bluuebunny/crossref_metadata_2025 huggingface dataset:
Nitrogen and oxygen isotopic constraints on the origin of atmospheric nitrate in coastal Antarctica
Authors: Savarino J. , Kaiser J. , Morin S. , Sigman D. M. , Thiemens M. H.
Published: April 2010

Abstract. Throughout the year 2001, size-segregated aerosol samples were collected continuously for 10 to 15 days at the French Antarctic Station Dumont d'Urville (DDU) (66°40' S, l40°01' E, 40 m above mean sea level). The nitrogen and oxygen isotopic ratios of particulate nitrate at DDU exhibit seasonal variations that are among the most extreme observed for nitrate on Earth. Associated with a late winter increase in the concentration of particulate nitrate, δ18O and Δ17O reach as high as 111.5 versus VSMOW and 41.1, respectively. These are best explained as a signal of stratospheric input, with halogen radicals extracting 17O- and 18O-rich terminal oxygen from ozone and incorporating it into the nitrogen oxides, even if we are unable to reproduce this high Δ17O value with the current knowledge of isotopic anomaly transfers during chemical reactions. This failure calls for the evaluation of polar ozone isotopic composition. During the late springtime peak in particulate nitrate, its δ15N falls to −46.9 versus atmospheric N2, suggesting that this peak results from snow re-emission, which has been shown previously to enrich in δ15N the retained nitrate fraction in continental Antarctic sites.
First comparison of a global microphysical aerosol model with size-resolved observational aerosol statistics
Authors: Spracklen D. V. , Pringle K. J. , Carslaw K. S. , Mann G. W. , Manktelow P. , Heintzenberg J.
Published: April 2010

Abstract. A statistical synthesis of marine aerosol measurements from experiments in four different oceans is used to evaluate a global aerosol microphysics model (GLOMAP). We compare the model against observed size resolved particle concentrations, probability distributions, and the temporal persistence of different size particles. We attempt to explain the observed size distributions in terms of sulfate and sea spray and quantify the possible contributions of anthropogenic sulfate and carbonaceous material to the number and mass distribution. The model predicts a bimodal size distribution that agrees well with observations as a grand average over all regions, but there are large regional differences. Notably, observed Aitken mode number concentrations are more than a factor 10 higher than in the model for the N Atlantic but a factor 7 lower than the model in the NW Pacific. We also find that modelled Aitken mode and accumulation mode geometric mean diameters are generally smaller in the model by 10–30%. Comparison with observed free tropospheric Aitken mode distributions suggests that the model underpredicts growth of these particles during descent to the MBL. Recent observations of a substantial organic component of free tropospheric aerosol could explain this discrepancy. We find that anthropogenic continental material makes a substantial contribution to N Atlantic marine boundary layer (MBL) aerosol, with typically 60–90% of sulfate across the particle size range coming from anthropogenic sources, even if we analyse air that has spent an average of >120 h away from land. However, anthropogenic primary black carbon and organic carbon particles do not explain the large discrepancies in Aitken mode number. Several explanations for the discrepancy are suggested. The lack of lower atmospheric particle formation in the model may explain low N Atlantic particle concentrations. However, the observed and modelled particle persistence at Cape Grim in the Southern Ocean, does not reveal a diurnal cycle consistent with a photochemically driven local particle source. We also show that a physically based cloud drop activation scheme is needed to explain the observed change in accumulation mode geometric mean diameter with particle number.
Solid particles in the tropical lowest stratosphere
Authors: Nielsen J. K. , Larsen N. , Cairo F. , Di Donfrancesco G. , Rosen J. M. , Durry G. , Held G. , Pommereau J. P.
Published: April 2010

Abstract. We report in situ and remote observations proving occasional occurrence of solid particles in the tropical lowest stratosphere, far away from deep convective events. The particles were found during field campaigns in Southeast Brazil (49.03 W 22.36 S). They occur in the altitude range from 17.5 to 20.8 km, at temperatures up to at least 10 K above the expected frost point temperature. While stability of ice particles at these altitudes is unexpected from a theoretical point of view, it is argued that these observations are indications of tropospheric air masses penetrating into the stratosphere during convective overshoots. It is concluded that the intrusion of tropospheric air must have carried a large amount of water with it, which effectively hydrated the lowest stratosphere, and consequently suppressed sublimation. This conclusion is further supported by a separate water vapor mixing ratio profile obtained at the same observation site.
Global contrail radiative forcing and the impact of diurnal variations of air the impact of diurnal variations of air
Authors: Stuber N. , Forster P.
Published: April 2010

Abstract. We combined high resolution aircraft flight data from the EU Fifth Framework Programme project AERO2k with analysis data from the ECMWF's integrated forecast system to calculate diurnally resolved 3-D contrail cover. Calibrating for the 1992 contrail cover in the Bakan area (eastern-Atlantic/western-Europe), we obtained a global, annual mean contrail cover due to persistent, line-shaped contrails of 0.04%. Adopting a contrail visible optical depth of 0.1, this contrail cover results in a global, annual mean radiative forcing of 2.0 mW/m2 for all-sky and 2.1 mW/m2 for clear sky conditions. Less than 40% of the global distance travelled by aircraft is due to flights during local night time. Yet, due to the cancellation of shortwave and longwave effects during daytime, night-flights contribute a disproportional 60 to 76% to the annual mean forcing. In general, regions with a significant local contrail radiative forcing are also regions for which night time flights amount to less than half of the daily total of flights. Neglecting diurnal variations in air traffic/contrail cover by assuming a diurnal mean contrail cover can therefore increase the global mean radiative forcing by up to 30%. Scaling the 1992 forcing for the year 2000 fuel usage and accounting for differences in contrail optical depth, our forcing estimate is at the lower end but within the range of the most recent results. This reinforces the finding that some earlier published estimates of contrail radiative forcing are likely to be too large. Our study builds confidence in the calculation of contrail radiative forcing. Once the amount and optical properties of contrails are known there is relatively little uncertainty about their radiative effects. However, global model calculations of contrail radiative forcing crucially rely on scaling their contrail cover with observations. We therefore see the urgent need for an update of area mean contrail cover values derived from multi-year analyses of observational data.
Modelling the direct effect of aerosols in the solar near-infrared on a planetary scale
Authors: Hatzianastassiou N. , Matsoukas C. , Fotiadi A. , Drakakis E. , Stackhouse P. W. , Koepke P. , Pavlakis K.G. , Hatzidimitriou D. , Vardavas I.
Published: April 2010

Abstract. We used a spectral radiative transfer model to compute the direct radiative effect (DRE) of natural plus anthropogenic aerosols in the solar near-infrared (IR), between 0.85–10 µm, namely, their effect on the outgoing near-IR radiation at the top of atmosphere (TOA, ΔFTOA), on the atmospheric absorption of near-IR radiation (ΔFatmab) and on the surface downward and absorbed near-IR radiation (ΔFsurf, and ΔFsurfnet, respectively). The computations were performed on a global scale (over land and ocean) under all-sky conditions, using spectral aerosol optical properties taken from the Global Aerosol Data Set (GADS) supplemented by realistic data for the rest of surface and atmospheric parameters. The computed aerosol DRE, averaged over the 12-year period 1984–1995 for January and July, shows that aerosols produce a planetary cooling by increasing the scattered near-IR radiation back to space (by up to 6 Wm−2), they warm the atmosphere (by up to 7 Wm−2) and cool the surface (by up to 12 Wm−2). However, they can also slightly warm the Earth-atmosphere system or cool the atmosphere (by less than 1 Wm−2) over limited areas. The magnitude of the near-IR aerosol DRE is smaller than that of the combined ultraviolet (UV) and visible DRE, but it is still energetically important, since it contributes to the total shortwave (SW) DRE by 22–31%. On a global mean basis, the DREs ΔFTOA, ΔFatmab, ΔFsurf, and ΔFsurfnet are equal to about 0.48, 0.37, −1.03 and −0.85 Wm−2, i.e. their magnitude is similar to that of climate forcing associated with increasing concentrations of greenhouse gases. The aerosol induced near-IR surface cooling combined with the atmospheric warming, affects the thermal dynamics of the Earth-atmosphere system, by increasing the atmospheric stability, decreasing thus cloud formation, and precipitation, especially over desertification threatened regions such as the Mediterranean basin. This, together with the fact that the sign of near-IR aerosol DRE is sometimes opposite to that of UV-visible DRE, demonstrates the importance of performing detailed spectral computations to provide estimates of the climatic role of aerosols for the Earth-atmosphere system.
Influence of different convection parameterisations in a GCM
Authors: Tost H. , Jöckel P. , Lelieveld J.
Published: April 2010

Abstract. In global models of the atmosphere convection is parameterised, since the typical scale of this process is smaller than the model resolution. Here we address some of the uncertainties arising from the selection of different algorithms to simulate this process. Four different parameterisations for atmospheric convection, all used in state-of-the-art models, are implemented in the model system ECHAM5/MESSy for a consistent inter-comparison and evaluation against observations. Relatively large differences are found in the simulated precipitation patterns, whereas simulated water vapour columns distributions are quite similar and close to observations. The effects on the hydrological cycle and on the simulated meteorological conditions are discussed.
Differences between the QBO in the first and in the second half of the ERA-40 reanalysis
Authors: Punge H. J. , Giorgetta M. A.
Published: April 2010

Abstract. The representation of the quasi-biennial oscillation (QBO) is investigated in the ERA-40 reanalysis. In the lower stratosphere, where there is a good number of observations, the representation of the QBO is equally well throughout the record. However, strong differences between the first and the second half of the zonal wind data set are found in the upper stratosphere, with a typical offset of −10 m/s in the equatorial zonal wind in the earlier part versus the later part of the ERA-40 data set. The strength of the QBO is also affected. Possible explanations are discussed. The identified change of the assimilated wind profiles over time in ERA-40 requires a careful use of equatorial upper stratospheric winds from the reanalysis for validation purposes.
Waterfalls as sources of small charged aerosol particles
Authors: Laakso L. , Hirsikko A. , Grönholm T. , Kulmala M. , Luts A. , Parts T.-E.
Published: April 2010

Abstract. In this study, we measured (to our knowledge, for the first time) the mobility distributions of cluster and intermediate ions with an ion spectrometer near a waterfall. We observed that the concentration of negative 1.5–10 nm ions was one-hundred fold compared to a reference point 100 m away from the waterfall. Also the concentration of positive intermediate ions was found to be higher by a factor of ten compared to the reference point. The increased concentration of negative intermediate ions is assumed to be due to the so-called waterfall effect. In the waterfall effect, autoionization causes free charges inside water droplets. These charges fluctuate and cause surface protrusions which produce free ions when a droplet collides with an obstacle. Differences between the polarities are supposed to be caused by the different interaction volumes of positive and negative ions and the formation of magical clusters in water. In addition to the waterfall effect, we assume that some of the ions of both polarities are formed in droplet breakup due to uneven distribution of charges inside the droplet.
Seasonal variability of measured Ozone production efficiencies in the lower free troposphere of Central Europe
Authors: Zanis P. , Ganser A. , Zellweger C. , Henne S. , Steinbacher M. , Staehelin J.
Published: April 2010

Abstract. In this study we present the seasonal variability of ozone production efficiencies (EN), defined as the net number of ozone molecules produced per molecule of nitrogen oxides (nitrogen oxide (NO) + nitrogen dioxide (NO2)=NOx) oxidized to NOz (total reactive nitrogen (NOy)-NOx) for a seven-year period (1998–2004) at the Swiss high-alpine research station Jungfraujoch (JFJ), 3580 m a.s.l. This dataset is a unique long-term data series of nitrogen levels in the free troposphere over Central Europe and hence it offers an excellent opportunity to perform such an analysis and provide further evidence to the photochemical origin of the ozone spring maximum at locations of the northern hemisphere distant from nearby pollution sources. Experimentally derived daily EN values have been selected for 571 days out of the 2557 days from 1998 to 2004, from which an average ozone production efficiency of 18.8±1.3 molecules of O3 produced per molecule of NOx oxidized was calculated. This value indicates the great potential and importance of photochemical ozone production in the free troposphere. The monthly means of experimentally derived daily EN values show a seasonal variation with lower values from May to August, which can be probably attributed to more efficient vertical transport of polluted air masses from the atmospheric boundary layer up to JFJ. In agreement, theoretically derived monthly EN values show similar seasonal variation. The ratio NOy/CO, a parameter to assess the aging process that has occurred in an air parcel, was used as a criterion to disaggregate the 571 selected days between undisturbed and disturbed free tropospheric (FT). The monthly means of experimentally derived EN values for the undisturbed FT conditions show a distinct seasonal cycle with higher values in the cold season from November to April. The EN values for undisturbed FT conditions are particularly higher than the respective monthly EN values for disturbed FT conditions from February to October. It should be noted that the monthly EN values of March (EN=35.8) and April (EN=34.9) are among the highest values throughout the year for undisturbed FT conditions at JFJ. These results highlight the key and possibly the dominant role for photochemistry in the observed build-up of tropospheric ozone in the winter-spring transition period.
Aerosol-cloud interaction inferred from MODIS satellite data and global aerosol models
Authors: Myhre G. , Stordal F. , Johnsrud M. , Kaufman Y. J. , Rosenfeld D. , Storelvmo T. , Kristjansson J. E. , Berntsen T. K. , Myhre A. , Isaksen I. S.A.
Published: April 2010

Abstract. We have used the Modis satellite data and two global aerosol models to investigate relationships between aerosol optical depth (AOD) and cloud parameters that may be affected by the aerosol concentration. The relationships that are studied are mainly between AOD on the one hand and cloud cover, cloud liquid water path, and water vapour on the other. Additionally, cloud droplet effective radius, cloud optical depth, cloud top pressure and aerosol Ångström exponent, have been analysed in a few cases. In the Modis data we found as in earlier studies an enhancement in the cloud cover with increasing AOD. We find it likely that most of the strong increase in cloud cover with AOD, at least for AOD<0.2, is a result of aerosol-cloud interactions and prolonged cloud lifetime. Meteorology seems not to be a cause for the increase in cloud cover with AOD in this range. When water uptake of the aerosols is not taken into account in the models the modelled cloud cover actually decreases with AOD. Part of the relationship found in the Modis data for AOD>0.2 can be explained by larger water uptake close to clouds since relative humidity is higher in regions with higher cloud cover. The efficiency of the hygroscopic growth depends on aerosol type, hygroscopic nature of the aerosol, the relative humidity, and to some extent the cloud screening. By analysing the Ångström exponent we find that the hygroscopic growth of the aerosol is not likely to be a main contributor to the cloud cover increase with AOD. Since the largest increase in cloud cover with AOD is for low AOD (~0.2) and thus also for low cloud cover, cloud contamination is not likely to play a large role. However, interpretation of the complex relationships between AOD and cloud parameters should be made with great care and further work is clearly needed.
Technical note: A numerical test-bed for detailed ice nucleation studies in the AIDA cloud simulation chamber
Authors: Cotton R. J. , Benz S. , Field P. R. , Möhler O. , Schnaiter M.
Published: April 2010

Abstract. The AIDA (Aerosol Interactions and Dynamics in the Atmosphere) aerosol and cloud chamber of Forschungszentrum Karlsruhe can be used to test the ice forming ability of aerosols. The AIDA chamber is extensively instrumented including pressure, temperature and humidity sensors, and optical particle counters. Expansion cooling using mechanical pumps leads to ice supersaturation conditions and possible ice formation. In order to describe the evolving chamber conditions during an expansion, a detailed microphysics size-resolving parcel model was modified to account for diabatic heat and moisture interactions with the chamber walls. Model results are shown for a series of expansions where the initial chamber temperature ranged from −20°C to −60°C and which used desert dust as ice forming nuclei. During each expansion, the initial formation of ice particles was clearly observed. For the colder expansions there were two clear ice nucleation episodes. In order to test the ability of the model to represent the changing chamber conditions and to give confidence in the observations of chamber temperature and humidity, and ice particle concentration and mean size, ice particles were simply added as a function of time so as to reproduce the observations of ice crystal concentration. The time interval and chamber conditions over which ice nucleation occurs is therefore accurately known, and enables the model to be used as a test bed for different representations of ice formation.
Two-years of NO3 radical observations in the boundary layer over the Eastern Mediterranean
Authors: Vrekoussis M. , Mihalopoulos N. , Gerasopoulos E. , Kanakidou M. , Crutzen P. , Lelieveld J.
Published: April 2010

Abstract. This is the first study that investigates the seasonal variability of nitrate (NO3) radicals in the marine boundary layer over the East Mediterranean Sea. An extensive data set of NO3 radical observations on the north coast of Crete for more than two years (June 2001–September 2003) is presented here. NO3 radicals follow a distinct seasonal dependency with maximum mixing ratios in summer (5.6±1.2 pptv) and minimum in winter (1.2±1.2 pptv). Episodes with high NO3 mixing ratios have been encountered mainly in polluted air masses originating from mainland Greece, Central and East Europe, and Turkey. Ancillary measurements of ozone, nitrogen dioxide (NO2 and meteorological parameters have been conducted and used to explain the observed NO3 variability. The acquired NO2 nighttime observations provide the up-to-date most complete overview of NO2 temporal variability in the area. The data show that the NO3 nighttime mixing ratios are primarily dependent on NO2 (positive correlation) and relative humidity (negative correlation) and to a lesser extend on temperature (positive correlation). As inferred from these observations, on average the major sink of NO3 radicals in the area is the heterogeneous reaction of dinitrogen pentoxide (N2O5) on aqueous particles whereas the homogeneous gas phase reactions of NO3 are most important during spring and summer. NO 3 chemistry in the area significantly contributes to VOC oxidation and to the nighttime formation of peroxy radicals, nitric acid and particulate nitrate.
Mirror image hydrocarbons from Tropical and Boreal forests
Authors: Williams J. , Yassaa N. , Bartenbach S. , Lelieveld J.
Published: April 2010

Abstract. Monoterpenes, emitted in large quantities by trees to attract pollinators and repel herbivores, can exist in mirror image forms called enantiomers. In this study such enantiomeric pairs have been measured in ambient air over extensive forest ecosystems in South America and northern Europe. For the dominant monoterpene, α-pinene, the (–)-form was measured in large excess over the (+)-form over the Tropical rainforest, whereas the reverse was observed over the Boreal forest. Interestingly, over the Tropical forest (–)-α-pinene did not correlate with its own enantiomer, but correlated well with isoprene. The results indicate a remarkable ecosystem scale enantiomeric fingerprint and a nexus between the biosphere and atmosphere.
Hygroscopic properties of ultrafine aerosol particles in the boreal forest: diurnal variation, solubility and the influence of sulfuric acid
Authors: Ehn M. , Petäjä T. , Aufmhoff H. , Aalto P. , Hämeri K. , Arnold F. , Laaksonen A. , Kulmala M.
Published: April 2010

Abstract. Freshly formed atmospheric aerosol particles are neither large enough to efficiently scatter incoming solar radiation nor able to act as cloud condensation nuclei. As the particles grow larger, their hygroscopicity determines the limiting size after which they are important in both of the aforementioned processes. The condensing species resulting in growth alter the hygroscopicity of the particles. We have measured hygroscopic growth of aerosol particles present in a boreal forest, along with the very hygroscopic atmospheric trace gas sulfuric acid. The focus was on days with new particle formation by nucleation. The measured hygroscopic growth factors (GF) correlated positively with gaseous phase sulfuric acid concentrations. This correlation had a strong size dependency; the smaller the particle, the more condensing sulfuric acid is bound to alter the GF due to initially smaller mass. In addition, water uptake of nucleation mode particles was monitored during new particle formation events and followed during their growth to Aitken mode sizes. As the modal diameter increased, the solubility of the particles decreased. This indicated that initially more hygroscopic particles transformed into less hygroscopic or even hydrophobic particles. A similar behavior was seen also during days with no particle formation, with GF decreasing during the evenings and increasing during early morning. This can be tentatively explained by day- and nighttime differences in the hygroscopicity of condensable vapors.
Defintion of “banner clouds" based on time lapse movies
Authors: Schween J. H. , Kuettner J. , Reinert D. , Reuder J. , Wirth V.
Published: April 2010

Abstract. Banner clouds appear on the leeward side of a mountain and resemble a banner or a flag. This article provides a comprehensive definition of "banner clouds". It is based primarily on an extensive collection of time lapse movies, but previous attempts at an explanation of this phenomenon are also taken into account. The following ingredients are considered essential: the cloud must be attached to the mountain but not appear on the windward side; the cloud must originate from condensation of water vapour contained in the air (rather than consist of blowing snow); the cloud must be persistent; and the cloud must not be of convective nature. The definition is illustrated and discussed with the help of still images and time lapse movies taken at Mount Zugspitze in the Bavarian Alps.
Atmospheric deuterium fractionation: HCHO and HCDO yields in the CH2DO+O2 reaction
Authors: Nilsson E. , Johnson M. S. , Taketani F. , Matsumi Y. , Hurley M. D. , Wallington T. J.
Published: April 2010

Abstract. The formation of formaldehyde via hydrogen atom transfer from the methoxy radical to molecular oxygen is a key step in the atmospheric photochemical oxidation of methane, and in the propagation of deuterium from methane to molecular hydrogen. We report the results of the first investigation of the branching ratio for HCHO and HCDO formation in the CH2DO+O2 reaction. Labeled methoxy radicals (CH2DO) were generated in a photochemical reactor by photolysis of CH2DONO. HCHO and HCDO concentrations were measured using FTIR spectroscopy. Significant deuterium enrichment was seen in the formaldehyde product, from which we derive a branching ratio of 88.2±1.1% for HCDO and 11.8±1.1% for HCHO. The implications of this fractionation on the propagation of deuterium in the atmosphere are discussed.
Evolving mass spectra of the oxidized component of organic aerosol: results from aerosol mass spectrometer analyses of aged diesel emissions
Authors: Sage A. M. , Weitkamp E. A. , Robinson A. L. , Donahue N. M.
Published: April 2010

Abstract. The species and chemistry responsible for secondary organic aerosol (SOA) formation remain highly uncertain. Laboratory studies of the oxidation of individual, high-flux SOA precursors do not lead to particles with mass spectra (MS) matching those of ambient aged organic material. And, the complexity of real organic particles challenges efforts to identify their chemical origins. We have previously hypothesized that SOA can form from the atmospheric oxidation of a large suite of precursors with varying vapor pressures. Here, we support this hypothesis by using an aerosol mass spectrometer to track the chemical evolution of diesel exhaust as it is photochemically oxidized in an environmental chamber. With explicit knowledge of the condensed-phase MS of the primary emissions from our engine, we are able to decompose each recorded MS into contributing primary and secondary spectra throughout the experiment. We find that the SOA MS becomes increasingly oxidized as a function of time, eventually reaching a final MS that closely resembles that of ambient aged organic particulate matter. This observation is consistent with the idea that lower vapor pressure, semi-volatile organic emissions can form condensable products with fewer generations of oxidation, and therefore, they form relatively less oxidized SOA very quickly.
Intercomparison of ground-based ozone and NO2 measurements during the MANTRA 2004 campaign
Authors: Fraser A. , Bernath P. F. , Blatherwick R. D. , Drummond J. R. , Fogal P. F. , Fu D. , Goutail F. , Kerzenmacher T. E. , McElroy C. T. , Midwinter C. , Olson J. R. , Strong K. , Walker K. A. , Wunch D. , Young I. J.
Published: April 2010

Abstract. The MANTRA (Middle Atmosphere Nitrogen TRend Assessment) 2004 campaign took place in Vanscoy, Saskatchewan, Canada (52° N, 107° W) from 3 August to 15 September, 2004. In support of the main balloon launch, a suite of five zenith-sky and direct-Sun-viewing UV-visible ground-based spectrometers was deployed, primarily measuring ozone and NO2 total columns. Three Fourier transform spectrometers (FTSs) that were part of the balloon payload also performed ground-based measurements of several species, including ozone. Ground-based measurements of ozone and NO2 differential slant column densities from the zenith-viewing UV-visible instruments are presented herein. They are found to partially agree within NDACC (Network for the Detection of Atmospheric Composition Change) standards for instruments certified for process studies and satellite validation. Vertical column densities of ozone from the zenith-sky UV-visible instruments, the FTSs, a Brewer spectrophotometer, and ozonesondes are compared, and found to agree within the combined error estimates of the instruments (15%). NO2 vertical column densities from two of the UV-visible instruments are compared, and are also found to agree within combined error (15%).
Measuring the specific surface area of snow with X-ray tomography and gas adsorption: comparison and implications for surface smoothness
Authors: Kerbrat M. , Pinzer B. , Huthwelker T. , Gäggeler H. W. , Ammann M. , Schneebeli M.
Published: April 2010

Abstract. Chemical and physical processes, such as heterogeneous chemical reactions, light scattering, and metamorphism occur in the natural snowpack. To model these processes in the snowpack, the specific surface area (SSA) is a key parameter. In this study, two methods, computed tomography and methane adsorption, which have intrinsically different spatial resolutions –molecular and 30 μm, respectively – were used to determine the SSA of identical natural snow samples. The two methods give identical results, with an uncertainty of 3%. This implies that the surface of natural snow is smooth up to a scale of about 30 μm and that for optical methods a voxel size of 10 μm is sufficient to capture all structural features of natural snow. This smoothness can be physically explained by calculating sublimation and surface diffusion on the snow particles. The methane adsorption method is superior to computed tomography for very fresh snow, but thin layers typical for natural snowpacks can not be resolved. Computed tomography can measure SSA in layers of less than 1 mm thickness, and is therefore advantageous in layered snowpacks.
CUACE/Dust – an integrated system of observation and modeling systems for operational dust forecasting in Asia
Authors: Gong S. L. , Zhang X. Y.
Published: April 2010

Abstract. An integrated sand and dust storm (SDS) forecasting system – CUACE/Dust (the Chinese Unified Atmospheric Chemistry Environment for Dust) has been developed, which consists of a comprehensive dust aerosol module with emission, dry/wet depositions and other atmospheric dynamic processes, and a data assimilation system (DAS) using observational data from the CMA (China Meteorological Administration) ground dust monitoring network and retrieved dust information from a Chinese geostationary satellite – FY-2C. This is the first time that a combination of surface network observations and satellite retrievals of the dust aerosol has been successfully used in the real time operational forecasts in East Asia through a DAS. During its application for the operational SDS forecasts in East Asia for spring 2006, this system captured the major 31 SDS episodes observed by both surface and satellite observations. Analysis shows that the seasonal mean threat score (TS) for 0–24 h forecast over the East Asia in spring 2006 increased from 0.22 to 0.31 by using the DAS, a 41% enhancement. The time series of the forecasted dust concentrations for a number of representative stations for the whole spring 2006 were also evaluated against the surface PM10 monitoring data, showing a very good agreement in terms of the SDS timing and magnitudes near source regions where dust aerosols dominate. This is a summary paper for a special issue of ACP featuring the development and results of the forecasting system.
Small-scale mixing processes enhancing troposphere-to-stratosphere transport by pyro-cumulonimbus storms
Authors: Luderer G. , Trentmann J. , Hungershöfer K. , Herzog M. , Fromm M. , Andreae M. O.
Published: April 2010

Abstract. Deep convection induced by large forest fires is an efficient mechanism for transport of aerosol particles and trace gases into the upper troposphere and lower stratosphere (UT/LS). For many pyro-cumulonimbus clouds (pyroCbs) as well as other cases of severe convection without fire forcing, radiometric observations of cloud tops in the thermal infrared (IR) reveal characteristic structures, featuring a region of relatively high brightness temperatures (warm center) surrounded by a U-shaped region of low brightness temperatures. We performed a numerical simulation of a specific case study of pyroCb using a non-hydrostatic cloud resolving model with a two-moment cloud microphysics parameterization and a prognostic turbulence scheme. The model is able to reproduce the thermal IR structure as observed from satellite radiometry. Our findings establish a close link between the observed temperature pattern and small-scale mixing processes atop and downwind of the overshooting dome of the pyroCb. Such small-scale mixing processes are strongly enhanced by the formation and breaking of a stationary gravity wave induced by the overshoot. They are found to enhance the stratospheric penetration of the smoke by up to 30 K and thus are of major significance for irreversible transport of forest fire smoke into the lower stratosphere.
What do 14CO measurements tell us about OH?
Authors: Krol M. C. , Fokke Meirink J. , Bergamaschi P. , Mak J. E. , Lowe D. , Jöckel P. , Houweling S. , Röckmann T.
Published: April 2010

Abstract. The possible use of 14CO measurements to constrain hydroxyl radical (OH) concentrations in the atmosphere is investigated.14CO is mainly produced in the upper atmosphere from cosmic radiation. During transport to measurement locations at the Earth's surface 14CO is oxidized by OH. In this paper, the sensitivity of 14CO mixing ratio measurements to the 3-D OH distribution is assessed with the TM5 model. Simulated 14CO mixing ratios compare reasonably well with atmospheric measurements taken at five locations worldwide. As a next step, the sensitivity of 14CO measurements to OH is calculated with the adjoint TM5 model. For our sensitivity calculations the adjoint methodology outlined in the paper offers computational advantages compared to forward model calculations. The results indicate that 14CO measurements, especially those taken in the tropics, are sensitive to OH in a spatially confined region. Moreover, the OH sensitivity at a certain location varies strongly over time due to meteorological variability. On average, 14CO measurements are about 5 times more sensitive to OH at high latitudes than to OH in the tropics. Moreover, the measurements are sensitive to OH in the main 14CO source region in the upper atmosphere. It will therefore be difficult to assign model-measurement discrepancies to either the 14CO source uncertainty or to the OH sink. Nevertheless, the large distance between the 14CO source region and the measurement locations should allow the retrieval of information on OH. Specifically, the sensitivity to OH in the lower atmosphere during a relatively short time span may offer the possibility to constrain local OH. These efforts will strongly depend on the number of measurements available and on our ability to accurately model the 14CO transport.
Formation and transport of photooxidants over Europe during the July 2006 heat wave – observations and GEM-AQ model simulations
Authors: Struzewska J. , Kaminski J. W.
Published: April 2010

Abstract. The European heat wave of July 2006 was caused by advection of very hot and dry air from North Africa. Air masses were transported on the western edge of the high pressure system, centred over Eastern Europe, resulting in exceptionally high air temperatures over large areas of the continent. In the first two weeks of July 2006 a severe heat wave affected Central and North-Eastern Europe. We present a synoptic analysis of the July 2006 European heat wave, resulting formation and transport of photooxidants over Europe as simulated by the on-line tropospheric chemistry model GEM-AQ. The model was executed on a global variable grid with ~15 km resolution over the entire European continent. Modelling results have been compared with surface observations and vertical soundings of meteorological and air quality parameters. We find that the calculated error measures, as well as analysis of time series and trends for selected quantities, indicate good model performance over the simulation period. The spatial pattern of two exposure indicators – SOMO35 and AOT40 – showed similarities with the high temperatures distribution. The zone of highest exposure was located over Central Europe along the advection path of the hot African air mass. The exposure to high ozone concentrations in higher latitudes was reduced by the dilution and aging of polluted air masses advected from over Central Europe.
Short- and medium-term atmospheric effects of very large solar proton events
Authors: Jackman C. H. , Marsh D. R. , Vitt F. M. , Garcia R. R. , Fleming E. L. , Labow G. J. , Randall C. E. , López-Puertas M. , Funke B.
Published: April 2010

Abstract. Solar eruptions sometimes produce protons, which impact the Earth's atmosphere. These solar proton events (SPEs) generally last a few days and produce high energy particles that precipitate into the Earth's atmosphere. The protons cause ionization and dissociation processes that ultimately lead to an enhancement of odd-hydrogen and odd-nitrogen in the polar cap regions (>60° geomagnetic latitude). We have used the Whole Atmosphere Community Climate Model (WACCM3) to study the atmospheric impact of SPEs over the period 1963–2005. The very largest SPEs were found to be the most important and caused atmospheric effects that lasted several months to years after the events. We present the short- and medium-term (days to a few months) atmospheric influence of the four largest SPEs in the past 45 years (August 1972; October 1989; July 2000; and October–November 2003) as computed by WACCM3 and observed by satellite instruments. The polar effects can be summarized as follows: 1) Mesospheric NOx (NO+NO2) increased by over 50 ppbv and mesospheric ozone decreased by over 30% during these very large SPEs; 2) upper stratospheric and lower mesospheric NOx increased by over 10 ppbv and was transported during polar night down to the middle stratosphere in a few weeks; 3) mid- to upper stratospheric ozone decreased over 20%; and 4) enhancements of HNO3, HOCl, ClO, ClONO2, and N2O5 were indirectly caused by the very large SPEs, although the model results suggest impacts at higher altitudes than indicated by the measurements for the October–November 2003 SPE period.
PM speciation and sources in Mexico during the MILAGRO-2006 Campaign
Authors: Querol X. , Pey J. , Minguillón M. C. , Pérez N. , Alastuey A. , Viana M. , Moreno T. , Bernabé R. M. , Blanco S. , Cárdenas B. , Vega E. , Sosa G. , Escalona S. , Ruiz H. , Artíñano B.
Published: April 2010

Abstract. Levels of PM10, PM2.5 and PM1 and chemical speciation of PM10 and PM2.5 were measured during the MILAGRO campaign (1 to 31 March 2006, but extended in some cases until 6 April) at four urban, one suburban, two rural, two rural background sites with different degree of industrial influence in the Mexico City Metropolitan Area (MCMA) and adjacent regions. PM10 and PM2.5 daily levels varied between 50–56 μg/m³ and 24–46 μg/m³ at the urban sites, 22–35 μg/m³ and 13–25 μg/m³ at the rural sites, and 75 μg/m³ and 31 $\\mu $g/m³ at the industrial hotspot, respectively; lower than those recorded at some Asian mega-cities and similar to those recorded at other South American cities. At the urban sites, hourly PM2.5 and PM1 concentrations showed a marked impact of road traffic emissions (at rush hours), with levels of coarse PM remaining elevated during daytime. At the suburban and rural sites, different PM daily patterns were registered according to the influence of the pollution plume from MCMA and also on local soil resuspension. The speciation studies showed that mineral matter accounted for 25–27% of bulk PM10 at the urban sites and a higher proportion (up to 43%) at the suburban and rural sites. This pattern is repeated in PM2.5, with 15% at urban and 28% at suburban and rural sites. Carbonaceous compounds accounted for a similar proportion at the urban sites (24–32% in PM10, and up to 37% in PM2.5), markedly reduced at the suburban and rural sites (17% in PM10, and 23–38% in PM2.5). The secondary inorganic aerosols accounted for 10–20% of bulk PM10 at urban, suburban, rural and industrial sites, with a higher proportion (40%) at the industrial background site. A relatively high proportion of nitrate in rural sites was present in the coarse fraction. Typically anthropogenic elements (As, Cr, Zn, Cu, Pb, Sn, Sb, Ba, among others) showed considerably high levels at the urban sites; however levels of particulate Hg and crustal trace elements (Rb, Ti, La, Sc, Ga) were generally higher at the suburban site. Principal component analysis identified three common factors: crustal, regional background and road traffic. Moreover, some specific factors were obtained for each site.
The effect of the total solar eclipse of 29 March 2006 on meteorological variables in Greece
Authors: Founda D. , Melas D. , Lykoudis S. , Lisaridis I. , Gerasopoulos E. , Kouvarakis G. , Petrakis M. , Zerefos C.
Published: April 2010

Abstract. This paper examines the effect of the total solar eclipse of 29 March 2006 on meteorological variables across Greece. Integrated micrometeorological measurements were conducted at Kastelorizo, a small island within the path of totality, and other sites within the Greek domain, with various degrees of solar obscuration. The observations showed a dramatic reduction in the incoming global radiation and subsequent, pronounced changes in surface air temperature with the lowest temperature values occurring about 15 min after the full phase. The amplitude of the air temperature drop was not analogous to the obscuration percentage but was principally determined by the surrounding environment (mainly the sea influence), the background meteorological conditions and local cloudiness. Surface wind-speed decreased in most sites as a result of the cooling and stabilization of the atmospheric boundary layer. This perturbation provided a unique opportunity to apply a sensitivity analysis on the effect of the eclipse to the Weather Research and Forecast (WRF) numerical mesoscale meteorological model. Strong anomalies, not associated with a dynamic response, were simulated over land especially in surface air temperature. The simulated temperature drop pattern was consistent with the observations.
Spectral absorption properties of atmospheric aerosols
Authors: Bergstrom R. W. , Pilewskie P. , Russell P. B. , Redemann J. , Bond T. C. , Quinn P. K. , Sierau B.
Published: April 2010

Abstract. We have determined the solar spectral absorption optical depth of atmospheric aerosols for specific case studies during several field programs (three cases have been reported previously; two are new results). We combined airborne measurements of the solar net radiant flux density and the aerosol optical depth with a detailed radiative transfer model for all but one of the cases. The field programs (SAFARI 2000, ACE Asia, PRIDE, TARFOX, INTEX-A) contained aerosols representing the major absorbing aerosol types: pollution, biomass burning, desert dust and mixtures. In all cases the spectral absorption optical depth decreases with wavelength and can be approximated with a power-law wavelength dependence (Absorption Angstrom Exponent or AAE). We compare our results with other recent spectral absorption measurements and discuss the limitations in using the AAE for calculating the solar absorption. We also discuss the resulting spectral single scattering albedo for these cases.
Design of and initial results from a highly instrumented reactor for atmospheric chemistry (HIRAC)
Authors: Glowacki D. R. , Goddard A. , Hemavibool K. , Malkin T. L. , Commane R. , Anderson F. , Bloss W. J. , Heard D. E. , Ingham T. , Pilling M. J. , Seakins P. W.
Published: April 2010

Abstract. The design of a Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) is described and initial results obtained from HIRAC are presented. The ability of HIRAC to perform in-situ laser-induced fluorescence detection of OH and HO2 radicals with the Fluorescence Assay by Gas Expansion (FAGE) technique establishes it as internationally unique for a chamber of its size and pressure/temperature variable capabilities. In addition to the FAGE technique, HIRAC features a suite of analytical instrumentation, including: a multipass FTIR system; a conventional gas chromatography (GC) instrument and a GC instrument for formaldehyde detection; and NO/NO2, CO, O3, and H2O vapour analysers. Ray tracing simulations and measurements of the blacklamp flux have been utilized to develop a detailed model of the radiation field within HIRAC. Comparisons between the analysers and the FTIR coupled to HIRAC have been performed, and HIRAC has also been used to investigate pressure dependent kinetics of the chlorine atom reaction with ethene and the reaction of O3 and t-2-butene. The results obtained are in good agreement with literature recommendations and Master Chemical Mechanism predictions. HIRAC thereby offers a highly instrumented platform with the potential for: (1) high precision kinetics investigations over a range of atmospheric conditions; (2) detailed mechanism development, significantly enhanced according to its capability for measuring radicals; and (3) field instrument intercomparison, calibration, development, and investigations of instrument response under a range of atmospheric conditions.
Size distributions of elemental carbon in a coastal urban atmosphere in South China: characteristics, evolution processes, and implications for the mixing state
Authors: Huang Xiao-Feng , Yu Jian Zhen
Published: April 2010

Abstract. Elemental carbon (EC), as one of the primary light-absorbing components in the atmosphere, has a significant impact on both regional and global climate. The environmental impacts of EC are strongly dependent on its particle size. Little is known about the size distribution characteristics of EC particles in the ambient environments of China. We here report size distributions of EC in the urban area of Shenzhen in South China. EC consistently exhibited two modes, a fine and a coarse mode. The majority of EC (~80%) in this coastal metropolitan city resided in particles smaller than 3.2 μm in diameter. The fine mode peaked at around either 0.42 μm or 0.75 μm. While the mode at 0.42 μm could be ascribed to fresh vehicular emissions in this region, the mode at 0.75 μm had to be a result of particle growth from smaller EC particles. We made a theoretical investigation of the particle growth processes that were responsible for EC particles to grow from 0.42 μm to 0.75 μm in the atmosphere. Our calculations indicate that the EC peak at 0.75 μm could not be produced through either coagulation or H2SO4 condensation; both were too slow to lead to significant EC growth. Hygroscopic growth was also calculated to be impossible. Instead, addition of sulfate through in-cloud processing was found to be able to significantly grow EC particles to explain the EC peak at 0.75 μm. We also estimated from the EC size distributions the mixing state of EC. In the droplet size, at least 45–60% of EC mass in the summer samples and 68% of EC mass in the winter samples was internally mixed with sulfate as a result of in-cloud processing. Such information on EC needs to be considered in modeling aerosol optical properties in this region. Our results also suggest that the in-cloud processing of primary EC particles could enhance light absorbing capacities through mixing EC and sulfate.