bluuebunny/crossref_metadata_2025 huggingface dataset:
ENSO surface longwave radiation forcing over the tropical Pacific
Authors: Pavlakis K. G. , Hatzidimitriou D. , Drakakis E. , Matsoukas C. , Fotiadi A. , Hatzianastassiou N. , Vardavas I.
Published: April 2010

Abstract. We have studied the spatial and temporal variation of the surface longwave radiation (downwelling and net) over a 21-year period in the tropical and subtropical Pacific Ocean (40 S–40 N, 90 E–75 W). The fluxes were computed using a deterministic model for atmospheric radiation transfer, along with satellite data from the ISCCP-D2 database and reanalysis data from NCEP/NCAR (acronyms explained in main text), for the key atmospheric and surface input parameters. An excellent correlation was found between the downwelling longwave radiation (DLR) anomaly and the Niño-3.4 index time-series, over the Niño-3.4 region located in the central Pacific. A high anti-correlation was also found over the western Pacific (15–0 S, 105–130 E). There is convincing evidence that the time series of the mean downwelling longwave radiation anomaly in the western Pacific precedes that in the Niño-3.4 region by 3–4 months. Thus, the downwelling longwave radiation anomaly is a complementary index to the SST anomaly for the study of ENSO events and can be used to asses whether or not El Niño or La Niña conditions prevail. Over the Niño-3.4 region, the mean DLR anomaly values range from +20 Wm−2 during El Niño episodes to −20 Wm−2 during La Niña events, while over the western Pacific (15–0 S, 105–130 E) these values range from −15 Wm−2 to +10 Wm−2, respectively. The long- term average (1984–2004) distribution of the net downwelling longwave radiation at the surface over the tropical and subtropical Pacific for the three month period November-December-January shows a net thermal cooling of the ocean surface. When El Niño conditions prevail, the thermal radiative cooling in the central and south-eastern tropical Pacific becomes weaker by 10 Wm−2 south of the equator in the central Pacific (7–0 S, 160–120 W) for the three-month period of NDJ, because the DLR increase is larger than the increase in surface thermal emission. In contrast, the thermal radiative cooling over Indonesia is enhanced by 10 Wm−2 during the early (August–September–October) El Niño phase.
Model sensitivity studies regarding the role of the retention coefficient for the scavenging and redistribution of highly soluble trace gases by deep convective cloud systems
Authors: Salzmann M. , Lawrence M. G. , Phillips V. T. J. , Donner L. J.
Published: April 2010

Abstract. The role of the retention coefficient (i.e. the fraction of a dissolved trace gas which is retained in hydrometeors during freezing) for the scavenging and redistribution of highly soluble trace gases by deep convective cloud systems is investigated using a modified version of the Weather Research and Forecasting (WRF) model. Results from cloud system resolving model runs (in which deep convection is initiated by small random perturbations in association with so-called "large scale forcings (LSF)") for a tropical oceanic (TOGA COARE) and a mid-latitude continental case (ARM) are compared to two runs in which bubbles are used to initiate deep convection (STERAO, ARM). In the LSF runs, scavenging is found to almost entirely prevent a highly soluble tracer initially located in the lowest 1.5 km of the troposphere from reaching the upper troposphere, independent of the retention coefficient. The release of gases from freezing hydrometeors leads to mixing ratio increases in the upper troposphere comparable to those calculated for insoluble trace gases only in the two runs in which bubbles are used to initiate deep convection. A comparison of the two ARM runs indicates that using bubbles to initiate deep convection may result in an overestimate of the influence of the retention coefficient on the vertical transport of highly soluble tracers. It is, however, found that the retention coefficient plays an important role for the scavenging and redistribution of highly soluble trace gases with a (chemical) source in the free troposphere and also for trace gases for which even relatively inefficient transport may be important. The large difference between LSF and bubble runs is attributed to differences in dynamics and microphysics in the inflow regions of the storms. The dependence of the results on the model setup indicates the need for additional model studies with a more realistic initiation of deep convection, e.g., considering effects of orography in a nested model setup.
Nine years of UV aerosol optical depth measurements at Thessaloniki, Greece
Authors: Kazadzis S. , Bais A. , Amiridis V. , Balis D. , Meleti C. , Kouremeti N. , Zerefos C. S. , Rapsomanikis S. , Petrakakis M. , Kelesis A. , Tzoumaka P. , Kelektsoglou K.
Published: April 2010

Abstract. Spectral measurements of the aerosol optical depth (AOD) and the Ångström coefficient were conducted at Thessaloniki, Greece (40.5° N, 22.9° E) between January 1997 and December 2005 with a Brewer MKIII double-monochromator spectroradiometer. The dataset was compared with collocated measurements of a second spectroradiometer (Brewer MKII) and a CIMEL sun-photometer, showing correlations of 0.93 and 0.98, respectively. A seasonal variation of the AOD was observed at Thessaloniki, with AOD values at 340 nm of 0.52 and 0.28 for August and December respectively. Back trajectories of air masses for up to 4 days were used to assess the influence of long-range transport from various regions to the aerosol load over Thessaloniki. It is shown that part of the observed seasonality can be attributed to air masses with high AOD originating from North-Eastern and Eastern directions during summertime. The analysis of the long-term record (9 years) of AOD showed a downward tendency. A similar decreasing tendency was found in the record of the PM$_{10}$ aerosol measurements, which are conducted near the surface at 4 air-quality monitoring stations in the area of the city of Thessaloniki.
Centennial evolution of the atmospheric methane budget: what do the carbon isotopes tell us?
Authors: Lassey K. R. , Etheridge D. M. , Lowe D. C. , Smith A. M. , Ferretti D. F.
Published: April 2010

Abstract. Little is known about how the methane source inventory and sinks have evolved over recent centuries. New and detailed records of methane mixing ratio and isotopic composition (12CH4, 13CH4 and 14CH4) from analyses of air trapped in polar ice and firn can enhance this knowledge. We use existing bottom-up constructions of the source history, including "EDGAR"-based constructions, as inputs to a model of the evolving global budget for methane and for its carbon isotope composition through the 20th century. By matching such budgets to atmospheric data, we examine the constraints imposed by isotope information on those budget evolutions. Reconciling both 12CH4 and 13CH4 budgets with EDGAR-based source histories requires a combination of: a greater proportion of emissions from biomass burning and/or of fossil methane than EDGAR constructions suggest; a greater contribution from natural such emissions than is commonly supposed; and/or a significant role for active chlorine or other highly-fractionating tropospheric sink as has been independently proposed. Examining a companion budget evolution for 14CH4 exposes uncertainties in inferring the fossil-methane source from atmospheric 14CH4 data. Specifically, methane evolution during the nuclear era is sensitive to the cycling dynamics of "bomb 14C" (originating from atmospheric weapons tests) through the biosphere. In addition, since ca. 1970, direct production and release of 14CH4 from nuclear-power facilities is influential but poorly quantified. Atmospheric 14CH4 determinations in the nuclear era have the potential to better characterize both biospheric carbon cycling, from photosynthesis to methane synthesis, and the nuclear-power source.
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) determined from field measurements of 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.
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 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 higher than a reference point 100 m away from the waterfall. Also, the concentration of positive intermediate ions was found to be higher than that at the reference point by a factor of ten. This difference was observed only at the smallest sizes; above 10 nm the difference was insignificant.
Secondary aerosol formation from atmospheric reactions of aliphatic amines
Authors: Murphy S. M. , Sorooshian A. , Kroll J. H. , Ng N. L. , Chhabra P. , Tong C. , Surratt J. D. , Knipping E. , Flagan R. C. , Seinfeld J. H.
Published: April 2010

Abstract. Although aliphatic amines have been detected in both urban and rural atmospheric aerosols, little is known about the chemistry leading to particle formation or the potential aerosol yields from reactions of gas-phase amines. We present here the first systematic study of aerosol formation from the atmospheric reactions of amines. Based on laboratory chamber experiments and theoretical calculations, we evaluate aerosol formation from reaction of OH, ozone, and nitric acid with trimethylamine, methylamine, triethylamine, diethylamine, ethylamine, and ethanolamine. Entropies of formation for alkylammonium nitrate salts are estimated by molecular dynamics calculations enabling us to estimate equilibrium constants for the reactions of amines with nitric acid. Though subject to significant uncertainty, the calculated dissociation equilibrium constant for diethylammonium nitrate is found to be sufficiently small to allow for its atmospheric formation, even in the presence of ammonia which competes for available nitric acid. Experimental chamber studies indicate that the dissociation equilibrium constant for triethylammonium nitrate is of the same order of magnitude as that for ammonium nitrate. All amines studied form aerosol when photooxidized in the presence of NOx with the majority of the aerosol mass present at the peak of aerosol growth consisting of aminium (R3NH+) nitrate salts, which repartition back to the gas phase as the parent amine is consumed. Only the two tertiary amines studied, trimethylamine and triethylamine, are found to form significant non-salt organic aerosol when oxidized by OH or ozone; calculated organic mass yields for the experiments conducted are similar for ozonolysis (15% and 5% respectively) and photooxidation (23% and 8% respectively). The non-salt organic aerosol formed appears to be more stable than the nitrate salts and does not quickly repartition back to the gas phase.
Global 2-D intercomparison of sectional and modal aerosol modules
Authors: Weisenstein D. K. , Penner J. E. , Herzog M. , Liu X.
Published: April 2010

Abstract. We present an intercomparison of several aerosol modules, sectional and modal, in a global 2-D model in order to differentiate their behavior for tropospheric and stratospheric applications. We model only binary sulfuric acid-water aerosols in this study. Three versions of the sectional model and three versions of the modal model are used to test the sensitivity of background aerosol mass and size distribution to the number of bins or modes and to the prescribed width of the largest mode. We find modest sensitivity to the number of bins (40 vs. 150) used in the sectional model. Aerosol mass is found to be reduced in a modal model if care is not taken in selecting the width of the largest lognormal mode, reflecting differences in sedimentation in the middle stratosphere. The size distributions calculated by the sectional model can be better matched by a modal model with four modes rather than three modes in most but not all situations. A simulation of aerosol decay following the 1991 eruption of Mt. Pinatubo shows that the representation of the size distribution can have a signficant impact on model-calculated aerosol decay rates in the stratosphere. Between 1991 and 1995, aerosol extinction and surface area density calculated by two versions of the modal model adequately match results from the sectional model. Calculated effective radius for the same time period shows more intermodel variability, with a 20-bin sectional model performing much better than any of the modal models.
Three years of global carbon monoxide from SCIAMACHY: comparison with MOPITT and first results related to the detection of enhanced CO over cities
Authors: Buchwitz M. , Khlystova I. , Bovensmann H. , Burrows J. P.
Published: April 2010

Abstract. Carbon monoxide (CO) is an important atmospheric constituent affecting air quality and climate. SCIAMACHY on ENVISAT is currently the only satellite instrument that can measure the vertical column of CO with nearly equal sensitivity at all altitudes down to the Earth's surface because of its near-infrared nadir observations of reflected solar radiation. Here we present three years' (2003–2005) of SCIAMACHY CO columns consistently retrieved with the latest version of our retrieval algorithm (WFMDv0.6). We describe the retrieval method and discuss the multi-year global CO data set focusing on a comparison with the operational CO column data product of MOPITT. We found reasonable to good agreement (~20%) with MOPITT, with the best agreement for 2004. We present detailed results for various regions (Europe, Middle East, India, China) and discuss to what extent enhanced levels of CO can be detected over populated areas including individual cities. The expected CO signal from cities is close to or even below the detection limit of individual measurements. We show that cities can be identified when averaging long time series.
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 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.
Application of synchrotron radiation for measurement of iron red-ox speciation in atmospherically processed aerosols
Authors: Majestic B. J. , Schauer J. J. , Shafer M. M.
Published: April 2010

Abstract. In this study, ambient atmospheric particulate matter samples were collected using a size-resolved impactor sampler from three urban sites. The purpose of this study is to gain a better understanding of transformations of aerosol-bound iron as it is processed in the atmosphere. Thus, the aerosol samples were artificially aged to represent long-term transport (10 to 40 days) or short-term transport (1 to 10 days) and were measured for iron at several time points. At each time point, iron was measured in each size fraction using three different techniques; 1) inductively coupled plasma-mass spectrometry (ICPMS) for total iron, 2) x-ray absorbance near edge structure (XANES) spectroscopy for the measurement of total Fe(II) and Fe(III), and 3) a wet-chemical method to measure soluble Fe(II) and Fe(III). Prior to aging, the XANES spectroscopy results show that a majority (>60% for each size fraction) of the total iron in the PM is in the form of Fe(III). Fe(III) was shown to be a significant fraction of the soluble iron (sometimes >50%), but the relative significance of Fe(III) was found to vary depending on the site. Overall, the total soluble iron depended on the sampling site, but values ranged from less than 1% up to about 20% of the total iron. Over the course of the 40 day aging period, we found moderate changes in the relative Fe(II)/Fe(III) content. A slight increase was noted in the coarse (>2.5 µm) fraction and a slight decrease in the 0.25 to 0.5 µm fraction. The soluble fraction generally showed (excepting one day) a decrease of soluble Fe(II) prior to 10 days of aging, followed by a relatively constant concentration. In the short-term transport condition, we found that the sub-micron fraction of soluble Fe(II) spikes at 1 to 3 days of aging, then decreases to near the initial value at around 6 to 10 days. Very little change in soluble Fe(II) was observed in the super-micron fraction.
Impact of an improved shortwave radiation scheme in the MAECHAM5 General Circulation Model
Authors: Cagnazzo C. , Manzini E. , Giorgetta M. A. , Forster P. M. De F. , Morcrette J. J.
Published: April 2010

Abstract. In order to improve the representation of ozone absorption in the stratosphere of the MAECHAM5 general circulation model, the spectral resolution of the shortwave radiation parameterization used in the model has been increased from 4 to 6 bands. Two 20-years simulations with the general circulation model have been performed, one with the standard and the other with the newly introduced parameterization respectively, to evaluate the temperature and dynamical changes arising from the two different representations of the shortwave radiative transfer. In the simulation with the increased spectral resolution in the radiation parameterization, a significant warming of almost the entire model domain is reported. At the summer stratopause the temperature increase is about 6 K and alleviates the cold bias present in the model when the standard radiation scheme is used. These general circulation model results are consistent both with previous validation of the radiation scheme and with the offline clear-sky comparison performed in the current work with a discrete ordinate 4 stream scattering line by line radiative transfer model. The offline validation shows a substantial reduction of the daily averaged shortwave heating rate bias (1–2 K/day cooling) that occurs for the standard radiation parameterization in the upper stratosphere, present under a range of atmospheric conditions. Therefore, the 6 band shortwave radiation parameterization is considered to be better suited for the representation of the ozone absorption in the stratosphere than the 4 band parameterization. Concerning the dynamical response in the general circulation model, it is found that the reported warming at the summer stratopause induces stronger zonal mean zonal winds in the middle atmosphere. These stronger zonal mean zonal winds thereafter appear to produce a dynamical feedback that results in a dynamical warming (cooling) of the polar winter (summer) mesosphere, caused by an increased downward (upward) circulation in the winter (summer) hemisphere. In addition, the comparison of the two simulations performed with the general circulation model shows that the increase in the spectral resolution of the shortwave radiation and the associated changes in the cloud optical properties result in a warming (0.5–1 K) and moistening (3%–12%) of the upper tropical troposphere. By comparing these modeled differences with previous works, it appears that the reported changes in the solar radiation scheme contribute to improve the model mean temperature also in the troposphere.
Simulating organic species with the global atmospheric chemistry general circulation model ECHAM5/MESSy1: a comparison of model results with observations
Authors: Pozzer A. , Jöckel P. , Tost H. , Sander R. , Ganzeveld L. , Kerkweg A. , Lelieveld J.
Published: April 2010

Abstract. The atmospheric-chemistry general circulation model ECHAM5/MESSy1 is evaluated with observations of different organic ozone precursors. This study continues a prior analysis which focused primarily on the representation of atmospheric dynamics and ozone. We use the results of the same reference simulation and apply a statistical analysis using data from numerous field campaigns. The results serve as a basis for future improvements of the model system. ECHAM5/MESSy1 generally reproduces the spatial distribution and the seasonal cycle of carbon monoxide (CO) very well. However, for the background in the Northern Hemisphere we obtain a negative bias (mainly due to an underestimation of emissions from fossil fuel combustion), and in the high latitude Southern Hemisphere a yet unexplained positive bias. The model results agree well with observations of alkanes, whereas severe problems in the simulation of alkenes and isoprene are present. For oxygenated compounds the results are ambiguous: The model results are in good agreement with observations of formaldehyde, but systematic biases are present for methanol and acetone. The discrepancies between the model results and the observations are explained (partly) by means of sensitivity studies.
Validation of MIPAS ClONO2 measurements
Authors: Höpfner M. , von Clarmann T. , Fischer H. , Funke B. , Glatthor N. , Grabowski U. , Kellmann S. , Kiefer M. , Linden A. , Milz M. , Steck T. , Stiller G. P. , Bernath P. , Blom C. E. , Blumenstock Th. , Boone C. , Chance K. , Coffey M. T. , Friedl-Vallon F. , Griffith D. , Hannigan J. W. , Hase F. , Jones N. , Jucks K. W. , Keim C. , Kleinert A. , Kouker W. , Liu G. Y. , Mahieu E. , Mellqvist J. , Mikuteit S. , Notholt J. , Oelhaf H. , Piesch C. , Reddmann T. , Ruhnke R. , Schneider M. , Strandberg A. , Toon G. , Walker K. A. , Warneke T. , Wetzel G. , Wood S. , Zander R.
Published: April 2010

Abstract. Altitude profiles of ClONO2 retrieved with the IMK (Institut für Meteorologie und Klimaforschung) science-oriented data processor from MIPAS/Envisat (Michelson Interferometer for Passive Atmospheric Sounding on Envisat) mid-infrared limb emission measurements between July 2002 and March 2004 have been validated by comparison with balloon-borne (Mark IV, FIRS2, MIPAS-B), airborne (MIPAS-STR), ground-based (Spitsbergen, Thule, Kiruna, Harestua, Jungfraujoch, Izaña, Wollongong, Lauder), and spaceborne (ACE-FTS) observations. With few exceptions we found very good agreement between these instruments and MIPAS with no evidence for any bias in most cases and altitude regions. For balloon-borne measurements typical absolute mean differences are below 0.05 ppbv over the whole altitude range from 10 to 39 km. In case of ACE-FTS observations mean differences are below 0.03 ppbv for observations below 26 km. Above this altitude the comparison with ACE-FTS is affected by the photochemically induced diurnal variation of ClONO2. Correction for this by use of a chemical transport model led to an overcompensation of the photochemical effect by up to 0.1 ppbv at altitudes of 30–35 km in case of MIPAS-ACE-FTS comparisons while for the balloon-borne observations no such inconsistency has been detected. The comparison of MIPAS derived total column amounts with ground-based observations revealed no significant bias in the MIPAS data. Mean differences between MIPAS and FTIR column abundances are 0.11±0.12×1014 cm−2 (1.0±1.1%) and −0.09±0.19×1014 cm−2 (−0.8±1.7%), depending on the coincidence criterion applied. χ2 tests have been performed to assess the combined precision estimates of MIPAS and the related instruments. When no exact coincidences were available as in case of MIPAS – FTIR or MIPAS – ACE-FTS comparisons it has been necessary to take into consideration a coincidence error term to account for χ2 deviations. From the resulting χ2 profiles there is no evidence for a systematic over/underestimation of the MIPAS random error analysis.
The direct effect of aerosols on solar radiation based on satellite observations, reanalysis datasets, and spectral aerosol optical properties from Global Aerosol Data Set (GADS)
Authors: Hatzianastassiou N. , Matsoukas C. , Drakakis E. , Stackhouse P. W. , Koepke P. , Fotiadi A. , Pavlakis K. G. , Vardavas I.
Published: April 2010

Abstract. A global estimate of the seasonal direct radiative effect (DRE) of natural plus anthropogenic aerosols on solar radiation under all-sky conditions is obtained by combining satellite measurements and reanalysis data with a spectral radiative transfer model and spectral aerosol optical properties taken from the Global Aerosol Data Set (GADS). The estimates are obtained with detailed spectral model computations separating the ultraviolet (UV), visible and near-infrared wavelengths. The global distribution of spectral aerosol optical properties was taken from GADS whereas data for clouds, water vapour, ozone, carbon dioxide, methane and surface albedo were taken from various satellite and reanalysis datasets. Using these aerosol properties and other related variables, we generate climatological (for the 12-year period 1984–1995) monthly mean aerosol DREs. The global annual mean DRE on the outgoing SW radiation at the top of atmosphere (TOA, ΔFTOA) is −1.62 W m−2 (with a range of −15 to 10 W m−2, negative values corresponding to planetary cooling), the effect on the atmospheric absorption of SW radiation (ΔFatmab) is 1.6 W m−2 (values up to 35 W m−2, corresponding to atmospheric warming), and the effect on the surface downward and absorbed SW radiation (ΔFsurf, and ΔFsurfnet, respectively) is −3.93 and −3.22 W m−2 (values up to −45 and −35 W m−2, respectively, corresponding to surface cooling). According to our results, aerosols decrease/increase the planetary albedo by −3 to 13% at the local scale, whereas on planetary scale the result is an increase of 1.5%. Aerosols can warm locally the atmosphere by up to 0.98 K day−1, whereas they can cool the Earth's surface by up to −2.9 K day−1. Both these effects, which can significantly modify atmospheric dynamics and the hydrological cycle, can produce significant planetary cooling on a regional scale, although planetary warming can arise over highly reflecting surfaces. The aerosol DRE at the Earth's surface compared to TOA can be up to 15 times larger at the local scale. The largest aerosol DRE takes place in the northern hemisphere both at the surface and the atmosphere, arising mainly at ultraviolet and visible wavelengths.
Humidity observations in the Arctic troposphere over Ny-Ålesund, Svalbard based on 15 years of radiosonde data
Authors: Treffeisen R. , Krejci R. , Ström J. , Engvall A. C. , Herber A. , Thomason L.
Published: April 2010

Abstract. Water vapour is an important component in the radiative balance of the polar atmosphere. We present a study covering fifteen years of data of tropospheric humidity profiles measured with standard radiosondes at Ny-Ålesund (78°55' N 11°52' E) during the period from 1991 to 2006. It is well-known that relative humidity measurements are less reliable at low temperatures when measured with standard radiosondes. The data was corrected for errors and used to determine key characteristic features of the vertical and temporal relative humidity evolution in the Arctic troposphere over Ny-Ålesund. We present frequencies of occurrence of ice-supersaturation layers in the troposphere, their vertical span, temperature and statistical distribution. Supersaturation with respect to ice shows a clear seasonal behaviour. In winter, (October–February) it occurred in 19% of all cases and less frequently in spring (March–May 12%), and summer (June–September, 9%). Finally, the results are compared with findings from the SAGE II satellite instrument on subvisible clouds.
Global peroxyacetyl nitrate (PAN) retrieval in the upper troposphere from limb emission spectra of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS)
Authors: Glatthor N. , von Clarmann T. , Fischer H. , Funke B. , Grabowski U. , Höpfner M. , Kellmann S. , Kiefer M. , Linden A. , Milz M. , Steck T. , Stiller G. P.
Published: April 2010

Abstract. We use limb emission spectra of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) onboard the ENVIronmental SATellite (ENVISAT) to derive the first global distribution of peroxyacetyl nitrate (PAN) in the upper troposphere. PAN is generated in tropospheric air masses polluted by fuel combustion or biomass burning and acts as a reservoir and carrier of NOx in the cold free troposphere. PAN exhibits continuum-like broadband structures in the mid-infrared region and was retrieved in a contiguous analysis window covering the wavenumber region 775–800 cm−1. The interfering species CCl4, HCFC-22, H2O, ClONO2, CH3CCl3 and C2H2 were fitted along with PAN, whereas pre-fitted profiles were used to model the contribution of other contaminants like ozone. Sensitivity tests consisting in retrieval without consideration of PAN demonstrated the existence of PAN signatures in MIPAS spectra obtained in polluted air masses. The analysed dataset consists of 10 days between 4 October and 1 December 2003. This period covers the end of the biomass burning season in South America and South and East Africa, in which generally large amounts of pollutants are produced and distributed over wide areas of the southern hemispheric free troposphere. Indeed, elevated PAN amounts of 200–700 pptv were measured in a large plume extending from Brasil over the Southern Atlantic, Central and South Africa, the South Indian Ocean as far as Australia at altitudes between 8 and 16 km. Enhanced PAN values were also found in a much more restricted area between northern subtropical Africa and India. The most significant northern midlatitude PAN signal was detected in an area at 8 km altitude extending from China into the Chinese Sea. The average mid and high latitude PAN amounts found at 8 km were around 125 pptv in the northern, but only between 50 and 75 pptv in the southern hemisphere. The PAN distribution found in the southern hemispheric tropics and subtropics is highly correlated with the jointly fitted acetylene (C2H2), which is another pollutant produced by biomass burning, and agrees reasonably well with the CO plume detected during end of September 2003 at the 275 hPa level (~10 km) by the Measurement of Pollution in the Troposphere (MOPITT) instrument on the Terra satellite. Similar southern hemispheric PAN amounts were also observed by previous airborne measurements performed in September/October 1992 and 1996 above the South Atlantic and the South Pacific, respectively.
Predicting terrestrial 222Rn flux using gamma dose rate as a proxy
Authors: Szegvary T. , Leuenberger M. C. , Conen F.
Published: April 2010

Abstract. 222Rn is commonly used as a natural tracer for validating climate models. To improve such models a better source term for 222Rn than currently used is necessary. The aim of this work is to establish a method for mapping this source term by using a commonly measured proxy, the gamma dose rate (GDR). Automatic monitoring of GDR has been networked in 25 European countries by the Institute for Environment and Sustainability at the Joint Research Centre (JRC IES) in Ispra, Italy, using a common data format. We carried out simultaneous measurements of 222Rn flux and GDR at 63 locations in Switzerland, Germany, Finland and Hungary in order to cover a wide range of GDR. Spatial variations in GDR resulted from different radionuclide concentrations in soil forming minerals. A relatively stable fraction (20%) of the total terrestrial GDR originates from the 238U decay series, of which 222Rn is a member. Accordingly, spatial variation in terrestrial GDR was found to describe almost 60% of the spatial variation in 222Rn flux. Furthermore, temporal variation in GDR and 222Rn was found to be correlated. Increasing soil moisture reduces gas diffusivity and the rate of 222Rn flux but it also decreases GDR through increased shielding of photons. Prediction of 222Rn flux through GDR for individual measurement points is imprecise but un-biased. Verification of larger scale prediction showed that estimates of mean 222Rn fluxes were not significantly different from the measured mean values.
Retrieval of aerosol complex refractive index from a synergy between lidar, sunphotometer and in situ measurements during LISAIR experiment
Authors: Raut J.-C. , Chazette P.
Published: April 2010

Abstract. Particulate pollutant exchanges between the streets and the Planetary Boundary Layer (PBL), and their daily evolution linked to human activity were studied in the framework of the LIdar pour la Surveillance de l'AIR (LISAIR) experiment. This program lasted from 10 to 30 May 2005. A synergetic approach combining dedicated active (lidar) and passive (sunphotometer) remote sensors as well as ground based in situ instrumentation (nephelometer, aethalometer and particle sizers) was used to investigate urban aerosol optical properties within Paris. Aerosol complex refractive indices were assessed to be 1.56–0.034 i at 355 nm and 1.59–0.040 i at 532 nm, thus leading to single-scattering albedo values between 0.80 and 0.88. These retrievals are consistent with soot components in the aerosol arising from traffic exhausts indicating that these pollutants have a radiative impact on climate. We also discussed the influence of relative humidity on aerosol properties. A good agreement was found between vertical extinction profile derived from lidar backscattering signal and retrieved from the coupling between radiosounding and ground in situ measurements.
Technical Note: The effect of sensor resolution on the number of cloud-free observations from space
Authors: Krijger J. M. , van Weele M. , Aben I. , Frey R.
Published: April 2010

Abstract. Air quality and surface emission inversions are likely to be focal points for future satellite missions on atmospheric composition. Most important for these applications is sensitivity to the atmospheric composition in the lowest few kilometers of the troposphere. Reduced sensitivity by clouds needs to be minimized. In this study we have quantified the increase in number of useful footprints, i.e. footprints which are sufficient cloud-free, as a function of sensor resolution (footprint area). High resolution (1 km×1 km) MODIS TERRA cloud mask observations are aggregated to lower resolutions. Statistics for different thresholds on cloudiness are applied. For each month in 2004 four days of MODIS data are analyzed. Globally the fraction of cloud-free observations drops from 16% at 100 km2 resolution to only 3% at 10 000 km2 if not a single MODIS observation within a footprint is allowed to be cloudy. If up to 5% or 20% of a footprint is allowed to be cloudy, the fraction of cloud-free observations is 9% or 17%, respectively, at 10 000 km2 resolution. The probability of finding cloud-free observations for different sensor resolutions is also quantified as a function of geolocation and season, showing examples over Europe and northern South America (ITCZ).
Ambient sesquiterpene concentration and its link to air ion measurements
Authors: Bonn B. , Hirsikko A. , Hakola H. , Kurtén T. , Laakso L. , Boy M. , Dal Maso M. , Mäkelä J. M. , Kulmala M.
Published: April 2010

Abstract. Ambient air ion size distributions have been measured continuously at the Finnish boreal forest site in Hyytiälä since spring 2003. In general, these measurements show a maximum of air ions below 1.0 nm in diameter. But this physical characterization does not provide any information about the ion's chemical composition, which is one key question regarding the explanation of nucleation events observed. In this study we propose a link of the observed maximum of negative air ions between 0.56 and 0.75 nm to the so-called stabilised Criegee biradical, formed in the reaction of biogenic sesquiterpenes with ozone and predominantly destroyed by its reaction with ambient water vapour. Calculations of the electron and proton affinities of 120 kJ mol−1 (1.24 eV) and of 960 kJ mol−1 support this link. Other possible candidates such as sulphuric acid derived clusters are unable to explain the observations made. By using this approach, we are able to calculate the ambient concentration of sesquiterpenes at the air ion instrument inlet with a high time resolution on the daily and seasonal scale. The estimated concentration is found to reveal the same seasonal pattern as emission measurements conducted at shoot level. As expected for biogenic VOCs, the concentration is obtained highest during summer (maximum values of about 100 pptv) and smallest during winter (minimum less than 1 pptv). Because of the sesquiterpenes high reactivity and its low ambient concentrations, this approach can be a first step in understanding their emission and their impact on atmospheric chemistry in more detail. The findings presented are highly relevant for emission budgets too, since boreal forests are extended over large areas of the globe.
Statistical variability of top of atmosphere cloud-free shortwave aerosol radiative effect
Authors: Jones T. A. , Christopher S. A.
Published: April 2010

Abstract. The statistical variability of globally averaged MODIS aerosol optical thickness at 0.55 μm (AOT) and top of atmosphere CERES cloud-free shortwave radiative effect (SWRE) is presented. Statistical variability is defined as the robustness of globally averaged statistics relative to data distribution. At the CERES footprint level, which we label "raw data", both the AOT and SWRE data derived from clear-sky CERES-SSF products show significant deviations from a normal distribution as evidenced by high skewness values. The spatial and temporal distribution of the data is also not uniform, with a greater concentration of data occurring in aerosol heavy-regions. As a result, globally averaged AOT and SWRE are overestimated when derived from raw data alone. To compensate, raw data are gridded into 2×2 degree grid-cells (called "gridded" data) to reduce the effect of spatial non-uniformity. However, the underlying non-normal distribution remains and manifests itself by increasing the uncertainty of grid-cell values. Globally averaged AOT and SWRE derived from a gridded dataset are substantially lower than those derived from raw data alone. The range of globally averaged AOT and SWRE values suggests that up to a 50% statistical variability exists, much of which is directly tied to how the data are manipulated prior to averaging. This variability increases when analyzing aerosol components (e.g. anthropogenic) since component AOT (and SWRE) may not exist at all locations were AOT is present. As a result, regions where a particular component AOT does not exist must either not be included in the global average or have data within these regions set to null values. However, each method produces significantly different results. The results of this work indicate simple mean and standard deviation statistics do not adequately describe global aerosol climate forcing data sets like the one used here. We demonstrate that placing raw observations on to a uniform grid is a necessary step before calculating global statistics. However, this by no means eliminates uncertainty in globally averaged AOT and SWRE values, while adding its own set of assumptions. When reporting any globally averaged statistic, it is important to report corresponding distribution and coverage information, in the form of skewness values, probability density functions, and spatial distribution plots, to help quantify its usefulness and robustness.
Lightning-produced NOx over Brazil during TROCCINOX: airborne measurements in tropical and subtropical thunderstorms and the importance of mesoscale convective systems
Authors: Huntrieser H. , Schlager H. , Roiger A. , Lichtenstern M. , Schumann U. , Kurz C. , Brunner D. , Schwierz C. , Richter A. , Stohl A.
Published: April 2010

Abstract. During the TROCCINOX field experiments in February–March 2004 and February 2005, airborne in situ measurements of NO, NOy, CO, and O3 mixing ratios and the J(NO2) photolysis rate were carried out in the anvil outflow of thunderstorms over southern Brazil. Both tropical and subtropical thunderstorms were investigated, depending on the location of the South Atlantic convergence zone. Tropical air masses were discriminated from subtropical ones according to the higher equivalent potential temperature (Θe) in the lower and mid troposphere, the higher CO mixing ratio in the mid troposphere, and the lower wind velocity in the upper troposphere within the Bolivian High (north of the subtropical jet stream). During thunderstorm anvil penetrations, typically at 20–40 km horizontal scales, NOx mixing ratios were distinctly enhanced and the absolute mixing ratios varied between 0.2–1.6 nmol mol−1 on average. This enhancement was mainly attributed to NOx production by lightning and partly due to upward transport from the NOx-richer boundary layer. In addition, CO mixing ratios were occasionally enhanced, indicating upward transport from the boundary layer. For the first time, the composition of the anvil outflow from a large, long-lived mesoscale convective system (MCS) advected from northern Argentina and Uruguay was investigated in more detail. Over a horizontal scale of about 400 km, NOx, CO and O3 absolute mixing ratios were significantly enhanced in these air masses in the range of 0.6–1.1, 110–140 and 60–70 nmol mol−1, respectively. Analyses from trace gas correlations and a Lagrangian particle dispersion model indicate that polluted air masses, probably from the Buenos Aires urban area and from biomass burning regions, were uplifted by the MCS. Ozone was distinctly enhanced in the aged MCS outflow, due to photochemical production and entrainment of O3-rich air masses from the upper troposphere – lower stratosphere region. The aged MCS outflow was transported to the north, ascended and circulated, driven by the Bolivian High over the Amazon basin. In the observed case, the O3-rich MCS outflow remained over the continent and did not contribute to the South Atlantic ozone maximum.
Technical Note: Characterization of a static thermal-gradient CCN counter
Authors: Frank G. P. , Dusek U. , Andreae M. O.
Published: April 2010

Abstract. The static (parallel-plate thermal-gradient) diffusion chamber (SDC) was one of the first instruments designed to measure cloud condensation nuclei (CCN) concentrations as a function of supersaturation. It has probably also been the most widely used type of CCN counter. This paper describes the detailed experimental characterization of a SDC CCN counter, including calibration with respect to supersaturation and particle number concentration. In addition, we investigated the proposed effect of lowered supersaturation because of water vapor depletion with increasing particle concentration. The results obtained give a better understanding why and in which way it is necessary to calibrate the SDC CCN counter. The calibration method is described in detail and can, in parts, be used for calibrations also for other types of CCN counters. We conclude the following: 1) it is important to experimentally calibrate SDC CCN counters with respect to supersaturation, and not only base the supersaturation on the theoretical description of the instrument; 2) the number concentration calibration needs to be performed as a function of supersaturation, also for SDC CCN counter using the photographic technique; and 3) we observed no evidence that water vapor depletion lowered the supersaturation.