bluuebunny/crossref_metadata_2025 huggingface dataset:
Assessment of high to low frequency variations of isoprene emission rates using a neural network approach
Authors: Boissard C. , Chervier F. , Dutot A. L.
Published: April 2010

Abstract. Using a statistical approach based on artificial neural networks, an emission algorithm (ISO_LF) accounting for high (instantaneous) to low (seasonal) frequency variations was developed for isoprene. ISO_LF was optimised using an isoprene emission data base (ISO-DB) specifically designed for this work. ISO-DB consists of 1321 emission rates collected in the literature, together with 34 environmental variables, measured or assessed using NCDC (National Climatic Data Center) or NCEP (National Centers for Environmental Predictions) meteorological databases. ISO-DB covers a large variety of emitters (25 species) and environmental conditions (10° S to 60° N). When only instantaneous environmental regressors (air temperature and photosynthetic active radiation, PAR) were used, a maximum of 60% of the overall isoprene variability was assessed and the highest emissions were underestimated. Considering a total of 9 high (instantaneous) to low (up to 3 weeks) frequency regressors, ISO_LF accounts for up to 91% of the isoprene emission variability, whatever the emission range, species or climate. Diurnal and seasonal variations are correctly reproduced for \\textit{Ulex europaeus} with a maximum factor of discrepancy of 4. ISO-LF was found to be mainly sensitive to air temperature cumulated over 3 weeks T21 and to instantaneous light L0 and air temperature T0 variations. T21, T0 and L0 only accounts for 76% of the overall variability. The use of ISO-LF for non stored monoterpene emissions was shown to give poor results.
Validation of the Atmospheric Chemistry Experiment (ACE) version 2.2 temperature using ground-based and space-borne measurements
Authors: Sica R. J. , Izawa M. R. M. , Walker K. A. , Boone C. , Petelina S. V. , Argall P. S. , Bernath P. , Burns G. B. , Catoire V. , Collins R. L. , Daffer W. H. , De Clercq C. , Fan Z. Y. , Firanski B. J. , French W. J. R. , Gerard P. , Gerding M. , Granville J. , Innis J. L. , Keckhut P. , Kerzenmacher T. , Klekociuk A. R. , Kyrö E. , Lambert J. C. , Llewellyn E. J. , Manney G. L. , McDermid I. S. , Mizutani K. , Murayama Y. , Piccolo C. , Raspollini P. , Ridolfi M. , Robert C. , Steinbrecht W. , Strawbridge K. B. , Strong K. , Stübi R. , Thurairajah B.
Published: April 2010

Abstract. An ensemble of space-borne and ground-based instruments has been used to evaluate the quality of the version 2.2 temperature retrievals from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS). The agreement of ACE-FTS temperatures with other sensors is typically better than 2 K in the stratosphere and upper troposphere and 5 K in the lower mesosphere. There is evidence of a systematic high bias (roughly 3–6 K) in the ACE-FTS temperatures in the mesosphere, and a possible systematic low bias (roughly 2 K) in ACE-FTS temperatures near 23 km. Some ACE-FTS temperature profiles exhibit unphysical oscillations, a problem fixed in preliminary comparisons with temperatures derived using the next version of the ACE-FTS retrieval software. Though these relatively large oscillations in temperature can be on the order of 10 K in the mesosphere, retrieved volume mixing ratio profiles typically vary by less than a percent or so. Statistical comparisons suggest these oscillations occur in about 10% of the retrieved profiles. Analysis from a set of coincident lidar measurements suggests that the random error in ACE-FTS version 2.2 temperatures has a lower limit of about ±2 K.
Aerosols' influence on the interplay between condensation, evaporation and rain in warm cumulus cloud
Authors: Altaratz O. , Koren I. , Reisin T. , Kostinski A. , Feingold G. , Levin Z. , Yin Y.
Published: April 2010

Abstract. A numerical cloud model is used to study the influence of aerosol on the microphysics and dynamics of moderate-sized, coastal, convective clouds that develop under the same meteorological conditions. The results show that polluted convective clouds start their precipitation later and precipitate less than clean clouds but produce larger rain drops. The evaporation process is more significant at the margins of the polluted clouds (compared to the clean cloud) due to a higher drop surface area to volume ratio and it is mostly from small drops. It was found that the formation of larger raindrops in the polluted cloud is due to a more efficient collection process.
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 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.
Hydrogen isotope fractionation in the photolysis of formaldehyde
Authors: Rhee T. S. , Brenninkmeijer C. A. M. , Röckmann T.
Published: April 2010

Abstract. Experiments investigating the isotopic fractionation in the formation of H2 by the photolysis of CH2O under tropospheric conditions are reported and discussed. The deuterium (D) depletion in H2 produced is 500(±20)‰ with respect to the parent CH2O. We also observed that complete photolysis of CH2O under atmospheric conditions produces H2 that has virtually the same isotopic ratio as that of the parent CH2O. These findings imply that there must be a very strong concomitant isotopic enrichment in the radical channel (CH2O + hν → CHO + H) as compared to the molecular channel (CH2O + hν → H2 + CO) of the photolysis of CH2O in order to balance the relatively small isotopic fractionation in the competing reaction of CH2O with OH. Using a 1-box photochemistry model we calculated the isotopic fractionation factor for the radical channel to be 0.22(±0.08), which is equivalent to a 780(±80)‰ enrichment in D of the remaining CH2O. When CH2O is in photochemical steady state, the isotopic ratio of the H2 produced is determined not only by the isotopic fractionation occurring during the photolytical production of H2 (αm) but also by overall fractionation for the removal processes of CH2O (αf), and is represented by the ratio of αm/αf. Applying the isotopic fractionation factors relevant to CH2O photolysis obtained in the present study to the troposphere, the ratio of αm/αf varies from ~0.8 to ~1.2 depending on the fraction of CH2O that reacts with OH and that produces H2. This range of αm/αf can render the H2 produced from the photochemical oxidation of CH4 to be enriched in D (with respect to the original CH4) by the factor of 1.2–1.3 as anticipated in the literature.
Some considerations about Ångström exponent distributions
Authors: Wagner F. , Silva A. M.
Published: April 2010

Abstract. A simulation study has been performed in order to show the influence of the aerosol optical thickness (AOT) distribution together with the corresponding error distribution on the resulting Ångström exponent (AE) distribution. It will be shown that the Ångström exponent frequency of occurrence distribution is only normal distributed when the relative error at the two wavelengths used for estimation of the Ångström exponent is the same. In all other cases a shift of the maximum of the AE-distribution will occur. It will be demonstrated that the Ångström exponent (or the maximum of an AE distribution) will be systematically over- or underestimated depending on whether the relative error of the shorter wavelength is larger or smaller compared with the relative error of the longer wavelength. In such cases the AE distribution are also skewed.
Performance of the meteorological radiation model during the solar eclipse of 29 March 2006
Authors: Psiloglou B. E. , Kambezidis H. D.
Published: April 2010

Abstract. Various solar broadband models have been developed in the last half of the 20th century. The driving demand has been the estimation of available solar energy at different locations on earth for various applications. The motivation for such developments, though, has been the ample lack of solar radiation measurements at global scale. Therefore, the main goal of such codes was to generate artificial solar radiation series or calculate the availability of solar energy at a place. One of the broadband models to be developed in the late 80's was the Meteorological Radiation Model (MRM). The main advantage of MRM over other similar models was its simplicity in acquiring and using the necessary input data, i.e., air temperature, relative humidity, barometric pressure and sunshine duration from any of the many meteorological stations. The present study describes briefly the various steps (versions) of MRM and in greater detail the latest version 5. To show the flexibility and great performance of the MRM, a harsh test of the code under the (almost total) solar eclipse conditions of 29 March 2006 over Athens was performed and comparison of its results with real measurements was made. From this hard comparison it is shown that the MRM can simulate solar radiation during a solar eclipse event as effectively as on a typical day. The value of this comparison is further enhanced if it said that the sky was cloudy almost all the duration of the solar eclipse event.
Technical Note: Measuring tropospheric OH and HO2 by laser-induced fluorescence at low pressure – a comparison of calibration techniques
Authors: Dusanter S. , Vimal D. , Stevens P. S.
Published: April 2010

Abstract. The hydroxyl radical (OH) is one of the most important oxidants in the atmosphere, as it is involved in many reactions that affect regional air quality and global climate change. Because of its high reactivity, measurements of OH radical concentrations in the atmosphere are difficult, and often require careful calibrations that rely on the production of a known concentration of OH at atmospheric pressure. The Indiana University OH instrument, based on the Fluorescence Assay by Gas Expansion technique (FAGE), has been calibrated in the laboratory using two different approaches: the production of OH from the UV-photolysis of water-vapor, and the steady-state production of OH from the reaction of ozone with alkenes. Both techniques are shown to agree within their experimental uncertainties, although the sensitivities derived from the ozone-alkene technique were systematically lower than those derived from the water-vapor UV-photolysis technique. The agreement between the two different methods improves the confidence of the water-vapor photolysis method as an accurate calibration technique for HOx instruments. Because several aspects of the mechanism of the gas phase ozonolysis of alkenes are still uncertain, this technique should be used with caution to calibrate OH instruments.
The measurement of aerosol optical properties at a rural site in Northern China
Authors: Yan P. , Tang J. , Huang J. , Mao J. T. , Zhou X. J. , Liu Q. , Wang Z. F. , Zhou H. G.
Published: April 2010

Abstract. Atmospheric aerosols contribute one of the largest sources of uncertainty in the estimation of climate forcing. During the period from April 2003 to January 2005, in situ measurements of aerosol optical properties were conducted at a rural site in Northern China, Shangdianzi Global Atmosphere Watch (GAW) regional station (SDZ). Based on the daily average data, the means (standard deviation, S.D.) of scattering and absorption coefficients for the entire period were 174.6 Mm−1 (189.1 Mm−1) and 17.5 Mm−1 (13.4 Mm−1), respectively. These values were approximately one third of the reported values for scattering coefficients and one fifth of those for absorption coefficients obtained in the Beijing urban area. The mean single scattering albedo (SSA) for the entire period was 0.88 (0.05), which was about 0.07 higher than the values reported for the Beijing urban area, and also higher than the values (0.85) used in the climate model simulation for China and India. Both the absorption and scattering coefficients showed the lowest values in winter (11.2 Mm−1 and 128.9 Mm−1, respectively), while the highest values appeared in summer for absorption coefficients (22.1 Mm−1) and in fall for scattering coefficients (208.2 Mm−1). The mean SSA were lowest in spring (0.85) and highest in winter (0.90). The daily variations of aerosol absorption and scattering coefficients were strongly influenced by synoptic changes throughout the observation period. A trajectory cluster analysis was applied to discern the source characteristics of aerosol optical properties for different air masses. The cluster mean scattering coefficients, absorption coefficients and SSA were all high when the air masses moved from SW and SE-E directions to the site and aerosols were influenced with heavy pollution from the dense population centers and industrial areas. The cluster mean SSA for air masses coming from the polluted areas was not only higher than those with the trajectories from the "clean" directions, but also higher than the reported values for the regions with high pollution emissions (such as Beijing urban area). This fact might reflect the substantial secondary aerosol production during the transport. The characteristics of aerosol optical properties measured at this rural site suggest the significant impacts of human activities on the regional aerosol.
VOC reactivity in central California: comparing an air quality model to ground-based measurements
Authors: Steiner A. L. , Cohen R. C. , Harley R. A. , Tonse S. , Goldstein A. H. , Millet D. B. , Schade G. W.
Published: April 2010

Abstract. Volatile organic compound (VOC) reactivity in central California is examined using a photochemical air quality model (the Community Multiscale Air Quality model; CMAQ) and ground-based measurements to evaluate the contribution of VOC to photochemical activity. We classify VOC into four categories: anthropogenic, biogenic, aldehyde, and other oxygenated VOC. Anthropogenic and biogenic VOC consist of primary emissions, while aldehydes and other oxygenated VOC include both primary anthropogenic emissions and secondary products from primary VOC oxidation. To evaluate the model treatment of VOC chemistry, we compare measured and modeled OH and VOC reactivities using the following metrics: 1) cumulative distribution functions of NOx concentration and VOC reactivity (ROH,VOC), 2) the relationship between ROH,VOC and NOx, 3) total OH reactivity (ROH,total) and speciated contributions, and 4) the relationship between speciated ROH,VOC and NOx. We find that the model predicts ROH,total to within 25–40% at three sites representing urban (Sacramento), suburban (Granite Bay) and rural (Blodgett Forest) chemistry. However in the urban area of Fresno, the model under predicts NOx and VOC emissions by a factor of 2–3. At all locations the model is consistent with observations of the relative contributions of total VOC. In urban areas, anthropogenic and biogenic ROH,VOC are predicted fairly well over a range of NOx conditions. In suburban and rural locations, anthropogenic and other oxygenated ROH,VOC relationships are reproduced, but measured biogenic and aldehyde ROH,VOC are often poorly characterized by measurements, making evaluation of the model with available data unreliable. In central California, 30–50% of the modeled urban VOC reactivity is due to aldehydes and other oxygenated species, and the total oxygenated ROH,VOC is nearly equivalent to anthropogenic VOC reactivity. In rural vegetated regions, biogenic and aldehyde reactivity dominates. This indicates that more attention needs to be paid to the accuracy of models and measurements of both primary emissions of oxygenated VOC and secondary production of oxygenates, especially formaldehyde and other aldehydes, and that a more comprehensive set of oxygenated VOC measurements is required to include all of the important contributions to atmospheric reactivity.
Vertical mixing in atmospheric tracer transport models: error characterization and propagation
Authors: Gerbig C. , Körner S. , Lin J. C.
Published: April 2010

Abstract. Imperfect representation of vertical mixing near the surface in atmospheric transport models leads to uncertainties in modelled tracer mixing ratios. When using the atmosphere as an integrator to derive surface-atmosphere exchange from mixing ratio observations made in the atmospheric boundary layer, this uncertainty has to be quantified and taken into account. A comparison between radiosonde-derived mixed layer heights and mixed layer heights derived from ECMWF meteorological data during May–June 2005 in Europe revealed random discrepancies of about 40% for the daytime with insignificant bias errors, and much larger values approaching 100% for nocturnal mixed layers with bias errors also exceeding 50%. The Stochastic Time Inverted Lagrangian Transport (STILT) model was used to propagate this uncertainty into CO2 mixing ratio uncertainties, accounting for spatial and temporal error covariance. Average values of 3 ppm were found for the 2 month period, indicating that this represents a large fraction of the overall uncertainty. A pseudo data experiment shows that the error propagation with STILT avoids biases in flux retrievals when applied in inversions. The results indicate that transport models driven by current generation data assimilation for meteorological fields is by far not sufficient for inversions of continental mixing ratio data. As a solution we suggest the use of better, higher resolution atmospheric models, and a modification of the overall sampling strategy.
Eddy covariance measurements of sea spray particles over the Atlantic Ocean
Authors: Norris S. , Brooks I. , de Leeuw G. , Smith M. H. , Moeman M. , Lingard J.
Published: April 2010

Abstract. Most estimates of sea spray aerosol source functions have used indirect means to infer the rate of production as a function of wind speed. Only recently has the technology become available to make high frequency measurements of aerosol concentration suitable for direct eddy correlation determination of the particle flux. This was accomplished in this study by combining a newly developed fast aerosol particle counter with an ultrasonic anemometer which allowed for eddy covariance measurements of size-segregated particle fluxes. The aerosol instrument is the Compact Lightweight Aerosol Spectrometer Probe (CLASP) – capable of measuring 8-channel size spectra for mean radii between 0.15 and 0.35 μm at 10 Hz. The first successful measurements were made during the WASFAB (Waves, Air Sea Fluxes, Aerosol and Bubbles) field campaign in October 2005 in Duck (NC, USA). The method and results are presented and comparisons are made with recent sea spray source functions from the literature.
Influence of altitude on ozone levels and variability in the lower troposphere: a ground-based study for western Europe over the period 2001–2004
Authors: Chevalier A. , Gheusi F. , Delmas R. , Ordó\\ nez C. , Sarrat C. , Zbinden R. , Thouret V. , Athier G. , Cousin J.-M.
Published: April 2010

Abstract. The PAES (French acronym for synoptic scale atmospheric pollution) network focuses on the chemical composition (ozone, CO, NOx/y and aerosols) of the lower troposphere (0–3000 m). Its high-altitude surface stations located in different mountainous areas in France complete the low-altitude rural MERA stations (the French contribution to the european program EMEP, European Monitoring and Evaluation Program). They are representative of pollution at the scale of the French territory because they are away from any major source of pollution. This study deals with ozone observations between 2001 and 2004 at 11 stations from PAES and MERA, in addition to 16 elevated stations located in mountainous areas of Switzerland, Germany, Austria, Italy and Spain. The set of stations covers a range of altitudes between 115 and 3550 m. The comparison between recent ozone mixing ratios with those of the last decade found in the literature for two high-elevation sites (Pic du Midi, 2877 m and Jungfraujoch, 3580 m) leads to a trend that has slowed down compared to old trends but remains positive. This could be attribuable to the reduction of ozone precursors at European scale, that however do not compensate an ozone increase at the global scale. Averaged levels of ozone increase with elevation in good agreement with data provided by the airborne observation system MOZAIC (Measurement of OZone and water vapour by Airbus In-service airCraft), showing a highly stratified ozone field in the lower troposphere, with a transition at about 1000 m asl between a sharp gradient (30 ppb/km) below but a gentler gradient (3 ppb/km) above. Ozone variability also reveals a clear transition between boundary-layer and free-tropospheric regimes at the same altitude. Below, diurnal photochemistry accounts for about the third of the variability in summer, but less than 20% above – and at all levels in winter – where ozone variability is mostly due to day-to-day changes (linked to weather conditions or synoptic transport). Monthly-mean ozone mixing-ratios show at all levels a minimum in winter and the classical summer broad maximum in spring and summer – which is actually the superposition of the tropospheric spring maximum (April–May) and regional pollution episodes linked to persistent anticyclonic conditions that may occur from June to September. To complement this classical result it is shown that summer maxima are associated with considerably more variability than the spring maximum. This ensemble of findings support the relevance of mountain station networks such as PAES for the long-term observation of free-tropospheric ozone over Europe.
The interaction of N2O5 with mineral dust: aerosol flow tube and Knudsen reactor studies
Authors: Wagner C. , Hanisch F. , Holmes N. , de Coninck H. , Schuster G. , Crowley J. N.
Published: April 2010

Abstract. The interaction of mineral dust with N2O5 was investigated using both airborne mineral aerosol (using an aerosol flow reactor with variable relative humidity) and bulk samples (using a Knudsen reactor at zero humidity). Both authentic (Saharan, SDCV) and synthetic dust samples (Arizona test dust, ATD and calcite, CaCO3) were used to derive reactive uptake coefficients (γ). The aerosol experiments (Saharan dust only) indicated efficient uptake, with e.g. a value of γ (SDCV)=(1.3±0.2)×10−2 obtained at zero relative humidity. The values of γ obtained for bulk substrates in the Knudsen reactor studies are upper limits due to assumptions of available surface area, but were in reasonable agreement with the AFT measurements, with: γ(SDCV)=(3.7±1.2)×10−2, γ(ATD)=(2.2±0.8)×10−2 and γ(CaCO3)=(5±2)×10−2. The errors quoted are statistical only. The results are compared to literature values and assessed in terms of their impact on atmospheric N2O5.
Influence of clouds on the spectral actinic flux density in the lower troposphere (INSPECTRO): overview of the field campaigns
Authors: Thiel S. , Ammannato L. , Bais A. , Bandy B. , Blumthaler M. , Bohn B. , Engelsen O. , Gobbi G. P. , Gröbner J. , Jäkel E. , Junkermann W. , Kazadzis S. , Kift R. , Kjeldstad B. , Kouremeti N. , Kylling A. , Mayer B. , Monks P. S. , Reeves C. E. , Schallhart B. , Scheirer R. , Schmidt S. , Schmitt R. , Schreder J. , Silbernagl R. , Topaloglou C. , Thorseth T. M. , Webb A. R. , Wendisch M. , Werle P.
Published: April 2010

Abstract. Ultraviolet radiation is the key factor driving tropospheric photochemistry. It is strongly modulated by clouds and aerosols. A quantitative understanding of the radiation field and its effect on photochemistry is thus only possible with a detailed knowledge of the interaction between clouds and radiation. The overall objective of the project INSPECTRO was the characterization of the three-dimensional actinic radiation field under cloudy conditions. This was achieved during two measurement campaigns in Norfolk (East Anglia, UK) and Lower Bavaria (Germany) combining space-based, aircraft and ground-based measurements as well as simulations with the one-dimensional radiation transfer model UVSPEC and the three-dimensional radiation transfer model MYSTIC. During both campaigns the spectral actinic flux density was measured at several locations at ground level and in the air by up to four different aircraft. This allows the comparison of measured and simulated actinic radiation profiles. In addition satellite data were used to complete the information of the three dimensional input data set for the simulation. A three-dimensional simulation of actinic flux density data under cloudy sky conditions requires a realistic simulation of the cloud field to be used as an input for the 3-D radiation transfer model calculations. Two different approaches were applied, to derive high- and low-resolution data sets, with a grid resolution of about 100 m and 1 km, respectively. The results of the measured and simulated radiation profiles as well as the results of the ground based measurements are presented in terms of photolysis rate profiles for ozone and nitrogen dioxide. During both campaigns all spectroradiometer systems agreed within ±10% if mandatory corrections e.g. stray light correction were applied. Stability changes of the systems were below 5% over the 4 week campaign periods and negligible over a few days. The J(O1D) data of the single monochromator systems can be evaluated for zenith angles less than 70°, which was satisfied by nearly all airborne measurements during both campaigns. The comparison of the airborne measurements with corresponding simulations is presented for the total, downward and upward flux during selected clear sky periods of both campaigns. The compliance between the measured (from three aircraft) and simulated downward and total flux profiles lies in the range of ±15%.
Estimation of the aerosol radiative forcing at ground level, over land, and in cloudless atmosphere, from METEOSAT-7 observation: method and first results
Authors: Elias T. , Roujean J.-L.
Published: April 2010

Abstract. A new method is proposed to estimate the spatial and temporal variability of the solar radiative flux reaching the surface (DSSF) over land, as well as the Aerosol Radiative Forcing (ARF), in cloud-free atmosphere. The objective of global applications of the method is fulfilled by using the visible broadband of METEOSAT-7 satellite which scans Europe and Africa on a half-hourly basis. The method relies on a selection of best correspondence between METEOSAT-7 radiance and DSSF computed with a radiative transfer code. The validation of DSSF is performed comparing retrievals with ground-based measurements acquired in two contrasted environments, i.e. an urban site near Paris and a continental background site in South East of France. The study is concentrated on aerosol episodes occurring around the 2003 summer heat wave, providing 42 cases of comparison for variable solar zenith angle (from 59° to 69°), variable aerosol type (biomass burning emissions and urban pollution), and variable aerosol optical thickness (a factor 6). The method reproduces measurements of DSSF within an accuracy assessment of 20 Wm−2 (5% in relative) in 70% of the cases, and within 40 Wm−2 in 90% of the cases. Considering aerosol is the main contributor in changing the measured radiance at the top of the atmosphere, DSSF temporal variability is assumed to be caused only by aerosols, and consequently the ARF at ground level and over land is also retrieved: ARF is computed as the difference between DSSF and a parameterised aerosol-free reference level. Retrievals are linearly correlated with the ground-based measurements of the aerosol optical thickness (AOT): sensitivity is included between 120 and 160 Wm−2 per unity of AOT at 440 nm. AOT being an instantaneous measure indicative of the aerosol columnar amount, we therefore prove the feasibility to infer instantaneous aerosol radiative impact at the ground level over land with METEOSAT-7 visible channel.
Ion-mediated nucleation as an important global source of tropospheric aerosols
Authors: Yu F. , Wang Z. , Luo G. , Turco R.
Published: April 2010

Abstract. Aerosol nucleation events have been observed at a variety of locations worldwide, and may have significant climatic and health implications. While ions have long been suggested as favorable nucleation embryos, their significance as a global source of particles has remained uncertain. Here, an ion-mediated nucleation (IMN) mechanism, which incorporates new thermodynamic data and physical algorithms, has been integrated into a global chemical transport model (GEOS-Chem) to study ion mediated particle formation in the global troposphere. The simulated annual mean results have been compared to a comprehensive set of data relevant to new particle formation around the globe. We show that predicted annual spatial patterns of particle nucleation rates agree reasonably well with land-, ship-, and aircraft-based observations. Our simulations show that, globally, IMN in the boundary layer is largely confined to two broad latitude belts: one in the northern hemisphere (~20° N–70° N), and one in the southern hemisphere (~30° S–90° S). In the middle latitude boundary layer over continentals, the annual mean IMN rates are generally above 104 cm−3day−1, with some hot spots reaching 105 cm−3day−1. Zonally-averaged vertical distribution of IMN rates indicates that IMN is significant in the tropical upper troposphere, whole middle latitude troposphere, and over Antarctica. The ratio of particle number annual source strength due to IMN to those associated with primary particle emission suggests that IMN contribution is important. Further research is needed to reduce modeling uncertainties and understand the contribution of nucleated particles to the abundance of cloud condensation nuclei.
Global distribution of mean age of stratospheric air from MIPAS SF6 measurements
Authors: Stiller G. P. , von Clarmann T. , Höpfner M. , Glatthor N. , Grabowski U. , Kellmann S. , Kleinert A. , Linden A. , Milz M. , Reddmann T. , Steck T. , Fischer H. , Funke B. , López-Puertas M. , Engel A.
Published: April 2010

Abstract. Global distributions of profiles of sulphur hexafluoride (SF6) have been retrieved from limb emission spectra recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat covering the period September 2002 to March 2004. Individual SF6 profiles have a precision of 0.5 pptv below 25 km altitude and a vertical resolution of 4–6 km up to 35 km altitude. These data have been validated versus in situ observations obtained during balloon flights of a cryogenic whole-air sampler. For the tropical troposphere a trend of 0.227±0.008 pptv/yr has been derived from the MIPAS data, which is in excellent agreement with the trend from ground-based flask and in situ measurements from the National Oceanic and Atmospheric Administration Earth System Research Laboratory, Global Monitoring Division. For the data set currently available, based on at least three days of data per month, monthly 5° latitude mean values have a 1σ standard error of 1%. From the global SF6 distributions, global daily and monthly distributions of the apparent mean age of air are inferred by application of the tropical tropospheric trend derived from MIPAS data. The inferred mean ages are provided for the full globe up to 90° N/S, and have a 1σ standard error of 0.25 yr. They range between 0 (near the tropical tropopause) and 7 years (except for situations of mesospheric intrusions) and agree well with earlier observations. The seasonal variation of the mean age of stratospheric air indicates episodes of severe intrusion of mesospheric air during each Northern and Southern polar winter observed, long-lasting remnants of old, subsided polar winter air over the spring and summer poles, and a rather short period of mixing with midlatitude air and/or upward transport during fall in October/November (NH) and April/May (SH), respectively, with small latitudinal gradients, immediately before the new polar vortex starts to form. The mean age distributions further confirm that SF6 is destroyed in the mesosphere to a considerable amount. Model calculations with the Karlsruhe simulation model of the middle atmosphere (KASIMA) chemical transport model agree well with observed global distributions of the mean age only if the SF6 sink reactions in the mesosphere are included in the model.
The validity of the kinetic collection equation revisited
Authors: Alfonso L. , Raga G. B. , Baumgardner D.
Published: April 2010

Abstract. The kinetic collection equation (KCE) describes the evolution of the average droplet spectrum due to successive events of collision and coalescence. Fluctuations and non-zero correlations present in the stochastic coalescence process would imply that the size distributions may not be correctly modelled by the KCE. In this study we expand the known analytical studies of the coalescence equation with some numerical tools such as Monte Carlo simulations of the coalescence process. The validity time of the KCE was estimated by calculating the maximum of the ratio of the standard deviation for the largest droplet mass over all the realizations to the averaged value. A good correspondence between the analytical and the numerical approaches was found for all the kernels studied. The expected values from analytical solutions of the KCE, were compared with true expected values of the stochastic collection equation (SCE) estimated with Gillespie's Monte Carlo algorithm and analytical solutions of the SCE, after and before the breakdown time. The possible implications for cloud physics are discussed, in particular the possibility of application of these results to kernels modified by turbulence and electrical processes.
Hygroscopic growth and activation of HULIS particles: experimental data and a new iterative parameterization scheme for complex aerosol particles
Authors: Ziese M. , Wex H. , Nilsson E. , Salma I. , Ocskay R. , Hennig T. , Massling A. , Stratmann F.
Published: April 2010

Abstract. The hygroscopic growth and activation of two HULIS and one Aerosol-Water-Extract sample, prepared from urban-type aerosol, were investigated. All samples were extracted from filters, redissolved in water and atomized for the investigations presented here. The hygroscopic growth measurements were done using LACIS (Leipzig Aerosol Cloud Interaction Simulator) together with a HH-TDMA (High Humidity Tandem Differential Mobility Analyzer). Hygroscopic growth was determined for relative humidities up to 99.75%. The critical diameters for activation were measured using LACIS for supersaturations between 2 and 10 per mill. All three samples showed a similar hygroscopic growth behaviour, and the two HULIS samples also were similar in their activation behavior, while the Aerosol-Water-Extract turned out to be more CCN active than the HULIS samples. The experimental data was used to derive parameterizations for the hygroscopic growth and activation of HULIS particles. The concept of ρion (Wex et al., 2007a) and the Szyszkowski-equation (Szyszkowski, 1908; Facchini et al., 1999) were used for parameterizing the Raoult and the Kelvin (surface tension) terms of the Köhler equation, respectively. This concept proved to be very successful for the HULIS samples in the saturation range from relative humidities larger than 98% up to activation. However it failed for the Aerosol-Water extract.
Average molecular weight of surfactants in aerosols
Authors: Latif M. T. , Brimblecombe P.
Published: April 2010

Abstract. Surfactants in atmospheric aerosols determined as methylene blue active substances (MBAS) and ethyl violet active substances (EVAS). The MBAS and EVAS concentrations can be correlated with surface tension as determined by pendant drop analysis. The effect of surface tension was more clearly indicated in fine mode aerosol extracts. The concentration of MBAS and EVAS was determined before and after ultrafiltration analysis using AMICON centrifuge tubes that define a 5000 Da (5 K Da) nominal molecular weight fraction. Overall, MBAS and to a greater extent EVAS predominates in fraction with molecular weight below 5 K Da. In case of aerosols collected in Malaysia the higher molecular fractions tended to be a more predominant. The MBAS and EVAS are correlated with yellow to brown colours in aerosol extracts. Further experiments showed possible sources of surfactants (e.g. petrol soot, diesel soot) in atmospheric aerosols to yield material having molecular size below 5 K Da except for humic acid. The concentration of surfactants from these sources increased after ozone exposure and for humic acids it also general included smaller molecular weight surfactants.
Unambiguous identification of esters as oligomers in secondary organic aerosol formed from cyclohexene and cyclohexene/α-pinene ozonolysis
Authors: Müller L. , Reinnig M.-C. , Warnke J. , Hoffmann T.
Published: April 2010

Abstract. The built-up of oligomeric compounds during secondary organic aerosol (SOA) formation is subject of atmospheric research since several years. New particle formation and especially the SOA mass yield might be influenced significantly by oligomer formation. However, the chemical nature of observed oligomers and their formation pathways are still unclear. In this paper, the structural characterization of certain dimeric compounds (esters) formed during the ozonolysis of cyclohexene and cyclohexene/α-pinene mixtures are presented. The identification is based on the comparison of the mass spectra and the retention times (LC) of the oligomeric products with synthesized reference compounds. Cyclohexene is used here as a model compound for terpenes as globally most important SOA precursors, since it possesses a simpler structure than the biogenic alkenes and therefore offers the possibility to get access to reference compounds for certain of its oxidation products. In addition to cyclohexene, the formation of esters could also be observed in experiments with α-pinene as reactant.
Cloud type comparisons of AIRS, CloudSat, and CALIPSO cloud height and amount
Authors: Kahn B. H. , Chahine M. T. , Stephens G. L. , Mace G. G. , Marchand R. T. , Wang Z. , Barnet C. D. , Eldering A. , Holz R. E. , Kuehn R. E. , Vane D. G.
Published: April 2010

Abstract. The precision of the two-layer cloud height fields derived from the Atmospheric Infrared Sounder (AIRS) is explored and quantified for a five-day set of observations. Coincident profiles of vertical cloud structure by CloudSat, a 94 GHz profiling radar, and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), are compared to AIRS for a wide range of cloud types. Bias and variability in cloud height differences are shown to have dependence on cloud type, height, and amount, as well as whether CloudSat or CALIPSO is used as the comparison standard. The CloudSat–AIRS biases and variability range from −4.3 to 0.5±1.2–3.6 km for all cloud types. Likewise, the CALIPSO–AIRS biases range from 0.6–3.0±1.2–3.6 km (−5.8 to −0.2±0.5–2.7 km) for clouds ≥7 km (<7 km). The upper layer of AIRS has the greatest sensitivity to Altocumulus, Altostratus, Cirrus, Cumulonimbus, and Nimbostratus, whereas the lower layer has the greatest sensitivity to Cumulus and Stratocumulus. Although the bias and variability generally decrease with increasing cloud amount, the ability of AIRS to constrain cloud occurrence, height, and amount is demonstrated across all cloud types for many geophysical conditions. In particular, skill is demonstrated for thin Cirrus, as well as some Cumulus and Stratocumulus, cloud types infrared sounders typically struggle to quantify. Furthermore, some improvements in the AIRS Version 5 operational retrieval algorithm are demonstrated. However, limitations in AIRS cloud retrievals are also revealed, including the existence of spurious Cirrus near the tropopause and low cloud layers within Cumulonimbus and Nimbostratus clouds. Likely causes of spurious clouds are identified and the potential for further improvement is discussed.
Cloud thermodynamic phase inferred from merged POLDER and MODIS data
Authors: Riedi J. , Marchant B. , Platnick S. , Baum B. , Thieuleux F. , Oudard C. , Parol F. , Nicolas J-M. , Dubuisson P.
Published: April 2010

Abstract. The global spatial and diurnal distribution of cloud properties is a key issue for understanding the hydrological cycle, and critical for advancing efforts to improve numerical weather models and general circulation models. Satellite data provides the best way of gaining insight into global cloud properties. In particular, the determination of cloud thermodynamic phase is a critical first step in the process of inferring cloud optical and microphysical properties from satellite measurements. It is important that cloud phase be derived together with an estimate of the confidence of this determination, so that this information can be included with subsequent retrievals (optical thickness, effective particle radius, and ice/liquid water content). In this study, we combine three different and well documented approaches for inferring cloud phase into a single algorithm. The algorithm is applied to data obtained by the MODIS (MODerate resolution Imaging Spectroradiometer) and POLDER3 (Polarization and Directionality of the Earth Reflectance) instruments. It is shown that this synergistic algorithm can be used routinely to derive cloud phase along with an index that helps to discriminate ambiguous phase from confident phase cases. The resulting product provides a semi-continuous confidence index ranging from confident liquid to confident ice instead of the usual discrete classification of liquid phase, ice phase, mixed phase (potential combination of ice and liquid particles), or simply unknown phase clouds. This approach is expected to be useful for cloud assimilation and modeling efforts while providing more insight into the global cloud properties derived from satellite data.
When does new particle formation not occur in the upper troposphere?
Authors: Benson D. R. , Young L.-H. , Lee S.-H. , Campos T. L. , Rogers D. C. , Jensen J.
Published: April 2010

Abstract. Recent aircraft studies showed that new particle formation is very active in the free troposphere and lower stratosphere. And, these observations lead to a new question: when does new particle formation not occur? Here, we provide case studies to show how convection and surface area affect new particle formation in the upper troposphere, using the measured aerosol size distributions during the NSF/NCAR GV Progressive Science Missions in December 2005. There were ten research flights, including three days of nighttime experiments, at latitudes from 18 to 52° N and altitudes up to 14 km. About 78% of the total samples showed the new particle formation feature with number concentrations of particles with diameters from 4 to 9 nm, 670±1270 cm−3, and the total particle number concentrations with diameters from 4 to 2000 nm, 920±1470 cm−3. Our case studies show that new particle formation was closely associated with convection and low surface areas of preexisting aerosol particles (<4 μm² cm−3). On the other hand, for the cases where no new particle formation events were observed, air masses usually did not experience a vertical motion and air often originated from either the upper troposphere or lower stratosphere where precursor concentrations are relatively low; in addition, it was also a general trend that non-event cases also had higher surface areas (~16 μm² cm−3). These observations are consistent with other observations during the Progressive Science Missions (Young et al., 2007). Because of the lower temperatures in this region (T<250 K), nucleation is thermodynamically favorable; but because of low aerosol precursor concentrations, nucleation is sensitive to aerosol precursor concentration and surface area. Under such conditions, convection (which brings higher concentrations of aerosol precursors and water vapor to higher altitudes) and low surface area play critical roles on whether new particle formation takes place or not. Latitude dependence of new particles also shows higher particle concentrations in the midlatitude tropopause region than in the subtropics, consistent with Hermann et al. (2003).
How quickly do cloud droplets form on atmospheric particles?
Authors: Ruehl C. R. , Chuang P. Y. , Nenes A.
Published: April 2010

Abstract. The influence of aerosols on cloud properties is an important modulator of the climate system. Traditional Köhler theory predicts the equilibrium concentration of cloud condensation nuclei (CCN); however, it is not known to what extent particles exist in the atmosphere that may be prevented from acting as CCN by kinetic limitations. We measured the rate of cloud droplet formation on atmospheric particles sampled at four sites across the United States during the summer of 2006: Great Smoky Mountain National Park, TN; Bondville, IL; Houston, TX; and the Atmospheric Radiation Measurement Program Southern Great Plains site near Lamont, OK. We express droplet growth rates with the mass accommodation coefficient (α), and report values of α measured in the field normalized to the mean α measured for lab-generated ammonium sulfate (AS) particles (i.e., α'=α/αAS). Overall, 61% of ambient CCN grew at a rate similar to AS. We report the fraction of CCN that were "low-α'" (α'<10−0.33). Of the 16 days during which these measurements were made, 7 had relatively few low-α'CCN (<16%), 7 had moderate low-α' fractions (31% to 62%), and 2 had large low-α' fractions (>77% during at least one ~30 min period). Day to day variability was greatest in Tennessee and Illinois, and low-α' CCN were most prevalent on days when back trajectories suggested that air was arriving from aloft. The highest fractions of low-α' CCN in Houston and Illinois occurred around local noon, and decreased later in the day. These results suggest that for some air masses, accurate quantification of CCN concentrations may need to account for kinetic limitations.