bluuebunny/crossref_metadata_2025 huggingface dataset:
The roles of dynamical variability and aerosols in cirrus cloud formation
Authors: Kärcher B. , Ström J.
Published: April 2010

Abstract. The probability of occurrence of ice crystal number densities in young cirrus clouds is examined based on airborne measurements. The observations have been carried out at midlatitudes in both hemispheres at equivalent latitudes (~52–55° N/S) during the same season (local autumn in 2000). The in situ measurements considered in the present study include temperatures, vertical velocities, and ice crystal concentrations, the latter determined with high precision and accuracy using a counterflow virtual impactor. Most young cirrus clouds typically contain high number densities (1–10 cm−3) of small (diameter <20 μm) ice crystals. This mode dominates the probability distributions in both hemispheres and is shown to be caused by rapid cooling rates associated with updraft speeds in the range 10–100 cm s-1. A second mode containing larger crystals extends from ~1 cm−3 to low concentrations close to the detection threshold (~3×104cm−3) and is associated with lower updraft speeds. Results of a statistical analysis provide compelling evidence that the dynamical variability of vertical air motions on the mesoscale is the key factor determining the observed probability distributions of pristine ice crystal concentrations in cirrus. Other factors considered are variations of temperature as well as size, number, and ice nucleation thresholds of the freezing aerosol particles. The variability in vertical velocities is likely caused by atmospheric waves. Inasmuch as gravity waves are widespread, mesoscale variability in vertical velocities can be viewed as a universa  feature of young cirrus clouds. Large-scale models that do not account for this subgrid-scale variability yield erroneous predictions of the variability of basic cirrus cloud properties. Climate change may bring about changes in the global distribution of updraft speeds, mean air temperatures, and aerosol properties. As shown in this work, these changes could significantly modify the probability distribution of cirrus ice crystal concentrations. This study emphasizes the key role of vertical velocities and mesoscale variability in vertical velocities in controlling cirrus properties. The results suggest that, in any effort to ascribe cause to trends of cirrus cloud properties, a careful evaluation of dynamical changes in cloud formation should be done before conclusions regarding the role of other anthropogenic factors, such as changes in aerosol composition, are made.
Atmospheric impact of the 1783–1784 Laki Eruption: Part II Climatic effect of sulphate aerosol
Authors: Highwood E. J. , Stevenson D. S.
Published: April 2010

Abstract. The long 1783–1784 eruption of Laki, in southern Iceland, was one of the first eruptions to have been linked to an observed climate anomaly, having been held responsible for cold temperatures over much of the Northern Hemisphere in the period 1783–1785. Results from the first climate model simulation of the impact of the 1783–1784 fissure eruption are presented. Using sulphate aerosol fields produced in a companion chemical transport model simulation by Stevenson et al. (2003), the radiative forcing and climate response due to the aerosol are calculated here using the Reading Intermediate General Circulation Model (IGCM). The peak Northern Hemisphere mean direct radiative forcing is −5.5 Wm−2 in August 1783. The radiative forcing dies away quickly as the emissions from the volcano decrease; however, a small forcing remains over the Mediterranean until March 1784. There is little forcing in the Southern Hemisphere. There is shown to be an uncertainty of at least 50% in the direct radiative forcing due to assumptions concerning relative humidity and the sophistication of the radiative transfer code used. The indirect effects of the Laki aerosol are potentially large but essentially unquantifiable at the present time. In the IGCM at least, the aerosol from the eruption produces a climate response that is spatially very variable. The magnitude of the Northern Hemisphere annual mean anomaly for 1783 is −0.21 K, statistically significant at the 95% level and in reasonable agreement with the available observations.
On extreme atmospheric and marine nitrogen fluxes and chlorophyll-a levels in the Kattegat Strait
Authors: Hasager C. B. , Carstensen J. , Ellermann T. , Gustafson B. G. , Hertel O. , Johnsson M. , Markager S. , Skjødth C.
Published: April 2010

Abstract. A retrospective analysis is carried out to investigate the importance of the vertical fluxes of nitrogen to the marine sea surface layer in which high chlorophyll a levels may cause blooms of harmful algae and subsequent turn over and oxygen depletion at the bottom of the sea. Typically nitrogen is the limiting factor for phytoplankton in the Kattegat Strait during summer periods (May to August) and the major nitrogen inputs come from the atmosphere and deep-water entrainment. The extreme reoccurence values of nitrogen from atmospheric wet and dry deposition and deep-water flux entrainments are calculated by the periodic maximum method and the results are successfully compared to a map of chlorophyll return periods based on in-situ observations. The one-year return of extreme atmospheric wet deposition is around 70 mg N m−2 day−1 and 30 mg N m−2 day−1 for deep-water entrainment. Atmospheric nitrogen dry deposition is insignificant in the context of algal blooms. At longer time-scales e.g. at 10-year return, the nitrogen deep-water entrainment is larger than the extreme of atmospheric wet deposition. This indicates that the pool of nitrogen released from the sea bottom by deep-water entrainment forced by high winds greatly exceeds the atmospheric pool of nitrogen washed out by precipitation. At the frontal zone of the Kattegat Strait and Skagerrak, the nitrogen deep-water entrainment is very high and this explains the high 10-year return chlorophyll level at 8 mg m−3 in Kattegat. In the southern part, the extreme chlorophyll level is only 4 mg m−3 according to the statistics of a multi-year time-series of water samples. The chlorophyll level varies greatly in time and space as documented by a series of SeaWiFS satellite maps (OC4v4 algorithm) of chlorophyll, ScanFish and buoy observations from an experimental period in Kattegat and it is recommended to sample in-situ chlorophyll observation collocated in time to the satellite overpasses of e.g. SeaWiFS and ENVISAT MERIS to ensure improved mapping of the chlorophyll levels in the Danish waters.
The detection of solar proton produced 14CO
Authors: Jöckel P. , Brenninkmeijer C. A. M. , Lawrence M. G. , Siegmund P.
Published: April 2010

Abstract. Major solar eruptions (coronal mass ejections) are accompanied by massive ejections of protons. When these charged particles head for the Earth through the interplanetary magnetic field with high flux and energy, a solar proton event (SPE) is recorded. Strong SPEs, in which energetic protons penetrate the atmosphere in large numbers are rare, but do have chemical effects (Crutzen, 1975; Jackman et al., 1990, 2001). They also have nucleonic effects by which they can almost instantaneously increase the atmospheric production of radio-nuclides, including 14C (radiocarbon), but this has never been demonstrated. We show, using satellite observations and modeling, that the 2nd most intensive set of SPEs on record, that of August-December 1989, must have caused detectable increases in atmospheric 14CO. This is confirmed by a sequence of peaks in the Baring Head (NZ) time series of 14CO observations (Brenninkmeijer, 1993), providing a unique indication of production of 14C by solar protons, and demonstrating the use of SPE 14CO as an atmospheric tracer.
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 j-values 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.
Trace gas measurements from infrared satellite for chemistry and climate applications
Authors: Clerbaux C. , Hadji-Lazaro J. , Turquety S. , Mégie G. , Coheur P.-F.
Published: April 2010

Abstract. Space-borne thermal infrared instruments working in the nadir geometry are providing spectroscopic measurements of species that impact on the chemical composition of the atmosphere and on the climate forcing: H2O, CO2, N2O, CH2, CFCs, O3, and CO. The atmospheric abundances obtained from the analysis of IMG/ADEOS measurements are discussed in order to demonstrate the potential scientific return to be expected from future missions using advanced infrared nadir sounders. Some strengths and limitations of passive infrared remote sensing from space are illustrated.
Rapid intercontinental air pollution transport associated with a meteorological bomb
Authors: Stohl A. , Huntrieser H. , Richter A. , Beirle S. , Cooper O. , Eckhardt S. , Forster C. , James P. , James N. , Wenig M. , Wagner T. , Burrows J. , Platt U.
Published: April 2010

Abstract. Intercontinental transport (ICT) of trace substances normally occurs on timescales ranging from a few days to several weeks. In this paper an extraordinary episode in November 2001 is presented, where pollution transport across the North Atlantic took only about one day. The transport mechanism, termed here an intercontinental pollution express highway, was exceptional, as it involved an explosively generated cyclone, a so-called meteorological "bomb". To the authors' knowledge, this is the first study describing pollution transport in a bomb. The discovery of this event was based on transport model calculations and satellite measurements of NO2, a species with a relatively short lifetime in the atmosphere, which could be transported that far only because of the high wind speeds produced by the bomb. A 15-year transport climatology shows that intercontinental express highways are about four times more frequent in winter than in summer, in agreement with bomb climatologies. The climatology furthermore reveals that intercontinental express highways may be important for the budget of short-lived substances in the remote troposphere. For instance, for a substance with a lifetime of 1 day, express highways may be responsible for about two thirds of the total ICT. A rough calculation suggests that express highways connecting North America with Europe enhance the average NOx mixing ratios over Europe, due to North American emissions, by about 2–3 pptv in winter.
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 or flux Jevof evaporation of H2O and its condensation, kcond, on a 1mm 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 with RT in kcal mol-1 for pure ice are Jev=1.6×1028+/−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 an HCl mole fraction in the range 3.2×10−5-6.4×10−3: Jev=6.4×1026+/−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 an HBr mole fraction smaller than 6.4×10−3:Jev=7.4×1025+/−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.
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 (1°×1°) 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 6°×9° resolution is too coarse to represent the polar vortex, whereas the higher resolutions, 3°×2° and 1°×1°, yield similar results, even with a 6°×9° resolution in the tropical region.
Characteristics, impacts and direct radiative forcing of aerosols at the ARM Southern Great Plains Central Facility
Authors: Iziomon M. G. , Lohmann U.
Published: April 2010

Abstract. Based on the analysis of five years of field measurements of aerosol and meteorological variables, we present the characteristics, impacts and direct radiative forcing of aerosols at the Southern Great Plains (SGP) Central Facility (CF) of the Atmospheric Radiation Measurement (ARM) Program. Annual means of total submicron aerosol concentration for particles with aerodynamic diameter (Dp) < 1 mm, particle concentration (Np) for aerosols with 0.1 <−Dp<−10 mm, submicron light absorption coefficient (σ_a) and single scattering albedo at the SGP CF amount to 5306±392 cm−2, 654&olusmn;290 cm−3, 2.0±0.7 Mm−1 and 0.94±0.02, respectively, while those of submicron total scattering coefficient, hemispheric backscatter fraction, submicrometer scattering fraction (Rsp), Angström exponent, hygroscopic growth factor and visibility at 550 nm are  36±2 Mm−1, 0.12± 0.01, 0.84±0.03, 2.25±0.09 and 1.84±0.10, 38±2 km, respectively. Although they exhibit a considerable year-to-year variability, ,i>Rsp, σp, Np and ozone show some increase over the period examined here. This increase is accompanied by a decline in annual precipitation, column integrated water vapor, relative humidity, and cloud cover (particularly from 1998 to 2001) at the site. In particular, the marked precipitation deficit at the SGP CF in 2000 and 2001 is evident of drought associated with La Nina conditions. We compare the diabatic impacts and optical characteristics for smoke and dust aerosols. In general, aerosols at the ARM site produce a net diabatic cooling, with an estimated direct radiative forcing ranging from about −0.7 W m−2 in winter to −2.4 W m−2 in summer.
Case study of a multi-layer aerosol structure in the eastern Mediterranean observed with the airborne polarized lidar ALEX during a STAAARTE campaign (7 June 1997)
Authors: Dulac F. , Chazette P.
Published: April 2010

Abstract. We present a case study of tropospheric aerosol transport in the eastern Mediterranean, based on airborne measurements obtained south of Greece on 7 June 1997. Airborne observations (backscattering lidar at 0.532 mm with polarization measurements, in situ particle counters/sizers, and standard meteorological measurements) are complemented by monitoring with Meteosat visible and infrared images and a ground-based sun-photometer, air-mass back-trajectory computations, and meteorological analyses. The vertical structure of the lower troposphere appears complex with horizontal variability and a superposition of several turbid layers from the surface up to the clean free troposphere which is found above 2 to 4 km in altitude. We identify the presence of depolarising dust from northern Africa in the most elevated turbid layer, which is relatively humid and has clouds embedded. The lowermost troposphere likely contains pollution water-soluble aerosols from eastern continental Greece, and an intermediate layer is found with a probable mixture of the two types of particles. The column optical depth at 0.55 mm estimated from Meteosat is in the range 0.15–0.35. It is used to constrain the aerosol backscattering-to-extinction ratio needed for the backscattering lidar data inversion. The column value of 0.017 sr−1 is found applicable to the various aerosol layers and allows us to derive the aerosol extinction vertical profile. The aerosol extinction coefficient ranges from 0.03 km−1 in the lower clean free troposphere to more than 0.25 km−1 in the marine boundary layer. Values are <0.1 km−1 in the elevated dust layer but its thickness makes it dominate the aerosol optical depth at some places.
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 a.s.l.) 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. Less important eruptions located in the Andes are also recorded and may also disturb background aerosol composition on a regional scale.
CO2 flux history 1982–2001 inferred from atmospheric data using a global inversion of atmospheric transport
Authors: Rödenbeck C. , Houweling S. , Gloor M. , Heimann M.
Published: April 2010

Abstract. Based on about 20 years of NOAA/CMDL's atmospheric CO2 concentration data and a global atmospheric tracer transport model, we estimate interannual variations and spatial patterns of surface CO2 fluxes in the period 01/1982–12/2000, by using a time-dependent Bayesian inversion technique. To increase the reliability of the estimated temporal features, particular care is exerted towards the selection of data records that are homogeneous in time. Fluxes are estimated on a grid-scale resolution (~8° latitude×10° longitude), constrained by a-priori spatial correlations, and then integrated over different sets of regions. The transport model is driven by interannually varying re-analysed meteorological fields. We make consistent use of unsmoothed measurements. In agreement with previous studies, land fluxes are estimated to be the main driver of interannual variations in the global CO2 fluxes, with the pace predominantly being set by the El Niño/La Niña contrast. An exception is a 2–3 year period of increased sink of atmospheric carbon after Mt. Pinatubo's volcanic eruption in 1991. The largest differences in fluxes between El Niño and La Niña are found in the tropical land regions, the main share being due to the Amazon basin. The flux variations for the Post-Pinatubo period, the 1997/1998 El Niño, and the 1999 La Niña events are exploited to investigate relations between CO2 fluxes and climate forcing. A rough comparison points to anomalies in precipitation as a prominent climate factor for short-term variability of tropical land fluxes, both through their role on NPP and through promoting fire in case of droughts. Some large flux anomalies seem to be directly related to large biomass burning events recorded by satellite observation. Global ocean carbon uptake shows a trend similar to the one expected if ocean uptake scales proportional to the anthropogenic atmospheric perturbation. In contrast to temporal variations, the longterm spatial flux distribution can be inferred with lesser robustness only. The tentative pattern estimated by the present inversion exhibits a northern hemisphere land sink on the order of 0.4 PgC/yr (for 01/1996–12/1999, non-fossil fuel carbon only) that is mainly confined to North America. Southern hemisphere land regions are carbon neutral, while the tropical land regions are taking up carbon (e.g., at a rate of 0.8 PgC/yr during 01/1996–2/1999). Ocean fluxes show larger uptake in the Northern mid to high latitudes than in the Southern mid latitude regions, in contrast to the estimates by Takahashi et al. (1999) based on in-situ measurements. On a regional basis, results that differ the most from previous estimates are large carbon uptake of 1 to 1.5 PgC/yr by the Southern temperate Pacific ocean region, weak outgassing from the Southern ocean, and a carbon source from eastern Europe.
Vertical profiles, optical and microphysical properties of Saharan dust layers determined by a ship-borne lidar
Authors: Immler F. , Schrems O.
Published: April 2010

Abstract. A unique data set of ship-borne lidar measurements of Saharan dust layers above the Atlantic ocean has been collected aboard the research vessel Polarstern with a mobile Aerosol Raman Lidar (MARL) during the LIMPIDO-campaign in June 2000. Extended Saharan dust layers have been observed in the region between 8.5° N and 34° N in an altitude range between 2 and 6 km. The continental, North African origin of the probed air masses is confirmed by 8-day backward trajectories. The Saharan dust is characterized by an optical depth in the range of 0.1 and 0.3, a depolarization around 10\\% and high lidar ratios of 45 sr at 532 nm and 75 sr at 355 nm. The backscattering by the dust particles at the UV-wavelength is relatively weak, resulting in a negative color index. From the measured optical properties the effective radius and the refractive index of the dust particles are derived using a new approach based on Mie Theory and non-spherical scattering calculations. The low backscatter coefficient observed at 355 nm is due to significant absorption which increases with decreasing wavelength. This finding agrees very well with results from satellite and sun photometer measurements. The effective radii decrease from about 3 mm base to 0.6 mm at the top of the dust plumes. The non-spherical shapes of the dust particles are responsible for the high values of the lidar ratios.
Synergetic measurements of aerosols over São Paulo, Brazil using LIDAR, sunphotometer and satellite data during the dry season
Authors: Landulfo E. , Papayannis A. , Artaxo P. , Castanho A. D. A. , de Freitas A. Z. , Sousa R. F. , Vieira Junior N. D. , Jorge M. P. M. P. , Sánchez-Ccoyllo O. R. , Moreira D. S.
Published: April 2010

Abstract. A backscattering LIDAR system, the first of this kind in Brazil, has been set-up in a suburban area in the city of São Paulo (23° 33' S, 46° 44' W) to provide the vertical profile of the aerosol backscatter coefficient at 532 nm up to an altitude of 4–6 km above sea level (a.s.l.). The measurements have been carried out during the second half of the so-called Brazilian dry season, September and October 2001 and during the first half of the dry season in August and September 2002. The LIDAR data are presented and analysed in synergy with aerosol optical thickness (AOT) measurements obtained by a CIMEL sun-tracking photometer in the visible spectral region and with satellite measurements obtained by the MODIS sensor. This synergetic approach has been used, not only to validate the LIDAR data, but also to derive a typical value (45 sr) of the so-called extinction-to-backscatter ratio (LIDAR ratio) during the dry season. The satellite data analysis offers additional information on the spatial distribution of aerosols over Brazil including the determination of aerosol source regions over the country. The LIDAR data were also used to retrieve the Planetary Boundary Layer (PBL) height, aerosol layering and the structure of the lower troposphere over the city of São Paulo. These first LIDAR measurements over the city of São Paulo during the dry season showed a significant variability of the AOT in the lower troposphere (0.5–5 km) at 532 nm. It was also found that the aerosol load is maximized in the 1–3 km height region, although up to 3 km thick aerosol layers were also detected in the 2.5–5.5 km region in certain cases. Three-dimensional 96-hours air mass back-trajectory analysis was also performed in selected cases to determine the source regions of aerosols around São Paulo during the dry season.
Inorganic bromine in the marine boundary layer: a critical review
Authors: Sander R. , Keene W. C. , Pszenny A. A. P. , Arimoto R. , Ayers G. P. , Baboukas E. , Cainey J. M. , Crutzen P. J. , Duce R. A. , Hönninger G. , Huebert B. J. , Maenhaut W. , Mihalopoulos N. , Turekian V. C. , Van Dingenen R.
Published: April 2010

Abstract. The cycling of inorganic bromine in the marine boundary layer (mbl) has received increased attention in recent years. Bromide, a constituent of sea water, is injected into the atmosphere in association with sea-salt aerosol by breaking waves on the ocean surface. Measurements reveal that supermicrometer sea-salt aerosol is depleted in bromine by about 50% relative to conservative tracers, whereas marine submicrometer aerosol is often enriched in bromine. Model calculations, laboratory studies, and field observations strongly suggest that these depletions reflect the chemical transformation of particulate bromide to reactive inorganic gases that influence the processing of ozone and other important constituents of marine air. However, currently available techniques cannot reliably quantify many \\chem{Br}-containing compounds at ambient concentrations and, consequently, our understanding of inorganic Br cycling over the oceans and its global significance are uncertain. To provide a more coherent framework for future research, we have reviewed measurements in marine aerosol, the gas phase, and in rain. We also summarize sources and sinks, as well as model and laboratory studies of chemical transformations. The focus is on inorganic bromine over the open oceans, excluding the polar regions. The generation of sea-salt aerosol at the ocean surface is the major tropospheric source producing about 6.2 Tg/a of bromide. The transport of  Br from continents (as mineral aerosol, and as products from biomass-burning and fossil-fuel combustion) can be of local importance. Transport of degradation products of long-lived Br-containing compounds from the stratosphere and other sources contribute lesser amounts. Available evidence suggests that, following aerosol acidification, sea-salt bromide reacts to form Br2 and BrCl that volatilize to the gas phase and photolyze in daylight to produce atomic Br and Cl. Subsequent transformations can destroy tropospheric ozone, oxidize dimethylsulfide (DMS) and hydrocarbons in the gas phase and S(IV) in aerosol solutions, and thereby potentially influence climate. The diurnal cycle of gas-phase \\Br and the corresponding particulate Br deficits are correlated. Higher values of Br in the gas phase during daytime are consistent with expectations based on photochemistry. Mechanisms that explain the widely reported accumulation of particulate Br in submicrometer aerosols are not yet understood. We expect that the importance of inorganic Br cycling will vary in the future as a function of both increasing acidification of the atmosphere (through anthropogenic emissions) and climate changes. The latter affects bromine cycling via meteorological factors including global wind fields (and the associated production of sea-salt aerosol), temperature, and relative humidity.
Role of NO3 radical in oxidation processes in the eastern Mediterranean troposphere during the MINOS campaign
Authors: Vrekoussis M. , Kanakidou M. , Mihalopoulos N. , Crutzen P. J. , Lelieveld J. , Perner D. , Berresheim H. , Baboukas E.
Published: April 2010

Abstract. During the MINOS campaign (28 July–18 August 2001) nitrate (NO3) radical was measured at Finokalia, on the north coast of Crete in South-East Europe using a long path (10.4 km) Differential Optical Absorption Spectroscopy instrument (DOAS). Hydroxyl (OH) radical was also measured by a Chemical Ionization Mass-Spectrometer (Berresheim et al., this issue). These datasets represent the first simultaneous measurements of OH and NO3 radicals in the area. NO3 radical concentrations ranged from less than 3·107 up to 9·108 radical·cm-3 with an average value of 1.1·108 radical·cm−3.  The observed NO3 mixing ratios are analyzed on the basis of the corresponding meteorological data and the volatile organic compound (VOC) observations simultaneously obtained at Finokalia station. The importance of the NO3 radical relatively to that of OH in the dimethylsulfide (DMS) and nitrate cycles is also investigated. The observed NO3 levels clearly regulate the diurnal variation of DMS. NO3 and N2O5 reactions account for about 21% of the total nitrate (HNO3(g) + NO−3(part)) production.
Rebuilding sources of linear tracers after atmospheric concentration concentration
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 this reconstruction of the complex space-time geometry of the sources. 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 to be rebuilt is non negative is also addressed. The experimental source ETEX1 is rebuilt in order to evaluate an impulse response of the algorithms.
The North Atlantic Oscillation controls air pollution transport to the Arctic
Authors: Eckhardt S. , Stohl A. , Beirle S. , Spichtinger N. , James P. , Forster C. , Junker C. , Wagner T. , Platt U. , Jennings S. G.
Published: April 2010

Abstract. This paper studies the interannual variability of pollution pathways from northern hemisphere (NH) continents into the Arctic. Using a 1-year model simulation of the dispersion of passive tracers representative of anthropogenic emissions from NH continents, we show that the North Atlantic Oscillation (NAO) exerts a strong control on the pollution transport into the Arctic, particularly in winter and spring. For tracer lifetimes of 5 (30) days, surface concentrations in the Arctic winter are enhanced by about 70% (30%) during high phases of the NAO (in the following referred to as NAO+) compared to its low phases (NAO−). This is mainly due to great differences in the pathways of European pollution during NAO+ and NAO− phases, respectively, but reinforced by North American pollution, which is also enhanced in the Arctic during NAO+  phases. In contrast, Asian pollution in the Arctic does not significantly depend on the NAO phase. The model results are confirmed using remotely-sensed NO2 vertical atmospheric columns obtained from seven years of satellite measurements, which show enhanced northward NO2 transport and reduced NO2 outflow into the North Atlantic from Central Europe during NAO+ phases. Surface measurements of carbon monoxide (CO) and black carbon at high-latitude stations further corroborate the overall picture of enhanced Arctic pollution levels during NAO+ phases.
Cirrus cloud occurrence as function of ambient relative humidity: A comparison of observations from the Southern and Northern Hemisphere midlatitudes 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. , Busen R. , Schumann U. , Haag W. , Hansson H. C.
Published: April 2010

Abstract. The occurrence frequency of cirrus clouds as function of ambient relative humidity over ice, based on in-situ observations performed during the INCA experiment, show a clear difference between the campaign carried out at Southern Hemisphere (SH) midlatitudes and the campaign carried out at Northern Hemisphere (NH) midlatitudes. At a given relative humidity above ice saturation, clouds are more frequent in the NH. At relative humidities near ice saturation, clouds defined as containing particles with sizes larger than 0.55 μm diameter and an integral number density above 0.2 cm−3 were present 70% of the time during the SH campaign, whereas clouds where present 95% of the time during the NH campaign. Using a size threshold of 1 μm diameter to define the presence of clouds result in a less frequent occurrence of 60% of the time in the SH campaign and 75% of the time in the NH campaign. The data show that the presence of particles is a common characteristic of cirrus clouds. Clouds at ice saturation defined as having crystal sizes of at least 5 μm diameter and a number density exceeding 0.001 cm−3 were present in about 80% of the time during the SH campaign, and almost 90% of the time during the NH campaign. The observations reveal a significant cloud presence fraction at humidities well below ice saturation. Local minima in the cloud presence fraction as a function of relative humidity are interpreted as systematic underestimation of cloud presence because cloud particles may become invisible to cloud probes. Based on this interpretation the data suggests that clouds in the SH form preferentially at relative humidities between 140 and 155%, whereas clouds in the NH formed at relative humidities less than 130%. A simple assumption about the probability to reach successively higher humidities in an ice supersaturated air parcel provides a model that explains the main trend of the cloud presence fraction as function of relative humidity. If adiabatic processes are assumed a cloud water content distribution can be derived from this probability model. The resulting distribution agrees well in shape compared to observations, but the observed mean cloud water content is less than expected from simply adiabatic processes.
Evidence of impact of aviation on cirrus cloud formation
Authors: Zerefos C. S. , Eleftheratos K. , Balis D. S. , Zanis P. , Tselioudis G. , Meleti C.
Published: April 2010

Abstract. This work examines changes in cirrus cloud cover in possible association with aviation activities at congested air corridors. The analysis is based on the latest version of the International Satellite Cloud Climatology Project D2 data set and covers the period 1984&ndash1998. Over areas with heavy air traffic, the effect of large-scale modes of natural climate variability such as ENSO, QBO and NAO as well as the possible influence of the tropopause variability, were first removed from the cloud data set in order to calculate long-term changes of observed cirrus cloudiness. The results show increasing trends in cirrus cloud coverage, between 1984 and 1998, over the high air traffic corridors of North America, North Atlantic and Europe, which in the summertime only over the North Atlantic are statistically significant at the 99.5% confidence level (2.6% per decade). In wintertime however, statistically significant changes at the 95% confidence level are found over North America, amounting to +2.1% per decade. Statistically significant increases at the 95% confidence level are also found for the annual mean cirrus cloud coverage over the North Atlantic air corridor (1.2% per decade). Over adjacent locations with lower air traffic, the calculated trends are statistically insignificant and in most cases negative both during winter and summer in regions studied. Moreover, it is shown that the longitudinal distribution of decadal changes in cirrus cloudiness along the latitude belt centered at the North Atlantic air corridor, parallels the spatial distribution of fuel consumption from highflying air traffic, providing an independent test of possible impact of aviation on contrail cirrus formation. Results from this study are compared with other studies and different periods of records and it appears as evidenced in this and in earlier studies that there exists general agreement on the aviation effect on high cloud trends.
Comment on evidence for surface-initiated homogenous nucleation
Authors: Kay J. E. , Tsemekhman V. , Larson B. , Baker M. , Swanson B.
Published: April 2010

Abstract. We investigate theoretical, laboratory, and atmospheric evidence for a recently proposed hypothesis: homogenous ice nucleation occurs at the surface, not in the volume, of supercooled water drops. Using existing thermodynamic arguments, laboratory experiments, and atmospheric data, we conclude that ice embryo formation at the surface cannot be confirmed or disregarded. Ice nucleation rates measured as a function of drop size in an air ambient could help distinguish between volume and surface nucleation rates.
Weekly cycle of NO2 by GOME measurements: A signature of anthropogenic sources
Authors: Beirle S. , Platt U. , Wenig M. , Wagner T.
Published: April 2010

Abstract. Nitrogen oxides (NO+NO2=NOx) are important trace gases in the troposphere with impact on human health, atmospheric chemistry and climate. Besides natural sources (lightning, soil emissions) and biomass burning, fossil fuel combustion is estimated to be responsible for about 50\\% of the total production of  NOx. Since human activity in industrialized countries largely follows an artificial seven-day cycle, fossil fuel combustion is expected to  be reduced during weekends. This "weekend effect" is well known from local, ground based measurements, but has never been analysed on a global scale before. The Global Ozone Monitoring Experiment (GOME) on board the ESA-satellite ERS-2 allows measurements of  NO2 column densities. Applying sophisticated algorithms, vertical column densities (VCD) of tropospheric NO2 can be determined. We demonstrate the statistical analysis of weekly cycles of tropospheric NO2 VCDs for different regions of the world. In the cycles of the industrialized regions and cities in the US, Europe and Japan a clear Sunday minimum of tropospheric NO2 VCD can be seen. Sunday NO2 VCDs are about 25–50% lower than working day levels. Metropolitan areas with other religious and cultural backgrounds (Jerusalem, Mecca) show different weekly patterns corresponding to different days of rest. In China, no weekly pattern can be found. The presence of a weekly cycle in the measured tropospheric NO2 VCD allows the identification of anthropogenic sources. In addition, the fraction of emissions subjected to a weekly cycle (mainly transport, power generation) with respect to a constant background (all kind of natural sources, biomass burning, heavy industry) can be estimated. Furthermore, we estimated the lifetime of tropospheric NO2 by analysing the mean weekly cycle over Germany in detail, obtaining a value of about 12 h.
Spatial variation of aerosol properties derived from satellite observations
Authors: Robles González C. , Schaap M. , de Leeuw G. , Builtjes P. J. H. , van Loon M.
Published: April 2010

Abstract. Aerosol optical depths (AOD) and Angström coefficients over Europe were retrieved using data from the ATSR-2 radiometer on board the ESA satellite ERS-2, for August 1997. Taking advantage of the nadir and forward view of the ATSR-2, the dual view algorithm was used over land to eliminate the influence of the surface reflection. Over sea the AOD was retrieved using only the forward observations. Retrieved aerosol optical properties are in good agreement with those from ground-based sunphotometers. The AOD and Angström coefficients together yield information on the column integrated effective aerosol distribution. Observed regional variations of the AOD and Angström coefficient are related to anthropogenic emissions of aerosol precursors such as SO2 and NOx in the major European industrial and urban areas, and their subsequent transformation into the aerosol phase. The influence of anthropogenic aerosols such as ammonium sulphate and ammonium nitrate on the total AOD is estimated using a regional chemistry transport model. Sulphate is estimated to contribute from 15% in very clean areas to 70% in polluted areas, the contribution of nitrate is between 5% and 25% over most of Europe. This paper shows the great importance of nitrate in summer over The Netherlands.
Aerosol-cirrus interactions: A number based phenomenon at all?
Authors: Seifert M. , Ström J. , Krejci R. , Minikin A. , Petzold A. , Gayet J.-F. , Schlager H. , Ziereis H. , Schumann U. , Ovarlez J.
Published: April 2010

Abstract. In situ measurements of the partitioning of aerosol particles within cirrus clouds were used to investigate aerosol-cloud interactions in ice clouds. The number density of interstitial aerosol particles (non-activated particles in between the cirrus crystals) was compared to the number density of cirrus crystal residuals. The data was obtained during the two INCA (Interhemispheric Differences in Cirrus Properties form Anthropogenic Emissions) campaigns, performed in the Southern Hemisphere (SH) and Northern Hemisphere (NH) midlatitudes. Different aerosol-cirrus interactions can be linked to the different stages of the cirrus lifecycle. Cloud formation is linked to positive correlations between the number density of interstitial aerosol (Nint) and crystal residuals (Ncvi), whereas the correlations are smaller or even negative in a dissolving cloud. Unlike warm clouds, where the number density of cloud droplets is positively related to the aerosol number density, we observed a rather complex relationship when expressing Ncvi as a function of Nint for forming clouds. The data sets are similar in that they both show local maxima in the Nint range 100 to 200 cm−3, where the SH-maximum is shifted towards the higher value. For lower number densities Nint and Ncvi are positively related. The slopes emerging from the data suggest that a tenfold increase in the aerosol number density corresponds to a 3 to 4 times increase in the crystal number density. As Nint increases beyond the ca. 100 to 200 cm−3, the mean crystal number density decreases at about the same rate for both data sets. For much higher aerosol number densities, only present in the NH data set, the mean Ncvi remains low. The situation for dissolving clouds presents two alternative interactions between aerosols and cirrus. Either evaporating clouds are associated with a source of aerosol particles, or air pollution (high aerosol number density) retards evaporation rates.
Online mass spectrometric aerosol measurements during the MINOS campaign (Crete, August 2001)
Authors: Schneider J. , Borrmann S. , Wollny A. G. , Bläsner M. , Mihalopoulos N. , Oikonomou K. , Sciare J. , Teller A. , Levin Z. , Worsnop D. R.
Published: April 2010

Abstract. Mass spectrometric analysis of volatile and semi-volatile (= non-refractory) aerosol particles have been performed during a field study in the summer Eastern Mediterranean. A size-resolved, quantitative mass spectrometric technique (the Aerodyne Aerosol Mass Spectrometer, AMS) has been used, and the results are compared to filter sampling methods and particle sizing techniques. The different techniques agree with the finding that the fine particle mode (d<1.2 μm) consisted mostly of ammonium sulfate and of organic material. The aerosol sulfate ranged between 2 and 12 μg/m3. Ammonium was closely correlated with sulfate, suggesting ammonium sulfate as the major aerosol component, but the mass ratio of ammonium to sulfate indicated that sulfate was not fully neutralized by ammonium. Trajectories indicate that the aerosol and/or its precursors originate from South-Eastern Europe. The source of the ammonium sulfate aerosol is most likely fossil fuel burning, whereas the organic aerosol may also originate from biomass burning. Ion series analysis of the organics fraction in the mass spectrometer indicated that the major component of the organics were oxygenated organics which are a marker for aged, photochemically processed aerosol or biomass burning aerosol. The non-refractory aerosol compounds, measured with the Aerosol Mass Spectrometer, contributed between 37 and 50% to the total aerosol mass in the fine mode. A second mass spectrometer for single particle analysis by laser ablation has been used for the first time in the field during this study and yielded results, which agree with filter samples of the coarse particle mode. This mode consisted of sea salt particles and dust aerosol.