bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultrathin Tropical Tropopause Clouds (UTTCs): II. Stabilization mechanisms
Authors: Luo B. P. , Peter Th. , Wernli H. , Fueglistaler S. , Wirth M. , Kiemle C. , Flentje H. , Yushkov V. A. , Khattatov V. , Rudakov V. , Thomas A. , Borrmann S. , Toci G. , Mazzinghi P. , Beuermann J. , Schiller C. , Cairo F. , Di Don-Francesco G. , Adriani A. , Volk C. M. , Strom J. , Noone K. , Mitev V. , MacKenzie R. A. , Carslaw K. S. , Trautmann T. , Santacesaria V. , Stefanutti L.
Published: April 2010

Abstract. Mechanisms by which subvisible cirrus clouds (SVCs) might contribute to dehydration close to the tropical tropopause are not well understood. Recently Ultrathin Tropical Tropopause Clouds (UTTCs) with optical depths around 10-4 have been detected in the western Indian ocean. These clouds cover thousands of square kilometers as 200-300 m thick distinct and homogeneous layer just below the tropical tropopause. In their condensed phase UTTCs contain only 1-5% of the total water, and essentially no nitric acid. A new cloud stabilization mechanism is required to explain this small fraction of the condensed water content in the clouds and their small vertical thickness. This work suggests a mechanism, which forces the particles into a thin layer, based on upwelling of the air of some mm/s to balance the ice particles, supersaturation with respect to ice above and subsaturation below the UTTC. In situ measurements suggest that these requirements are fulfilled. The basic physical properties of this mechanism are explored by means of a single particle model. Comprehensive 1-D cloud simulations demonstrate this stabilization mechanism to be robust against rapid temperature fluctuations of +/- 0.5 K. However, rapid warming (Δ T > 2 K) leads to evaporation of the UTTC, while rapid cooling (Δ T < -2 K) leads to destabilization of the particles with the potential for significant dehydration below the cloud
Occurrence of ozone anomalies over cloudy areas in TOMS version-7 level-2 data
Authors: Liu X. , Newchurch M. J. , Kim J. H.
Published: April 2010

Abstract. This study investigates anomalous ozone distributions over cloudy areas in Nimbus-7 (N7) and Earth-Probe (EP) TOMS version-7 data and analyzes the causes for ozone anomaly formation. A 5°-longitude by 5°-latitude region is defined to contain a Positive Ozone Anomaly (POA) or Negative Ozone Anomaly (NOA) if the correlation coefficient between total ozone and reflectivity is > 0.5 or < -0.5. The average fractions of ozone anomalies among all cloud fields are 31.8 ± 7.7% and 35.8 ± 7.7% in the N7 and EP TOMS data, respectively. Some ozone anomalies are caused by ozone retrieval errors, and others are caused by actual geophysical phenomena. Large cloud-height errors are found in the TOMS version-7 algorithm in comparison to the Temperature Humidity Infrared Radiometer (THIR) cloud data. On average, cloud-top pressures are overestimated by ~200 hPa (THIR cloud-top pressure < 200 hPa) for high-altitude clouds and underestimated by ~150 hPa for low-altitude clouds (THIR cloud-top pressure > 750 hPa). Most tropical NOAs result from negative errors induced by large cloud-height errors, and most tropical POAs are caused by positive errors due to intra-cloud ozone absorption enhancement. However, positive and negative errors offset each other, reducing the ozone anomaly occurrence in TOMS data. Large ozone/reflectivity slopes for mid-latitude POAs show seasonal variation consistent with total ozone fluctuation, indicating that they result mainly from synoptic and planetary wave disturbances. POAs with an occurrence fraction of 30--60% occur in regions of marine stratocumulus off the west coast of South Africa and off the west coast of South America. Both fractions and ozone/reflectivity slopes of these POAs show seasonal variations consistent with that in the tropospheric ozone. About half the ozone/reflectivity slope can be explained by ozone retrieval errors over clear and cloudy areas. The remaining slope may result from there being more ozone production because of rich ozone precursors and higher photolysis rates over high-frequency, low-altitude clouds than in clear areas. Ozone anomalies due to ozone retrieval errors have important implications in TOMS applications such as tropospheric ozone derivation and analysis of ozone seasonal variation.
The rate of water vapor evaporation from ice substrates in the presence of HCl and HBr: implications for the lifetime of atmospheric ice particles
Authors: Delval C. , Fluckiger B. , Rossi M. J.
Published: April 2010

Abstract. Using a multidiagnostic approach the rate Rev [ molec cm-3 s-1] or flux Jev [ molec cm-2 s-1] of evaporation of H2O and its corresponding rate constant for condensation, kcond [s-1 ], on a 1 µm thick ice film have been studied in the temperature range 190 to 240 K as well as in the presence of small amounts of HCl and HBr that left the vapor pressure of H2O on ice unchanged. The resulting Arrhenius expressions for pure ice are Jev = 1.6 · 10 28 ± 1 · exp  (- 10.3 ± 1.2/ RT)  [ molec cm-2 s-1] , kcond = 1.7 · 10 - 2 ± 1 · exp  (+ 1.6 ± 1.5/ RT ) [s -1], in the presence of a HCl mole fraction in the range 3.2 · 10 - 5 - 6.4 · 10 - 3 : Jev = 6.4 · 10 26 ± 1 · exp  (- 9.7 ± 1.2/ RT)  [ molec cm-2 s-1] , kcond = 2.8 · 10 - 2 ± 1 · exp ( + 1.5 ± 1.6 /RT)  [s -1], and a HBr mole fraction smaller than 6.4 · 10 - 3 : Jev = 7.4 · 10 25 ± 1 · exp ( - 9.1 ± 1.2 /RT)  [ molec cm-2 s-1] , kcond = 7.1 · 10 - 5 ± 1 · exp (+ 2.6 ± 1.5/ RT) [s -1]. The small negative activation energy for H2O condensation on ice points to a precursor mechanism. The corresponding enthalpy of sublimation is DHsubl = Eev - Econd = 11.9 ± 2.7 kcal mol-1 , DHsubl = 11.2 ± 2.8 kcal mol-1, and DHsubl = 11.7 ± 2.8 kcal mol-1 whose values are identical within experimental uncertainty to the accepted literature value of 12.3 kcal mol-1 . Interferometric data at 633 nm and FTIR absorption spectra in transmission support the kinetic results. The data are consistent with a significant lifetime enhancement for HCl- and HBr-contaminated ice particles by a factor of 3–6 and 10–20, respectively, for submonolayer coverages of HX once the fraction of the ice not contaminated by HX has evaporated.
Ozone decomposition on Saharan dust: an experimental investigation
Authors: Hanisch F. , Crowley J. N.
Published: April 2010

Abstract. The heterogeneous reaction between O3 and authentic Saharan dust surfaces was investigated in a Knudsen reactor at approx 296 K. O3 was destroyed on the dust surface and O2 was formed with conversion efficiencies of 1.0 and 1.3 molecules O2 per O3 molecule destroyed for unheated and heated samples, respectively. No O3 desorbed from exposed dust samples, showing that the uptake was irreversible. The uptake coefficients for the irreversible destruction of O3 on (unheated) Saharan dust surfaces depended on the O3 concentration and varied between 3.5 x 10-4 and 5.5 x 10-6 for the initial uptake coefficient (g0 approx 3 x 10-5 at 30 ppbv O3 STP) and between 4.8 x 10-5 and 2.2 x 10-6 for the steady-state uptake coefficient (gss approx 7 x10-6 at 30 ppbv O3 STP). At very high O3 concentrations the surface was deactivated, and O3 uptake ceased after a certain exposure period. Sample re-activation (i.e. de-passivation) was found to occur over periods of hours, after exposure to O3 had ceased, suggesting that re-activation processes play a role both in the laboratory and in the atmosphere.
Origin of anthropogenic hydrocarbons and halocarbons measured in the summertime european outflow (on Crete in 2001)
Authors: Gros V. , Williams J. , van Aardenne J. A. , Salisbury G. , Hofmann R. , Lawrence M. G. , von Kuhlmann R. , Lelieveld J. , Krol M. , Berresheim H. , Lobert J. M. , Atlas E.
Published: April 2010

Abstract. During the Mediterranean Intensive Oxidant Study MINOS in August 2001, 87 air samples were collected at the ground-based station Finokalia (35°19'N, 25°40'E) on the north coast of Crete and subsequently analysed by GC-MS. The analysis includes various hydrocarbons, organo-halogens, HCFCs and CFCs. These compounds have a wide variety of sources and sinks and a large range of atmospheric lifetimes. We evaluated the characteristics of the sampling site in terms of proximity to individual sources by plotting the measured variability of these species against lifetime. The resulting linear relationship suggests that the sampling site is representative of intermediate conditions between a remote site and one that is in the vicinity of a wide variety of sources. Our analysis of air mass origin and chemical ratios also shows that several distinct anthropogenic sources influenced the atmospheric composition over Crete. Propane observations are compared to a global model to assess the fossil fuel related emission inventory. Although the model reproduces the general pattern of the propane variations, the model mixing ratios are systematically too low by a factor of 1.5 to 3, probably due to an underestimation of the propane emissions from east European countries in the underlying global database EDGAR. Another important finding was that methyl chloroform, a compound banned under the Montreal protocol, showed significant enhancements from background, which were well correlated with CFC-113. This suggests continued use and emission of methyl chloroform by one or more European countries. We also discuss the observed variations of methyl bromide and suggest that the significant peak observed on 12 August 2001 reflects heavy agricultural use as a soil fumigant in Italy.
Estimated variability of below-cloud aerosol removal by rainfall for observed aerosol size distributions
Authors: Andronache C.
Published: April 2010

Abstract. Below-cloud scavenging (BCS) coefficients of aerosols by rainfall are estimated for reported aerosol size distributions measured during field experiments in various environments. The method employed is based on explicit calculations of the efficiency of collision between a raindrop and aerosol particles. Such BCS coefficients can be used in numerical models that describe: 1) the detailed evolution of aerosol size distribution and, 2) the evolution of total aerosol mass concentration. The effects of raindrop size distribution and aerosol size distribution variability on BCS coefficients are illustrated using observed data. Results show that BCS coefficient increases with rainfall rate and has a significant dependence on aerosol size distribution parameters. Thus, BCS is important for very small particles (with diameters less than 0.01 μm) and for coarse particles (with diameters larger than 2 µm). For rainfall rate R ~ 1 mm hr-1, the 0.5-folding time of these particles is of the order of one hour. It is shown that BCS is negligible for aerosol particles in the range [0.1-1] µm if compared with in-cloud scavenging rates for low and moderate rainfall rates ( R ~ 0.1-10 mm hr-1). The results indicate that a boundary layer aerosol size distribution with coarse mode is drastically affected very shortly after rain starts (in a fraction of one hour) and consequently, the below-cloud aerosol size distribution becomes dominated by particles in the accumulation mode.
Trace elements in South America aerosol during 20th century inferred from a Nevado Illimani ice core, Eastern Bolivian Andes (6350 m asl)
Authors: Correia A. , Freydier R. , Delmas R. J. , Simões J. C. , Taupin J.-D. , Dupré B. , Artaxo P.
Published: April 2010

Abstract. A 137 m ice core drilled in 1999 from Eastern Bolivian Andes at the summit of Nevado Illimani (16º 37' S, 67º 46' W, 6350 m asl) was analyzed at high temporal resolution, allowing a characterization of trace elements in Andean aerosol trapped in the ice during the 20th century. The upper 50 m of the ice core were dated by multi-proxy analysis of stable isotopes (d18O and d2H), 137Cs and Ca+2 content, electrical conductivity, and insoluble microparticle content, together with reference historical horizons from atmospheric nuclear tests and known volcanic eruptions. This 50 m section corresponds to a record of environmental variations spanning about 80 years from 1919 to 1999. It was cut in 744 sub-samples under laminar flow in a clean bench, which were analyzed by Ion Chromatography for major ionic concentration, by a particle counter for insoluble aerosol content, and by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for the concentration of 45 chemical species from Li to U. This paper focuses on results of trace element concentrations measured by ICP-MS. The high temporal resolution used in the analyses allowed classifying samples as belonging to dry or wet seasons. During wet season elemental concentrations are low and samples show high crustal enrichment factors. During dry seasons the situation is opposite, with high elemental concentrations and low crustal enrichments. For example, with salt lakes as main sources in the region, average Li concentration during the 20th century is 0.035 and 0.90 ng g-1 for wet and dry seasons, respectively. Illimani average seasonal concentration ranges cover the spectrum of elemental concentration measurements at another Andean ice core site (Sajama) for most soil-related elements. Regional crustal dust load in the deposits was found to be overwhelming during dry season, obfuscating the contribution of biomass burning material. Marked temporal trends from the onset of 20th century to more recent years were identified for the concentrations of several trace species of anthropic origin, especially for Cu, As, Zn, Cd, Co, Ni and Cr. Among these elements, Cu shows average wet season crustal enrichment factors above 103, while the others range between 102 to about 5x102. P and K show moderate average wet season enrichment factors, suggesting an impact of natural biogenic emissions from the Amazon Basin. Pb has multiple anthropic sources in the region, from mining activities in the beginning of 20th century to automotive fuel after 1950s. From the large number of samples analyzed from Illimani, it was possible to derive an effective chemical characterization of the deposited background Andean soil dust aerosol during 20th century.
Global distribution of total ozone and lower stratospheric temperature variations
Authors: Steinbrecht W. , Hassler B. , Claude H. , Winkler P. , Stolarski R. S.
Published: April 2010

Abstract. This study gives an overview of interannual variations of total ozone and 50 hPa temperature. It is based on newer and longer records from the 1979 to 2001 Total Ozone Monitoring Spectrometer (TOMS) and Solar Backscatter Ultraviolet (SBUV) instruments, and on US National Center for Environmental Prediction (NCEP) reanalyses. Multiple linear least squares regression is used to attribute variations to various natural and anthropogenic explanatory variables. Usually, maps of total ozone and 50 hPa temperature variations look very similar, reflecting a very close coupling between the two. As a rule of thumb, a 10 Dobson Unit (DU) change in total ozone corresponds to a 1 K change of 50 hPa temperature. Large variations come from the linear trend term, up to -30 DU or -1.5 K/decade, from terms related to polar vortex strength, up to 50 DU or 5 K (typical, minimum to maximum), from tropospheric meteorology, up to 30 DU or 3 K, or from the Quasi-Biennial Oscillation (QBO), up to 25 DU or 2.5 K. The 11-year solar cycle, up to 25 DU or 2.5 K, or El Niño/Southern Oscillation (ENSO), up to 10 DU or 1 K, are contributing smaller variations. Stratospheric aerosol after the 1991 Pinatubo eruption lead to warming up to 3 K at low latitudes and to ozone depletion up to 40 DU at high latitudes. Variations attributed to QBO, polar vortex strength, and to a lesser degree to ENSO, exhibit an inverse correlation between low latitudes and higher latitudes. Variations related to the solar cycle or 400 hPa temperature, however, have the same sign over most of the globe. Variations are usually zonally symmetric at low and mid-latitudes, but asymmetric at high latitudes. There, position and strength of the stratospheric anti-cyclones over the Aleutians and south of Australia appear to vary with the phases of solar cycle, QBO or ENSO.
The impact of monsoon outflow from India and Southeast Asia in the upper troposphere over the eastern Mediterranean
Authors: Scheeren H. A. , Lelieveld J. , Roelofs G. J. , Williams J. , Fischer H. , de Reus M. , de Gouw J. A. , Warneke C. , Holzinger R. , Schlager H. , Klüpfel T. , Bolder M. , van der Veen C. , Lawrence M.
Published: April 2010

Abstract. A major objective of the Mediterranean INtensive Oxidant Study (MINOS) was to investigate long-range transport of pollutants (notably ozone precursor species). Here we present trace gas measurements from the DLR (German Aerospace Organization) Falcon aircraft in the eastern Mediterranean troposphere. Ten day backward trajectories and a coupled chemistry-climate model (ECHAM4) were used to study the nature and origin of pollution observed in the upper troposphere between 6 and 13 km altitude. We focus on a large pollution plume encountered over the eastern Mediterranean between 1 and 12 August originating in South Asia (India and Southeast Asia), referred to as the Asian plume, associated with the Asian Summer Monsoon. Vertical as well as longitudinal gradients of methane, carbon monoxide, hydrocarbons including acetone, methanol, and acetonitrile, halocarbons, ozone and total reactive nitrogen (NOy) are presented, showing the chemical impact of the Asian plume compared to westerly air masses containing pollution from North America. The Asian plume is characterized by enhanced concentrations of biomass burning tracers (acetylene, methyl chloride, acetonitrile), notably from biofuel use. Concentrations of the new automobile cooling agent HFC-134a were significantly lower in the Asian plume than in air masses from North America. Relatively high levels of ozone precursors (CO, hydrocarbons) were found in both air masses, whereas lower ozone concentrations in the Asian plume suggest NOx-limited conditions. Consistently, ECHAM model simulations indicate that the expected future increase of NOx-emissions in Asia enhances the photochemical ozone production in the Asian plume. The size and location of the Asian plume near the tropopause provides an important potential for pollution transport into the lowermost stratosphere. We present observations indicative of Asian pollution transport into the lower stratosphere.
Study of the heterogeneous reaction of O3 with CH3SCH3 using the wetted-wall flowtube technique
Authors: Barcellos da Rosa M. , Behnke W. , Zetzsch C.
Published: April 2010

Abstract. This work presents the heterogeneous kinetics of the reaction of CH3SCH3 (dimethyl sulphide, DMS) with O3 (ozone) in aqueous solutions of different ionic strengths (0, 0.1 and 1.0M NaCl) using the wetted-wall flowtube (WWFT) technique. Henry's law coefficients of DMS on pure water and on different concentrations of NaCl (0.1M - 4.0M) in the WWFT from UV spectrophotometric measurements of DMS in the gas phase, using a numerical transport model of phase exchange, were determined to be H ±s (M atm-1) = 2.16±0.5 at 274.4 K, 1.47±0.3 at 283.4 K, 0.72±0.2 at 291 K, 0.57±0.1 at 303.4 K and 0.33±0.1 at 313.4 K on water, on 1.0M NaCl to be H = 1.57±0.4 at 275.7 K, 0.8±0.2 at 291 K and on 4.0M NaCl to be H = 0.44±0.1 at 275.7 K and 0.16±0.04 at 291 K, showing a significant effect of ionic strength, m, on the solubility of DMS according to the equation ln (H/M atm-1) = 4061 T-1 - 0.052 m2 - 50.9 m T-1 - 14.0. At concentrations of DMS(liq) above 50 mM, UV spectrophotometry of both O3(gas) and DMS(gas) enables us to observe simultaneously the reactive uptake of O3 on DMS solution and the gas-liquid equilibration of DMS along the WWFT. The uptake coefficient, g (gamma), of O3 on aqueous solutions of DMS, varying between 1 and 15·10-6, showed a square root-dependence on the aqueous DMS concentration (as expected for diffusive penetration into the surface film, where the reaction takes place in aqueous solution). The uptake coefficient was smaller on NaCl solution in accord with the lower solubility of O3. The heterogeneous reaction of O3(gas) with DMS(liq) was evaluated from the observations of the second order rate constant (kII) for the homogeneous aqueous reaction O3(liq) + DMS(liq) using a numerical model of radial diffusion and reactive penetration, leading to kII ± D kII (in units of 108 M-1 s-1) = 4.1±1.2 at 291.0 K, 2.15±0.65 at 283.4 K and 1.8±0.5 at 274.4 K. Aside from the expected influence on solubility and aqueous-phase diffusion coefficient of both gases there was no significant effect of ionic strength on kII, that was determined for 0.1M NaCl, leading to kII ± D kII (108 M-1 s-1) = 3.2±1.0 at 288 K, 1.7±0.5 at 282 K and 1.3±0.4 at 276 K, and for 1.0M NaCl, leading to 3.2±1.0 at 288 K, 1.3±0.4 at 282 K and 1.2±0.4 at 276 K, where the error limits are estimated from the output of the model calculations, taking the variability of individual runs at various DMS levels into account.
Effect of water addition and nitrogen fertilization on the fluxes of CH4, CO2, NOx, and N2O following five years of elevated CO2 in the Colorado Shortgrass Steppe
Authors: Mosier A. R. , Pendall E. , Morgan J. A.
Published: April 2010

Abstract. An open-top-chamber (OTC) CO2 enrichment (~720 mmol mol-1) study was conducted in the Colorado shortgrass steppe from April 1997 through October 2001. Aboveground plant biomass increased under elevated CO2 and soil moisture content was typically higher than under ambient CO2 conditions. Fluxes of CH4, CO2, NOx and N2O, measured weekly year round were not significantly altered by CO2 enrichment over the 55 month period of observation. During early summer of 2002, following the removal of the open-top-chambers from the CO2 enrichment sites in October 2001, we conducted a short term study to determine if soil microbial processes were altered in soils that had been exposed to double ambient CO2 concentrations during the growing season for the past five years. Microplots were established within each experimental site and 10 mm of water or 10 mm of water containing the equivalent of 10 g m-2 of ammonium nitrate-N was applied to the soil surface. Fluxes of CO2, CH4, NOx and N2O fluxes within control (unchambered), ambient CO2 and elevated CO2 OTC soils were measured at one to three day intervals for the next month. With water addition alone, CO2 and NO emission did not differ between ambient and elevated CO2 soils, while CH4 uptake rates were higher and N2O fluxes lower in elevated CO2 soils. Adding water and mineral N resulted in increased CO2 emissions, increased CH4 uptake and decreased NO emissions in elevated CO2 soils. The N addition study confirmed previous observations that soil respiration is enhanced under elevated CO2 and N immobilization is increased, thereby decreasing NO emission.
On the relationship between acetone and carbon monoxide in different air masses
Authors: de Reus M. , Fischer H. , Arnold F. , de Gouw J. , Holzinger R. , Warneke C , Williams J.
Published: April 2010

Abstract. Carbon monoxide and acetone measurements are presented for five aircraft measurement campaigns at mid-latitudes, polar and tropical regions in the northern hemisphere. Throughout all campaigns, free tropospheric air masses, which were influenced by anthropogenic emissions, showed a similar linear relation between acetone and CO, with a slope of 21-25 pptv acetone/ppbv CO. Measurements in the anthropogenically influenced marine boundary layer revealed a slope of 13-16 pptv acetone/ppbv CO. The different slopes observed in the marine boundary layer and the free troposphere indicate that acetone is emitted by the ocean in relatively clean air masses and taken up by the ocean in polluted air masses. In the lowermost stratosphere, a good correlation between acetone and CO was observed as well, however, with a much smaller slope (~5 pptv acetone/ppbv CO) compared to the troposphere. This is caused by the longer photochemical lifetime of CO compared to acetone in the lower stratosphere, due to the increasing photolytic loss of acetone and the decreasing OH concentration with altitude. No significant correlation between acetone and CO was observed over the tropical rain forest due to the large direct and indirect biogenic emissions of acetone. The common slopes of the linear acetone-CO relation in various layers of the atmosphere, during five field experiments, makes them useful for model calculations. Often a single observation of the acetone-CO correlation, determined from stratospheric measurements, has been used in box model applications. This study shows that different slopes have to be considered for marine boundary layer, free tropospheric and stratospheric air masses, and that the acetone-CO relation cannot be used for air masses which are strongly influenced by biogenic emissions.
Volcanic eruptions recorded in the Illimani ice core (Bolivia): 1918–1998 and Tambora periods
Authors: De Angelis M. , Simões J. , Bonnaveira H. , Taupin J.-D. , Delmas R. J.
Published: April 2010

Abstract. Acid layers of volcanic origin detected in polar snow and ice layers are commonly used to document past volcanic activity on a global scale or, conversely, to date polar ice cores. Although most cataclysmic eruptions of the last two centuries (Pinatubo, El Chichon, Agung, Krakatoa, Cosiguina, Tambora, etc.) occurred in the tropics, cold tropical glaciers have not been used for the reconstruction of past volcanism. The glaciochemical study of a 137 m ice core drilled in 1999 close to the summit of Nevado Illimani (Eastern Bolivian Andes, 16°37' S, 67°46' W, 6350 m asl) demonstrates, for the first time, that such eruptions are recorded by both their tropospheric and stratospheric deposits. An 80-year ice sequence (1918-1998) and the Tambora years have been analyzed in detail. In several cases, ash, chloride and fluoride were also detected. The ice records of the Pinatubo (1991), Agung (1963) and Tambora (1815) eruptions are discussed in detail. The potential impact of less important regional eruptions is discussed.
Freezing thresholds and cirrus cloud formation mechanisms inferred from in situ measurements of relative humidity
Authors: Haag W. , Kärcher B. , Ström J. , Minikin A. , Lohmann U. , Ovarlez J. , Stohl A.
Published: April 2010

Abstract. Factors controlling the microphysical link between distributions of relative humidity above ice saturation in the upper troposphere and lowermost stratosphere and cirrus clouds are examined with the help of microphysical trajectory simulations. Our findings are related to results from aircraft measurements and global model studies. We suggest that the relative humidities at which ice crystals form in the atmosphere can be inferred from in situ measurements of water vapor and temperature close to, but outside of, cirrus clouds. The comparison with concomitant measurements performed inside cirrus clouds provides a clue to freezing mechanisms active in cirrus. The analysis of field data taken at northern and southern midlatitudes in fall 2000 reveals distinct differences in cirrus cloud freezing thresholds. Homogeneous freezing is found to be the most likely mechanism by which cirrus form at southern hemisphere midlatitudes. The results provide evidence for the existence of heterogeneous freezing in cirrus in parts of the polluted northern hemisphere, but do not suggest that cirrus clouds in this region form exclusively on heterogeneous ice nuclei, thereby emphasizing the crucial importance of homogeneous freezing. The key features of distributions of upper tropospheric relative humidity simulated by a global climate model are shown to be in general agreement with both, microphysical simulations and field observations, delineating a feasible method to include and validate ice supersaturation in other large-scale atmospheric models, in particular chemistry-transport and weather forecast models.
Cirrus cloud occurrence as function of ambient relative humidity: a comparison of observations obtained during the INCA experiment
Authors: Ström J. , Seifert M. , Kärcher B. , Ovarlez J. , Minikin A. , Gayet J.-F. , Krejci R. , Petzold A. , Auriol F. , Haag W. , Busen R. , Schumann U. , Hansson H. C.
Published: April 2010

Abstract. Based on in-situ observations performed during the Interhemispheric differences in cirrus properties from anthropogenic emissions (INCA) experiment, we introduce and discuss the cloud presence fraction (CPF) defined as the ratio between the number of data points determined to represent cloud at a given ambient relative humidity over ice (RHI) divided by the total number of data points at that value of RHI. The CPFs are measured with four different cloud probes. Within similar ranges of detected particle sizes and concentrations, it is shown that different cloud probes yield results that are in good agreement with each other. The CPFs taken at Southern Hemisphere (SH) and Northern Hemisphere (NH) midlatitudes differ from each other. Above ice saturation, clouds occurred more frequently during the NH campaign. Local minima in the CPF as a function of RHI are interpreted as a systematic underestimation of cloud presence when cloud particles become invisible to cloud probes. Based on this interpretation, we find that clouds during the SH campaign formed preferentially at RHIs between 140 and 155%, whereas clouds in the NH campaign formed at RHIs somewhat below 130%. The data show that interstitial aerosol and ice particles coexist down to RHIs of 70-90%, demonstrating that the ability to distinguish between different particle types in cirrus conditions depends on the sensors used to probe the aerosol/cirrus system. Observed distributions of cloud water content differ only slightly between the NH and SH campaigns and seem to be only weakly, if at all, affected by the freezing aerosols.
The impact of model grid zooming on tracer transport in the 1999/2000 Arctic polar vortex
Authors: van den Broek M. M. P. , van Aalst M. K. , Bregman A. , Krol M. , Lelieveld J. , Toon G. C. , Garcelon S. , Hansford G. M. , Jones R. L. , Gardiner T. D.
Published: April 2010

Abstract. We have used a 3D chemistry transport model to evaluate the transport of HF and CH4 in the stratosphere during the Arctic winter of 1999/2000. Several model experiments were carried out with the use of a zoom algorithm to investigate the effect of different horizontal resolutions. Balloon-borne and satellite-borne observations of HF and CH4 were used to test the model. In addition, air mass descent rates within the polar vortex were calculated and compared to observations. Outside the vortex the model results agree well with the observations, but inside the vortex the model underestimates the observed vertical gradient in HF and CH4, even when the highest available resolution (1º x 1º) is applied. The calculated diabatic descent rates agree with observations above potential temperature levels of 450 K. These model results suggest that too strong mixing through the vortex edge could be a plausible cause for the model discrepancies, associated with the calculated mass fluxes, although other reasons are also discussed. Based on our model experiments we conclude that a global 6º x 9º resolution is too coarse to represent the polar vortex, whereas the higher resolutions, 3º x 2º and 1º x 1º, yield similar results, even with a 6º x 9º resolution in the tropical region.
Gaseous (DMS, MSA, SO2, H2SO4 and DMSO) and particulate (sulfate and methanesulfonate) sulfur species over the northeastern coast of Crete
Authors: Bardouki H. , Berresheim H. , Vrekoussis M. , Sciare J. , Kouvarakis G. , Oikonomou K. , Schneider J. , Mihalopoulos N.
Published: April 2010

Abstract. A detailed study of the levels, the temporal and diurnal variability of the main compounds involved in the biogenic sulfur cycle was carried out in Crete (Eastern Mediterranean) during the Mediterranean Intensive Oxidant Study (MINOS) field experiment in July-August 2001. Intensive measurements of gaseous dimethylsulfide (DMS), dimethylsulfoxide (DMSO), sulfur dioxide (SO2), sulfuric (H2SO4) and methanesulfonic acids (MSA) and particulate sulfate (SO42-) and methanesulfonate (MS-) have been performed during the campaign. Dimethylsulfide (DMS) levels ranged from 2.9 to 136 pmol·mol-1 (mean value of 21.7 pmol·mol-1) and showed a clear diurnal variation with daytime maximum. During nighttime DMS levels fall close or below the detection limit of 2 pmol·mol-1. Concurrent measurements of OH and NO3 radicals during the campaign indicate that NO3 levels can explain most of the observed diurnal variation of DMS. Dimethylsulfoxide (DMSO) ranged between 0.02 and 10.1 pmol·mol-1 (mean value of 1.7 pmol·mol-1) and presents a diurnal variation similar to that of DMS. SO2 levels ranged from 220 to 2970 pmol·mol-1 (mean value of 1030 pmol·mol-1), while nss-SO42- and MS- ranged from 330 to 7100 pmol·mol-1, (mean value of 1440 pmol·mol-1) and 1.1 to 37.5 pmol·mol-1 (mean value of 11.5 pmol·mol-1) respectively. Of particular interest are the measurements of gaseous MSA and H2SO4. MSA ranged from below the detection limit (3x104) to 3.7x107 molecules cm-3, whereas H2SO4 ranged between 1x105 and 9.0x107 molecules cm-3. The measured H2SO4 maxima are among the highest reported in literature and can be attributed to high insolation, absence of precipitation and increased SO2 levels in the area. From the concurrent SO2, OH, and H2SO4 measurements a sticking coefficient of 0.52±0.28 was calculated for H2SO4. From the concurrent MSA, OH, and DMS measurements the yield of gaseous MSA from the OH-initiated oxidation of DMS was calculated to range between 0.1-0.4%. This low MSA yield implies that gaseous MSA levels can not account for the observed MS- levels. Heterogeneous reactions of DMSO on aerosols should be considered to explain the observed levels of MS-.
Numerical simulations of homogeneous freezing processes in the aerosol chamber AIDA
Authors: Haag W. , Kärcher B. , Schaefers S. , Stetzer O. , Möhler O. , Schurath U. , Krämer M. , Schiller C.
Published: April 2010

Abstract. The homogeneous freezing of supercooled H2SO4/H2O aerosols in an aerosol chamber is investigated with a microphysical box model using the activity parameterization of the nucleation rate by Koop et al. (2000). The simulations are constrained by measurements of pressure, temperature, total water mixing ratio, and the initial aerosol size distribution, described in a companion paper Möhler et al. (2003). Model results are compared to measurements conducted in the temperature range between 194 and 235 K, with cooling rates in the range between 0.5 and 2.6 K min-1, and at air pressures between 170 and 1000 hPa. The simulations focus on the time history of relative humidity with respect to ice, aerosol size distribution, partitioning of water between gas and particle phase, onset times of freezing, freezing threshold relative humidities, aerosol chemical composition at the onset of freezing, and the number of nucleated ice crystals. The latter four parameters can be inferred from the experiments, the former three aid in interpreting the measurements. Sensitivity studies are carried out to address the relative importance of uncertainties of basic quantities such as temperature, total H2O mixing ratio, aerosol size spectrum, and deposition coefficient of H2O molecules on ice. The ability of the numerical simulations to provide detailed explanations of the observations greatly increases confidence in attempts to model this process under real atmospheric conditions, for instance with regard to the formation of cirrus clouds or polar stratospheric ice clouds, provided that accurate temperature and humidity measurements are available.
Climatological aspects of aerosol optical properties in Northern Greece
Authors: Gerasopoulos E. , Andreae M. O. , Zerefos C. S. , Andreae T. W. , Balis D. , Formenti P. , Merlet P. , Amiridis V. , Papastefanou C.
Published: April 2010

Abstract. Measurements of aerosol optical properties (aerosol optical depth, scattering and backscattering coefficients) have been conducted at two ground-based sites in Northern Greece, Ouranoupolis (40° 23' N, 23° 57' E, 170 m a.s.l.) and Thessaloniki (40° 38' N, 22° 57' E, 80 m a.s.l.), between 1999 and 2002. The frequency distributions of the observed parameters have revealed the presence of individual modes of high and low values, indicating the influence from different sources. At both sites, the mean aerosol optical depth at 500 nm was 0.23. Values increase considerably during summer when they remain persistently between 0.3 and 0.5, going up to 0.7-0.8 during specific cases. The mean value of 65±40 Mm-1 of the particle scattering coefficient at 550 nm reflects the impact of continental pollution in the regional boundary layer. Trajectory analysis has shown that higher values of aerosol optical depth and the scattering coefficient are found in the east sector (former Soviet Union countries, eastern Balkan countries), whereas cleaner conditions are found for the NW direction. The influence of Sahara dust events is clearly reflected in the Ångström exponents. About 45-60% of the observed diurnal variation of the optical properties was attributed to the growth of aerosols with humidity, while the rest of the variability is in phase with the evolution of the sea-breeze cell. The contribution of local pollution is estimated to contribute 35±10% to the average aerosol optical depth at the Thessaloniki site during summer. Finally, the aerosol scale height (aerosol optical depth divided by scattering coefficient) was found to be related to the height of the boundary layer with values between 0.5-1 km during winter and up to 2.5-3 km during summer.
Intercontinental transport of tropospheric ozone: a study of its seasonal variability across the North Atlantic utilizing tropospheric ozone residuals and its relationship to the North Atlantic Oscillation
Authors: Creilson J. K. , Fishman J. , Wozniak A. E.
Published: April 2010

Abstract. Using the empirically-corrected tropospheric ozone residual (TOR) technique, which utilizes coincident observations of total ozone from the Total Ozone Mapping Spectrometer (TOMS) and stratospheric ozone profiles from the Solar Backscattered Ultraviolet (SBUV) instruments, the seasonal and regional distribution of tropospheric ozone across the North Atlantic from 1979-2000 is examined. Its relationship to the North Atlantic Oscillation (NAO) is also analyzed as a possible transport mechanism across the North Atlantic. Monthly climatologies of tropospheric ozone for five different regions across the North Atlantic exhibit strong seasonality. The correlation between these monthly climatologies of the TOR and ozonesonde profiles at nearby sites in both eastern North America and western Europe are highly significant (R values of +0.98 and +0.96 respectively) and help to validate the use of satellite retrievals of tropospheric ozone. Distinct springtime interannual variability over North Atlantic Region 5 (eastern North Atlantic-western Europe) is particularly evident and exhibits similar variability to the positive phase of the NAO (R=+0.61, r=<0.01). Positive phases of the NAO are indicative of a stronger Bermuda-Azores high and a stronger Icelandic low and thus faster more zonal flow across the North Atlantic from west to east. This flow regime appears to be causing the transport of tropospheric ozone across the North Atlantic and onto Europe. The consequence of such transport is the impact on a downwind region's ability to meet their ozone attainment goals. This link between the positive phase of the NAO and increased tropospheric ozone over Region 5 could be an important tool for prediction of such pollution outbreaks.
A revised parameterization for gaseous dry deposition in air-quality models
Authors: Zhang L. , Brook J. R. , Vet R.
Published: April 2010

Abstract. A parameterization scheme for calculating gaseous dry deposition velocities in air-quality models is revised based on recent study results on non-stomatal uptake of O3 and SO2 over 5 different vegetation types. Non-stomatal resistance, which includes in-canopy aerodynamic, soil and cuticle resistances, for SO2 and O3 is parameterized as a function of friction velocity, relative humidity, leaf area index, and canopy wetness. Non-stomatal resistance for other chemical species is scaled to those of SO2 and O3 based on their chemical and physical characteristics. Stomatal resistance is calculated using a two-big-leaf stomatal resistance sub-model for all gaseous species of interest. The improvements in the present model compared to its earlier version include a newly developed non-stomatal resistance formulation, a realistic treatment of cuticle and ground resistance in winter, and the handling of seasonally-dependent input parameters. Model evaluation shows that the revised parameterization can provide more realistic deposition velocities for both O3 and SO2, especially for wet canopies. Example model output shows that the parameterization provides reasonable estimates of dry deposition velocities for different gaseous species, land types and diurnal and seasonal variations. Maximum deposition velocities from model output are close to reported measurement values for different land types. The current parameterization can be easily adopted into different air-quality models that require inclusion of dry deposition processes.
Operational mapping of atmospheric nitrogen deposition to the Baltic Sea
Authors: Hertel O. , Ambelas Skjøth C. , Brandt J. , Christensen J. H. , Frohn L. M. , Frydendall J.
Published: April 2010

Abstract. A new model system for mapping and forecasting nitrogen deposition to the Baltic Sea has been developed. The system is based on the Lagrangian variable scale transport-chemistry model ACDEP (Atmospheric Chemistry and Deposition model), and aims at delivering deposition estimates to be used as input to marine ecosystem models. The system is tested by comparison of model results to measurements from monitoring stations around the Baltic Sea. The comparison shows that observed annual mean ambient air concentrations and wet depositions are well reproduced by the model. Diurnal mean concentrations of NHx (sum of NH3 and NH4+) and NO2 are fairly well reproduced, whereas concentrations of total nitrate (sum of HNO3 and NO3-) are somewhat overestimated. Wet depositions of nitrate and ammonia are fairly well described for annual mean values, whereas the discrepancy is high for the monthly mean values and the wet depositions are rather poorly described concerning the diurnal mean values. The model calculations show that the annual atmospheric nitrogen deposition has a pronounced south--north gradient with depositions in the range about 1.0 T N km-2 in the south and 0.2 T N km-2 in the north. The results show that in 1999 the maximum diurnal mean deposition to the Danish waters appeared during the summer in the algae growth season. For the northern parts of the Baltic the highest depositions were distributed over most of the year. Total deposition to the Baltic Sea was for the year 1999 estimated to 318 kT N for an area of 464 406 km2 equivalent to an average deposition of 684 kg N/km2.
Investigation of airborne foot-and-mouth disease virus transmission during low-wind conditions in the early phase of the UK 2001 epidemic
Authors: Mikkelsen T. , Alexandersen S. , Astrup P. , Champion H. J. , Donaldson A. I. , Dunkerley F. N. , Gloster J. , Sørensen J. H. , Thykier-Nielsen S.
Published: April 2010

Abstract. Foot-and-mouth disease (FMD) is a highly contagious viral disease of cloven-hoofed domesticated and wild animals. The highly contagious nature of FMD is a reflection of the wide range of host species, the enormous quantities of virus liberated by infected animals, the range of excretions and secretions which can be infectious, the stability of the virus in the environment, the multiplicity of routes of infection and the very small doses of the virus that can initiate infection. One of the mechanisms of spread is the carriage of droplets and droplet nuclei exhaled in the breath of infected animals. Such spread can be rapid and extensive, and it is known in certain circumstances to have transmitted disease over a distance of several hundred kilometres. During the 2001 FMD epidemic in the United Kingdom (UK), atmospheric dispersion models were applied in real time in order to assess the potential for atmospheric dispersion of the disease. The operational value of such modelling is primarily to identify premises which may have been exposed so that the human resources for surveillance and disease control purposes are employed most effectively. The paper describes the combined modelling techniques and presents the results obtained of detailed analyses performed during the early stages of the UK 2001 epidemic. This paper investigates the potential for disease spread in relation to two outbreaks (Burnside Farm, Heddon-on-the-Wall and Prestwick Hall Farm, Ponteland, Northumberland). A separate paper (Gloster et al., 2002) provides a more detailed analysis of the airborne disease transmission in the vicinity of Burnside Farm. The combined results are consistent with airborne transmission of disease to livestock in the Heddon-on-the-Wall area. Local topography may have played a significant role in influencing the pattern of disease spread.
Rebuilding sources of linear tracers after atmospheric concentration measurements
Authors: Issartel J.-P.
Published: April 2010

Abstract. The identification of widespread sources of passive tracers out of atmospheric concentration measurements has become an important challenge of modern meteorology. The paper proposes some mathematical tracks to address the reconstruction of the complex space-time geometry of the sources of linear tracers. The methods are based upon the use of retroplumes. The inverse problem is addressed in a deterministic non statistical frame. The information obtained by local measurements is spread by introducing the concept of illumination. The constraint that the source is non negative is also addressed. The experimental source ETEX1 is rebuilt in order to evaluate an impulse response of the algorithms.
Intercomparison of aircraft instruments on board the C-130 and Falcon 20 over southern Germany during EXPORT 2000
Authors: Brough N. , Reeves C. E. , Penkett S. A. , Stewart D. J. , Dewey K. , Kent J. , Barjat H. , Monks P. S. , Ziereis H. , Stock P. , Huntrieser H. , Schlager H.
Published: April 2010

Abstract. In the summer 2000 EXPORT aircraft campaign (European eXport of Precursors and Ozone by long-Range Transport), two comprehensively instrumented research aircraft measuring a variety of chemical species flew wing tip to wing tip for a period of one and a quarter hours. During this interval a comparison was undertaken of the measurements of nitrogen oxide (NO), odd nitrogen species (NOy), carbon monoxide (CO) and ozone (O3). The comparison was performed at two different flight levels, which provided a 10-fold variation in the concentrations of both NO (10 to 1000 parts per trillion by volume (pptv)) and NOy (200 to over 2500 pptv). Large peaks of NO and NOy observed from the Falcon 20, which were at first thought to be from the exhaust of the C-130, were also detected on the 4 channel NOxy instrument aboard the C-130. These peaks were a good indication that both aircraft were in the same air mass and that the Falcon 20 was not in the exhaust plume of the C-130. Correlations and statistical analysis are presented between the instruments used on the two separate aircraft platforms. These were found to be in good agreement giving a high degree of correlation for the ambient air studied. Any deviations from the correlations are accounted for in the estimated inaccuracies of the instruments. These results help to establish that the instruments aboard the separate aircraft are reliably able to measure the corresponding chemical species in the range of conditions sampled and that data collected by both aircraft can be co-ordinated for purposes of interpretation.