated case and the model. Similarly, riming is shown to decrease in some cases and models whereas it increases in others which implies that a decrease in riming with increasing aerosol load is not a robust result. Furthermore, it is found that neither a decrease in cloud droplet coalescence nor a decrease in riming necessarily implies a decrease in precipitation due to compensation effects by other microphysical pathways. The simulations suggest that mixed-phase conditions play an important role in reducing the overall susceptibility of clouds and precipitation with respect to changes in the aerosols number concentrations. As a consequence the indirect aerosol effect on precipitation is suggested to be less pronounced or even inverted in regions with high terrain (e.g., the Alps or Rocky Mountains) or in regions where mixed-phase microphysics climatologically plays an important role for orographic precipitation.
Direct observation of aerosol particles in aged agricultural biomass burning plumes impacting urban atmospheres
Authors: Li W. Y. , Shao L. Y.
Published: April 2010

Abstract. Emissions from agricultural biomass burning (ABB) in northern China have a significant impact on the regional and the global climate. According to the Giovanni's Aerosol optical depth (AOD) map, the monthly average AOD at 550 nm in northern China in 2007 shows a maximum value of 0.7 in June, suggesting that episodes of severe aerosol pollution occurred in this region. Aerosol particles were collected in urban Beijing during regional brown hazes from 12 to 30 June, 2007. Transmission electron microscopy with energy-dispersive X-ray spectrometry characterized the morphology, composition, and mixing state of aerosol particles. Potassium salts (K2SO4 and KNO3), ammonium sulfate, soot, and organic particles predominated in fine particles (diameter <1 μm) collected from 12 to 20 June, 2007. In contrast, from 21 to 30 June, 2007, ammonium sulfate, soot, and organic particles were dominant. Potassium-dominant particles as a tracer of biomass burning, together with wildfire maps, show that intensive regional ABB in northern China from 10 to 20 June, 2007 contributed significantly to the regional haze. After long-range transport, ABB particles exhibited marked changes in their morphology, elemental composition, and mixing state. Heterogeneous reactions completely converted KCl particles from ABB into K2SO4 and KNO3. Soot particles were generally mixed with potassium salts, ammonium salts, and organic particles. In addition, the abundant aged organic particles and soluble salts emitted by ABB become more hygroscopic and increase their size during long-range transport, becoming in effect additional cloud condensation nuclei. The high AOD (average value at 2.2) during 12 to 20 June, 2007, in Beijing is partly explained by the hygroscopic growth of aged fine aerosol particles and by the strong absorption of internally mixed soot particles, both coming from regional ABB emissions.
Isoprene nitrates: preparation, separation, identification, yields, and atmospheric chemistry
Authors: Lockwood A. L. , Shepson P. B. , Fiddler M. N. , Alaghmand M.
Published: April 2010

Abstract. Isoprene is an important atmospheric volatile organic compound involved in ozone production and NOx (NO+NO2) sequestration and transport. Isoprene reaction with OH in the presence of NO can form either isoprene nitrates or convert NO to NO2 which can photolyze to form ozone. While it has been shown that isoprene nitrate production can represent an important sink for NOx in forest impacted environments, there is little experimental knowledge of the relative importance of the individual isoprene nitrate isomers, each of which has a different fate and reactivity. In this work, we have identified the 8 individual isomers and determined their total and individual production yields. The overall yield of isoprene nitrates at atmospheric pressure and 295 K was found to be 0.070(+0.025/–0.015). Three isomers, the (4,3)-IN, (1,2)-IN and Z-(4,1)-IN represent 90% of the total IN yield. We also determined the ozone rate constants for three of the isomers, and have calculated their atmospheric lifetimes, which range from ~1–2 h, making their oxidation products likely more important as atmospheric organic nitrates and sinks for nitrogen.
Terpenylic acid and related compounds: precursors for dimers in secondary organic aerosol from the ozonolysis of α and β-pinene
Authors: Yasmeen F. , Vermeylen R. , Szmigielski R. , Iinuma Y. , Böge O. , Herrmann H. , Maenhaut W. , Claeys M.
Published: April 2010

Abstract. In the present study, we have characterized the structure of a higher-molecular weight (MW) 358 α- and β-pinene dimeric secondary organic aerosol (SOA) product that received ample attention in previous molecular characterization studies. Based on mass spectrometric evidence for deprotonated molecules formed by electrospray ionization in the negative ion mode, we propose that diaterpenylic acid is a key monomeric unit for dimers of the ester type. It is shown that cis-pinic acid is esterified with the hydroxyl-containing diaterpenylic acid which can be explained through acid-catalyzed hydrolysis of the recently elucidated lactone-containing terpenylic acid and/or diaterpenylic acid acetate, both first-generation oxidation products. To a minor extent, higher-MW 358 and 344 diester products are formed containing other terpenoic acids as monomeric units, i.e., diaterpenylic acid instead of cis-pinic acid, and diaterebic acid instead of diaterpenylic acid. It is shown that the MW 358 diester and related MW 344 compounds, which can be regarded as processed SOA products, also occur in ambient fine (PM2.5) rural aerosol collected at night during the warm period of the 2006 summer field campaign conducted at K-puszta, Hungary, a rural site with coniferous vegetation. This indicates that, under ambient conditions, the higher-MW diesters are formed in the particle phase over a longer time-scale than that required for gas-to-particle partitioning of their monomeric precursors.
Quantifying pollution inflow and outflow over East Asia through coupling regional and global models
Authors: Lin M. , Holloway T. , Carmichael G. R. , Fiore A. M.
Published: April 2010

Abstract. Understanding the exchange processes between the atmospheric boundary layer and the free troposphere is crucial for estimating hemispheric transport of air pollution. Most studies of hemispheric air pollution transport have taken a large-scale perspective: using global chemical transport models and focusing on synoptic-scale export events. These global models have fairly coarse spatial and temporal resolutions, and thus have a limited ability to represent boundary layer processes and urban photochemistry. In support of United Nations Task Force on Hemispheric Transport of Air Pollution (TF HTAP; http://www.htap.org), this study employs two high-resolution atmospheric chemistry models (WRF-Chem and CMAQ; 36×36 km) coupled with a global model (MOZART; 1.9×1.9°) to examine the importance of fine-scale transport and chemistry processes in controlling pollution export and import over the Asian continent. We find that the vertical lifting and outflow of Asian pollution is enhanced in the regional models throughout the study period (March 2001) as contrast to the global model. Episodic outflow of CO, PAN, and O3 to the upper troposphere during cold frontal passages is twice as great in the WRF-Chem model as compared with the MOZART model. The TRACE-P aircraft measurements indicate that the pollution plumes in MOZART are too weak and too low in the altitude, which we attribute to the global model's inability to capture rapid deep convection that develops along the leading edge of the convergence band during frontal events. In contrast to pollution export from Asia, we find little difference in the regional vs. global model transport of European (EU) pollution into surface air over East Asia (EA). Instead, the local surface characteristics – sensitivity – strongly influence surface O3 responses. For instance, the O3 response to 20% decreases in EU emissions imported into our regional model domain is strongest (0.4–0.6 ppbv) over mountainous regions and weakest (0.1–0.3 ppbv) in megacities. The spatial averaged O3 response over EA estimated by our regional models is ~0.1 ppbv lower than global model estimates. Our results suggest that global models tend to underestimate the total budget of Asian pollutants exported to the free troposphere given their limited ability to properly capture vertical convection and lifting. Due to the compensating effects on surface O3 responses over downwind continents, future high-resolution hemispheric model analysis should provide additional insights into how the export and import processes interact, and will help to narrow the uncertainty of intercontinental source-receptor relationships.
Comparison of TOMS retrievals and UVMRP measurements of surface spectral UV radiation in the United States
Authors: Xu M. , Liang X.-Z. , Gao W. , Krotkov N.
Published: April 2010

Abstract. Surface noontime spectral ultraviolet (UV) irradiances during May–September of 2000–2004 from the total ozone mapping spectrometer (TOMS) satellite retrievals are systematically compared with the ground measurements at 27 climatological sites maintained by the USDA UV-B Monitoring and Research Program. The TOMS retrievals are evaluated by two cloud screening methods and local air quality conditions to determine their bias dependencies on spectral bands, cloudiness, aerosol loadings, and air pollution. Under clear-sky conditions, TOMS retrieval biases vary from −3.4% (underestimation) to 23.6% (overestimation). Averaged over all sites, the relative mean biases for 305, 311, 325, and 368 nm are respectively 15.4, 7.9, 7.6, and 7.0% (overestimation). The bias enhancement for 305 nm by approximately twice that of other bands likely results from absorption by gaseous pollutants (SO2, O3), and aerosols that are not included in the TOMS algorithm. For all bands, strong positive correlations of the TOMS biases are identified with aerosol optical depth, which explains nearly 50% of the variances of TOMS biases. The more restrictive in-situ cloud screening method reduces the biases by 3.4–3.9% averaged over all sites. This suggests that the TOMS biases from the in-situ cloud contamination may account for approximately 25% for 305 nm and 50% for other bands of the total bias. The correlation coefficients between total-sky and clear-sky biases across 27 sites are 0.92, 0.89, 0.83, and 0.78 for 305, 311, 325, and 368 nm, respectively. The results show that the spatial characteristics of the TOMS retrieval biases are systematic, representative of both clear and total-sky conditions.
Probabilistic model of shattering effect on in-cloud measurements
Authors: Shcherbakov V. , Gayet J.-F. , Febvre G. , Heymsfield A. J. , Mioche G.
Published: April 2010

Abstract. A probabilistic model of the ice shattering effects on in-cloud measurements performed with instruments designed with inlets, which have the circular cross-section, is presented. Applications are made for the Polar Nephelometer and PMS FSSP instruments. The model provides rough estimates of the effects on microphysical measurements and assigns the key parameters that govern the efficiency of ice shattering. It is shown that experimental data are less affected by the shattering for clouds that have a lower proportion of large particles. The effects on derived integral microphysical parameters are shown to be very sensitive to the effective diameter of the ice fragments. The smaller the fragments from a given cloud particle are, the higher their effects are. Errors on Polar Nephelometer measurements were evaluated. It is shown that the ice particle shattering leads to the overestimation of the extinction coefficient. For example, for a given distribution with the effective diameter of 68 μm and with fragment effective diameters of 10 μm the extinction is overestimated by 25%. With larger particles having an effective diameter of 89 μm, the error increases up to 37%. As for the FSSP-300 instrument, under the same conditions the extinction coefficient is overrated by 17% and the number particle concentration is overestimated by 30%. The discussion of the results points out the main hypothesis which may seriously limit the reliability of the modelling results. Nevertheless, the magnitudes of the errors on extinction and particle concentration are of the same orders of those reported in the literature from experimental assessments.
Using measurements for evaluation of black carbon modeling
Authors: Gilardoni S. , Vignati E. , Wilson J.
Published: April 2010

Abstract. The ever increasing use of air quality and climate model assessments to underpin economic, public health, and environmental policy decisions makes effective model evaluation critical. This paper discusses the properties of black carbon and light attenuation and absorption observations that are the key to a reliable evaluation of black carbon model and compares parametric and nonparametric statistical tools for the quantification of the agreement between models and observations. Black carbon concentrations are simulated with TM5/M7 global model from July 2002 to June 2003 at four remote sites (Alert, Jungfraujoch, Mace Head, and Trinidad Head) and two regional background sites (Bondville and Ispra). Equivalent black carbon (EBC) concentrations are calculated using light attenuation measurements from January 2000 to December 2005. Seasonal trends in the measurements are determined by fitting sinusoidal functions and the representativeness of the period simulated by the model is verified based on the scatter of the experimental values relative to the fit curves. When the resolution of the model grid is larger than 1°×1°, it is recommended to verify that the measurement site is representative of the grid cell. For this purpose, equivalent black carbon measurements at Alert, Bondville and Trinidad Head are compared to light absorption and elemental carbon measurements performed at different sites inside the same model grid cells. Comparison of these equivalent black carbon and elemental carbon measurements indicates that uncertainties in black carbon optical properties can compromise the comparison between model and observations. During model evaluation it is important to examine the extent to which a model is able to simulate the variability in the observations over different integration periods as this will help to identify the most appropriate timescales. The agreement between model and observation is accurately described by the overlap of probability distribution (PD) curves. Simple monthly median comparisons, the Student's t-test, and the Mann-Whitney test are discussed as alternative statistical tools to evaluate the model performance. The agreement measured by the Student's t-test, when applied to the logarithm of EBC concentrations, overestimates the higher PD agreements and underestimates the lower PD agreements; the Mann-Whitney test can be employed to evaluate model performance on a relative scale when the shape of model and experimental distributions are similar.
Cirrus clouds triggered by radiation, a multiscale phenomenon
Authors: Fusina F. , Spichtinger P.
Published: April 2010

Abstract. In this study, the influence of radiative cooling and small eddies on cirrus formation is investigated. For this purpose the non-hydrostatic, anelastic model EULAG is used with a recently developed and validated ice microphysics scheme (Spichtinger and Gierens, 2009a). Additionally, we implemented a fast radiation transfer code (Fu et al., 1998). Using idealized profiles with high ice supersaturations up to 144% and weakly stable stratifications with Brunt-Vaisala frequencies down to 0.018 s−1 within a supersaturated layer, the influence of radiation on the formation of cirrus clouds is remarkable. Due to the radiative cooling at the top of the ice supersaturated layer with cooling rates down to -3.5 K/d, the stability inside the ice supersaturated layer decreases with time. During destabilization, small eddies induced by Gaussian temperature fluctuations start to grow and trigger first nucleation. These first nucleation events then induce the growth of convective cells due to the radiative destabilization. The effects of increasing the local relative humidity by cooling due to radiation and adiabatic lifting lead to the formation of a cirrus cloud with IWC up to 33 mg/m3 and mean optical depths up to 0.36. In a more stable environment, radiative cooling is not strong enough to destabilize the supersaturated layer within 8 h; no nucleation occurs in this case. Overall triggering of cirrus clouds via radiation works only if the supersaturated layer is destabilized by radiative cooling such that small eddies can grow in amplitude and finally initialize ice nucleation. Both processes on different scales, small-scale eddies and large-scale radiative cooling are necessary.
Do vibrationally excited OH molecules affect middle and upper atmospheric chemistry?
Authors: von Clarmann T. , Hase F. , Funke B. , López-Puertas M. , Orphal J. , Sinnhuber M. , Stiller G. P. , Winkler H.
Published: April 2010

Abstract. Except for a few reactions involving electronically excited molecular or atomic oxygen or nitrogen, atmospheric chemistry modelling usually assumes that the temperature dependence of reaction rates is characterized by Arrhenius law involving kinetic temperatures. It is known, however, that in the upper atmosphere the vibrational temperatures may exceed the kinetic temperatures by several hundreds of Kelvins. This excess energy has an impact on the reaction rates. We have used upper atmospheric OH populations and reaction rate coefficients for OH(v=0...9)+O3 and OH(v=0...9)+O to estimate the effective (i.e. population weighted) reaction rates for various atmospheric conditions. We have found that the effective rate coefficient for OH(v=0...9)+O3 can be larger by a factor of up to 1020 than that involving OH in its vibrational ground state only. At altitudes where vibrationally excited states of OH are highly populated, the OH reaction is a minor sink of Ox and O3 compared to other reactions involving, e.g., atomic oxygen. Thus the impact of vibrationally excited OH on the ozone or Ox sink remains small. Among quiescent atmospheres under investigation, the largest while still small (less than 0.1%) effect was found for the polar winter upper stratosphere and mesosphere. The contribution of the reaction of vibrationally excited OH with ozone to the OH sink is largest in the upper polar winter stratosphere (up to 4%), while its effect on the HO2 source is larger in the lower thermosphere (up to 1% for polar winter and 1.7% for midlatitude night conditions). For OH(v=0...9)+O the rate coefficients differ by plus/minus a few percent only from those involving OH in its vibrational ground state. The effects on the odd oxygen sink are negative and can reach −0.7% (polar summer lowermost thermosphere), i.e. neglect of vibrational excitation overestimates the odd oxygen sink. The OH sink is overestimated by up to 2%. After a solar proton event, when upper atmospheric OH can be enhanced by an order of magnitude, the excess relative odd oxygen sink by OH(v=0...9)+O3 is estimated at up to 0.2%, and the excess relative OH sink by OH(v=0...9)+O3 can exceed 4% in the thermosphere.
High variability of the heterogeneous ice nucleation potential of oxalic acid dihydrate and sodium oxalate
Authors: Wagner R. , Möhler O. , Saathoff H. , Schnaiter M. , Leisner T.
Published: April 2010

Abstract. The heterogeneous ice nucleation potential of airborne oxalic acid dihydrate and sodium oxalate particles in the deposition and condensation mode has been investigated by controlled expansion cooling cycles in the AIDA aerosol and cloud chamber of the Karlsruhe Institute of Technology at temperatures between 244 and 228 K. Previous laboratory studies have highlighted the particular role of oxalic acid dihydrate as the only species amongst a variety of other investigated dicarboxylic acids to be capable of acting as a heterogeneous ice nucleus in both the deposition and immersion mode. We could confirm a high deposition mode ice activity for 0.03 to 0.8 μm sized oxalic acid dihydrate particles that were either formed by nucleation from a gaseous oxalic acid/air mixture or by rapid crystallisation of highly supersaturated aqueous oxalic acid solution droplets. The critical saturation ratio with respect to ice required for deposition nucleation was found to be less than 1.1 and the size-dependent ice-active fraction of the aerosol population was in the range from 0.1 to 22%. In contrast, oxalic acid dihydrate particles that had crystallised from less supersaturated solution droplets and had been allowed to slowly grow in a supersaturated environment from still unfrozen oxalic acid solution droplets over a time period of several hours were found to be much poorer heterogeneous ice nuclei. We speculate that under these conditions a crystal surface structure with less-active sites for the initiation of ice nucleation was generated. Such particles partially proved to be almost ice-inactive in both the deposition and condensation mode. At times, the heterogeneous ice nucleation ability of oxalic acid dihydrate significantly changed when the particles had been processed in preceding cloud droplet activation steps. Such behaviour was also observed for the second investigated species, namely sodium oxalate. Our experiments address the atmospheric scenario that coating layers of oxalic acid or its salts may be formed by physical and chemical processing on pre-existing particulates such as mineral dust and soot. Given the broad diversity of the observed heterogeneous ice nucleability of the oxalate species, it is not straightforward to predict whether an oxalate coating layer will improve or reduce the ice nucleation ability of the seed aerosol particles.
High-frequency urban measurements of hydrogen and carbon monoxide in the UK
Authors: Grant A. , Stanley K. F. , Henshaw S. J. , Shallcross D. E. , O'Doherty S.
Published: April 2010

Abstract. High-frequency measurements of atmospheric hydrogen (H2) and carbon monoxide (CO) were made at an urban site in the UK from mid-December 2008 until early March 2009. Very few measurements of these trace gases exist in the urban environment, particularly within the United Kingdom, but are an essential component in the assessment of anthropogenic emissions of H2 and CO. These data provide detailed information on urban time-series, diurnal cycles as well as sources and sinks of both H2 and CO at urban locations. High-frequency data were found to be strongly influenced by local meteorological conditions of wind speed and temperature. Diurnal cycles were found to follow transport frequency very closely due to the sites proximity to major carriageways, consequently a strong correlation was found between H2 and CO mole fractions. Background subtracted mean and rush hour H2/CO emission ratios of 0.50 and 0.53 were calculated, the scatter plot of which displayed an unusual two population pattern, the source of which could not be elucidated. H2 emissions from transport in the UK were estimated at 175 Gg/yr, with 7.8 Tg/yr of H2 produced from vehicle emissions globally. H2 and CO deposition velocities were calculated over stable periods when a clear decay of both species was observed. CO was found to have a much higher deposition velocity than H2, 1.3×10−3 and 2.2×10−4 m s−1, respectively, going against the law of molecular diffusivity. The source of this unusual result was investigated, however no conclusive evidence was found for increased loss of CO over H2 during stable night time inversion events.
Testing remote sensing on artificial observations: impact of drizzle and 3-D cloud structure on effective radius retrievals
Authors: Zinner T. , Wind G. , Platnick S. , Ackerman A. S.
Published: April 2010

Abstract. Remote sensing of cloud effective particle size with passive sensors like the Moderate Resolution Imaging Spectroradiometer (MODIS) is an important tool for cloud microphysical studies. As a measure of the radiatively relevant droplet size, effective radius can be retrieved with different combinations of visible through shortwave and midwave infrared channels. In practice, retrieved effective radii from these combinations can be quite different. This difference is perhaps indicative of different penetration depths and path lengths for the spectral reflectances used. In addition, operational liquid water cloud retrievals are based on the assumption of a relatively narrow distribution of droplet sizes; the role of larger precipitation particles in these distributions is neglected. Therefore, possible explanations for the discrepancy in some MODIS spectral size retrievals could include 3-D radiative transport effects, including sub-pixel cloud inhomogeneity, and/or the impact of drizzle formation. The possible factors of influence are isolated and investigated in detail by the use of simulated cloud scenes and synthetic satellite data: marine boundary layer cloud scenes from large eddy simulations (LES) with detailed microphysics are combined with Monte Carlo radiative transfer calculations that explicitly account for the detailed droplet size distributions as well as 3-D radiative transfer to simulate MODIS observations. The operational MODIS optical thickness and effective radius retrieval algorithm is applied to these and the results are compared to the given LES microphysics. We investigate two types of marine cloud situations each with and without drizzle from LES simulations: (1) a typical daytime stratocumulus deck at two times in the diurnal cycle and (2) one scene with scattered cumulus. Only small impact of drizzle formation on the retrieved domain average and on the differences between the three effective radius retrievals is noticed for both cloud scene types for different reasons. For the presumably typical overcast stratocumulus scenes, the optical thickness (8 to 9) is large enough to mask the drizzle rain rates at cloud bottom (up to 0.05 mm/h). The cumulus scene does not show much drizzle sensitivity either despite extended drizzle areas being directly visible from above (locally >1 mm/h), which is mainly due to characteristics of the standard retrieval approach. 3-D effects, on the other hand, produce large discrepancies between the 1.6 and 2.1 μm channel observations compared to 3.7 μm retrievals in the latter case. A general sensitivity of MODIS particle size data to drizzle formation is not corroborated by our results.
Large estragole fluxes from oil palms in Borneo
Authors: Misztal P. K. , Owen S. M. , Guenther A. B. , Rasmussen R. , Geron C. , Harley P. , Phillips G. J. , Ryan A. , Edwards D. P. , Hewitt C. N. , Nemitz E. , Siong J. , Heal M. R. , Cape J. N.
Published: April 2010

Abstract. During two field campaigns (OP3 and ACES), which ran in Borneo in 2008, we measured large emissions of estragole (methyl chavicol; IUPAC systematic name 1-allyl-4-methoxybenzene; CAS number 140-67-0) in ambient air above oil palm canopies (0.81 mg m−2 h−1 and 3.2 ppbv for mean midday fluxes and mixing ratios, respectively) and subsequently from flower enclosures. However, we did not detect this compound at a nearby rainforest. Estragole is a known attractant of the African oil palm weevil (Elaeidobius kamerunicus), which pollinates oil palms (Elaeis guineensis). There has been recent interest in the biogenic emissions of estragole but it is normally not included in atmospheric models of biogenic emissions and atmospheric chemistry despite its relatively high potential for secondary organic aerosol formation from photooxidation and high reactivity with OH radical. We report the first direct canopy-scale measurements of estragole fluxes from tropical oil palms by the virtual disjunct eddy covariance technique and compare them with previously reported data for estragole emissions from Ponderosa pine. Flowers, rather than leaves, appear to be the main source of estragole from oil palms; we derive a global estimate of estragole emissions from oil palm plantations of ~0.5 Tg y−1. The observed ecosystem mean fluxes (0.44 mg m−2 h−1) and mean ambient volume mixing ratios (3.0 ppbv) of estragole are the highest reported so far. The value for midday mixing ratios is not much different from the total average as, unlike other VOCs (e.g. isoprene), the main peak occurred in the evening rather than in the middle of the day. Despite this, we show that the estragole flux can be parameterised using a combination of a modified G06 algorithm for emission and a canopy resistance approach for deposition. However, the model underestimates the afternoon peak even though a similar approach works well for isoprene. Our measurements suggest that this biogenic compound may have an impact on regional atmospheric chemistry that previously has not been accounted for in models and could become more important in the future due to expansion of the areas of oil palm plantation.
IASI-METOP and MIPAS-ENVISAT data fusion
Authors: Ceccherini S. , Cortesi U. , Del Bianco S. , Raspollini P. , Carli B.
Published: April 2010

Abstract. The combination of data obtained with different sensors (data fusion) is a powerful technique that can provide target products of the best quality in terms of precision and accuracy, as well as spatial and temporal coverage and resolution. In this paper the results are presented of the data fusion of measurements of ozone vertical profile performed by two space-borne interferometers (IASI on METOP and MIPAS on ENVISAT) using the new measurement-space-solution method. With this method both the loss of information due to interpolation and the propagation of possible biases (caused by a priori information) are avoided. The data fusion products are characterized by means of retrieval errors, information gain, averaging kernels and number of degrees of freedom. The analysis is performed both on simulated and real measurements and the results demonstrate and quantify the improvement of data fusion products with respect to measurements of a single instrument.
Greenhouse gas relationships in the Indian summer monsoon plume measured by the CARIBIC passenger aircraft
Authors: Schuck T. J. , Brenninkmeijer C. A. M. , Baker A. K. , Slemr F. , Velthoven P. F. J. v. , Zahn A.
Published: April 2010

Abstract. During the Indian summer monsoon period 2008, regular measurement flights were performed by the CARIBIC aircraft between Germany and India. Measurements included whole air sampling and subsequent analysis for greenhouse gases (CH4, CO2, N2O, SF6) and in-situ measurements of CO, O3, and H2O. For all these tracers a distinct monsoon signature was observed in the longitude range 50–80° E south of 40° N at flight altitudes 8–12.5 km. The formation of a monsoon plume with enhanced mixing ratios was observed for CH4, N2O, SF6, CO and H2O. The plume began to develop in May and persisted through September, and maximum mixing ratios and maximum spatial extension of the plume were observed in August. For CO2 and O3 a minimum was observed. The amplitude of the CO2 seasonal cycle was larger than at comparable latitudes outside the monsoon region, and the CO2 spring maximum was observed with a delay of one month. Different tracer relationships show strong spatial variations across the plume. The comparison with NOAA ground station data shows a clear enhancement for CH4, N2O, SF6 and CO and a decrease in CO2 mixing ratios. Assuming seasonally invariant SF6 emissions based on the EDGAR 4.0 inventory, monthly emissions were estimated for the period June–September for CH4, N2O, and CO, and the CO2 uptake was estimated. While for N2O an emission decrease during the monsoon period was derived, emissions of CH4 were highest in September indicating a different seasonal cycle of emissions. Net CO2 uptake by the vegetation was largest in September.
Impact of West African Monsoon convective transport and lightning NOx production upon the upper tropospheric composition: a multi-model study
Authors: Barret B. , Williams J. E. , Bouarar I. , Yang X. , Josse B. , Law K. , Pham M. , Le Flochmoën E. , Liousse C. , Peuch V. H. , Carver G. D. , Pyle J. A. , Sauvage B. , van Velthoven P. , Schlager H. , Mari C. , Cammas J.-P.
Published: April 2010

Abstract. Within the African Monsoon Multidisciplinary Analysis (AMMA), we investigate the impact of nitrogen oxides produced by lightning (LiNOx) and convective transport during the West African Monsoon (WAM) upon the composition of the upper troposphere (UT) in the tropics. For this purpose, we have performed simulations with 4 state-of-the-art chemistry transport models involved within AMMA, namely MOCAGE, TM4, LMDz-INCA and p-TOMCAT. The model intercomparison is complemented with an evaluation of the simulations based on both spaceborne and airborne observations. The baseline simulations show important differences between the UT CO and O3 distributions simulated by each of the 4 models when compared to measurements of the African latitudinal transect from the MOZAIC program and to distributions measured by the Aura/MLS spaceborne sensor. We show that such model discrepancies can be explained by differences in the convective transport parameterizations and, more particularly, the altitude reached by convective updrafts (ranging between ~200–125 hPa). Concerning UT O3, the majority of models exhibit low concentrations compared to both MOZAIC and MLS observations south of the equator, with good agreement in the Northern Hemisphere. Sensitivity studies are performed to quantify the effect of deep convective transport and the influence of LiNOx production on the UT composition. These clearly indicate that the CO maxima and the elevated O3 concentrations south of the equator are due to convective uplift of air masses impacted by Southern African biomass burning, in agreement with previous studies. Moreover, during the WAM, LiNOx from Africa are responsible for the highest UT O3 enhancements (10–20 ppbv) over the tropical Atlantic between 10° S–20° N. Differences between models are primarily due to the performance of the parameterizations used to simulate lightning activity which are evaluated using spaceborne observations of flash frequency. Combined with comparisons of in-situ NO measurements we show that the models producing the highest amounts of LiNOx over Africa during the WAM (INCA and p-TOMCAT) capture observed NO profiles with the best accuracy, although they both overestimate lightning activity over the Sahel.
Enhancing effect of dimethylamine in sulfuric acid nucleation in the presence of water – a computational study
Authors: Loukonen V. , Kurtén T. , Ortega I. K. , Vehkamäki H. , Pádua A. A. H. , Sellegri K. , Kulmala M.
Published: April 2010

Abstract. We have studied the hydration of sulfuric acid – ammonia and sulfuric acid – dimethylamine clusters using quantum chemistry. We calculated the formation energies and thermodynamics for clusters of one ammonia or one dimethylamine molecule together with 1–2 sulfuric acid and 0–5 water molecules. The results indicate that dimethylamine enhances the addition of sulfuric acid to the clusters much more efficiently than ammonia when the number of water molecules in the cluster is either zero, or greater than two. Further hydrate distribution calculations reveal that practically all dimethylamine-containing two-acid clusters will remain unhydrated in tropospherically relevant circumstances, thus strongly suggesting that dimethylamine assists atmospheric sulfuric acid nucleation much more effectively than ammonia.