bluuebunny/crossref_metadata_2025 huggingface dataset:
Total aerosol effect: radiative forcing or radiative flux perturbation?
Authors: Lohmann U. , Rotstayn L. , Storelvmo T. , Jones A. , Menon S. , Quaas J. , Ekman A. M. L. , Koch D. , Ruedy R.
Published: April 2010

Abstract. Uncertainties in aerosol radiative forcings, especially those associated with clouds, contribute to a large extent to uncertainties in the total anthropogenic forcing. The interaction of aerosols with clouds and radiation introduces feedbacks which can affect the rate of precipitation formation. In former assessments of aerosol radiative forcings, these effects have not been quantified. Also, with global aerosol-climate models simulating interactively aerosols and cloud microphysical properties, a quantification of the aerosol forcings in the traditional way is difficult to define properly. Here we argue that fast feedbacks should be included because they act quickly compared with the time scale of global warming. We show that for different forcing agents (aerosols and greenhouse gases) the radiative forcings as traditionally defined agree rather well with estimates from a method, here referred to as radiative flux perturbations (RFP), that takes these fast feedbacks and interactions into account. Based on our results, we recommend RFP as a valid option to compare different forcing agents, and to compare the effects of particular forcing agents in different models.
Aerosol Single Scattering Albedo retrieval in the UV range: an application to OMI satellite validation
Authors: Ialongo I. , Buchard V. , Brogniez C. , Casale G. R. , Siani A. M.
Published: April 2010

Abstract. The aerosol Single Scattering Albedo (SSA) and Absorbing Aerosol Optical Depth (AAOD) at 320.1 nm are derived at Rome site by the comparison between Brewer and modelled spectra. The UVSPEC radiative transfer model is used to calculate the UV irradiances for different SSA values, taking into account as input data total ozone and Aerosol Optical Depth (AOD) obtained from Brewer spectral measurements. The accuracy in determining SSA depends on the aerosol amount and on Solar Zenith Angle (SZA) value: SSA uncertainty increases when AOD and SZA decrease. The monthly mean values of SSA and AAOD during the period January 2005–June 2008 are analysed, showing a monthly and seasonal variability. It is found that the SSA and AAOD averages are 0.80±0.08 and 0.056±0.028, respectively. AAOD retrievals are also used to quantify the error in the Ozone Monitoring Instrument (OMI) surface UV products due to absorbing aerosols, not included in the current OMI UV algorithm. OMI and Brewer UV irradiances at 324.1 nm and Erythemal Dose Rates (EDRs) under clear sky conditions, are compared as a function of AAOD. Three methods are considered to investigate on the applicability of an absorbing aerosol correction on OMI UV data at Rome site. Depending on the correction methodology, the bias value decreases from 18% to 2% for spectral irradiance at 324.1 nm and from 25% to 8% for EDR.
Study on the impact of sudden stratosphere warming in the upper mesosphere-lower thermosphere regions using satellite and HF radar measurements
Authors: Mbatha N. , Sivakumar V. , Malinga S. B. , Bencherif H. , Pillay S. R.
Published: April 2010

Abstract. The occurrence of a sudden stratospheric warming (SSW) excites disturbances in the mesosphere-lower thermospheric (MLT) wind and temperature. Here, we have examined the high frequency (HF) radar wind data from the South African National Antarctic Expedition, SANAE (72° S, 3° W), a radar which is part of the Super Dual Auroral Radar Network (SuperDARN). Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) on board the Thermosphere-Ionosphere-Mesosphere-Energetics and Dynamics (TIMED) satellite temperature data and National Centre for Environmental Prediction (NCEP) temperature and wind data are used to investigate the dynamical effects of the unprecedented September 2002 SSW in the Antarctica stratosphere and MLT. The mean zonal wind (from SANAE HF radar) at the MLT shows reversal approximately 7 days before the reversal at 10 hPa (from NCEP). This indicates that there was a downwards propagation of circulation disturbance. Westerly zonal winds dominate the winter MLT, but during the 2002 winter there are many periods of westward winds observed compared to other years. The normalised power spectrums of both meridional and zonal winds show presence of planetary waves (of ~14-day period) before the occurrence of the SSW. The SABER vertical temperature profiles indicated the cooling of the MLT region before the SSW event.
Parameterization of vertical diffusion and the atmospheric boundary layer height determination in the EMEP model
Authors: Jeričević A. , Kraljević L. , Grisogono B. , Fagerli H. , Večenaj Ž.
Published: April 2010

Abstract. This paper introduces two changes of the turbulence parameterization for the EMEP (European Monitoring and Evaluation Programme) Eulerian air pollution model: the replacement of the Blackadar in stable and O'Brien in unstable turbulence formulations with an analytical vertical diffusion profile (K(z)) called Grisogono, and a different mixing height determination, based on a bulk Richardson number formulation (RiB). The operational or standard (STD) and proposed new parameterization for eddy diffusivity have been validated in all stability conditions against the observed daily surface nitrogen dioxide (NO2), sulphur dioxide (SO2) and sulphate (SO42−) concentrations at different EMEP stations during the year 2001. A moderate improvement in the correlation coefficient and bias for NO2 and SO2 and a slight improvement for sulphate is found for the most of the analyzed stations with the Grisogono K(z) scheme, which is recommended for further application due to its scientific and technical advantages. The newly extended approach for the mechanical eddy diffusivity is applied to the Large Eddy Simulation data focusing at the bulk properties of the neutral and stable atmospheric boundary layer. A summary and extension of the previous work on the empirical coefficients in neutral and stable conditions is provided with the recommendations to the further model development. Special emphasis is given to the representation of the ABL in order to capture the vertical transport and dispersion of the atmospheric air pollution. Two different schemes for the ABL height determination are evaluated against the radiosounding data in January and July 2001, and against the data from the Cabauw tower, the Netherlands, for the same year. The validation of the ABL parameterizations has shown that the EMEP model is able to reproduce spatial and temporal mixing height variability. Improvements are identified especially in stable conditions with the new ABL height scheme based on the RiB number.
The ground-based FTIR network's potential for investigating the atmospheric water cycle
Authors: Schneider M. , Yoshimura K. , Hase F. , Blumenstock T.
Published: April 2010

Abstract. We present tropospheric H216O and HD16O/H216O vapour profiles measured by ground-based FTIR (Fourier Transform Infrared) spectrometers between 1996 and 2008 at a northern hemispheric subarctic and subtropical site (Kiruna, Northern Sweden, 68° N and Izaña, Tenerife Island, 28° N, respectively). We compare these measurements to an isotope incorporated atmospheric general circulation model (AGCM). If the model is nudged towards meteorological fields of reanalysis data the agreement is very satisfactory on time scales ranging from daily to inter-annual. Taking the Izaña and Kiruna measurements as an example we document the FTIR network's unique potential for investigating the atmospheric water cycle. At the subarctic site we find strong correlations between the FTIR data, on the one hand, and the Arctic Oscillation index and the northern Atlantic sea surface temperature, on the other hand. The Izaña FTIR measurements reveal the importance of the Hadley circulation and the Northern Atlantic Oscillation index for the subtropical middle/upper tropospheric water balance. We document where the AGCM is able to capture these complexities of the water cycle and where it fails.
Sediment records of highly variable mercury inputs to mountain lakes in Patagonia during the past millennium
Authors: Ribeiro Guevara S. , Meili M. , Rizzo A. , Daga R. , Arribére M.
Published: April 2010

Abstract. High Hg levels in the pristine lacustrine ecosystems of the Nahuel Huapi National Park, a protected zone situated in the Andes of Northern Patagonia, Argentina, have initiated further investigations on Hg cycling and source identification. Here we report Hg records in sedimentary sequences to identify atmospheric sources during the past millennium. In addition to global transport and deposition, a potential atmospheric Hg source to be considered is the local emissions associated with volcanic activity, because the Park is situated in the Southern Volcanic Zone. Two sediment cores were extracted from Lake Tonček, a small, high-altitude system reflecting mainly direct inputs associated with atmospheric contributions, and Lake Moreno Oeste, a much larger and deeper lake having an extended watershed covered mostly by native forest. The sedimentary sequences were dated based on both 210Pb and 137Cs profiles. In addition, tephra layers were identified and geochemically characterized for chronological application and to investigate any association of volcanic eruptions with Hg records. Hg concentrations in sediments were measured along with 32 other elements, as well as organic matter, subfossil chironomids, and biogenic silica. Observed background Hg concentrations, determined from the sequence domains with lower values, ranged from 50 to 100 ng g−1 dry weight (DW), whereas the surficial layers reached 200 to 500 ng g−1 DW. In addition to this traditional pattern, however, two deep domains in both sequences showed dramatically increased Hg levels reaching 400 to 650 ng g−1 DW; the upper dated to the 18th to 19th centuries, and the lower around the 13th century. These concentrations are not only elevated in the present profiles but also many-fold above the background values determined in other fresh water sediments, as were also the Hg fluxes, reaching 120 to 150 μg m−2 y−1 in Lake Tonček . No correlation was observed between Hg concentrations and the contents of organic matter, subfossil chironomids, biogenic silica, or the other elements determined. However, distinctly increased Hg concentrations were observed immediately above some tephra layers, suggesting a link to volcanic events. Extended fires might be another potential atmospheric source because the earlier Hg peaks coincide with reported charcoal peaks, whereas the upper Hg peaks coincide with evidences of extended forest fires from tree-ring data and historical records.
Analysis of global and regional CO burdens measured from space between 2000 and 2009 and validated by ground-based solar tracking spectrometers
Authors: Yurganov L. , McMillan W. , Grechko E. , Dzhola A.
Published: April 2010

Abstract. Interannual variations in AIRS and MOPITT retrieved CO burdens are validated, corrected, and compared with CO emissions from wild fires from the Global Fire Emission Dataset (GFED2) inventory. Validation of daily mean CO total column (TC) retrievals from MOPITT version 3 and AIRS version 5 is performed through comparisons with archived TC data from the Network for Detection of Atmospheric Composition Change (NDACC) ground-based Fourier Transform Spectrometers (FTS) between March 2000 and December 2007. MOPITT V3 retrievals exhibit an increasing temporal bias with a rate of 1.4–1.8% per year; thus far, AIRS retrievals appear to be more stable. For the lowest CO values in the Southern Hemisphere (SH), AIRS TC retrievals overestimate FTS TC by 20%. MOPITT's bias and standard deviation do not depend on CO TC absolute values. Empirical corrections are derived for AIRS and MOPITT retrievals based on the observed annually averaged bias versus the FTS TC. Recently published MOPITT V4 is found to be in a good agreement with MOPITT V3 corrected by us (with exception of 2000–2001 period). With these corrections, CO burdens from AIRS V5 and MOPITT V3 (as well as MOPITT V4) come into good agreement in the mid-latitudes of the Northern Hemisphere (NH) and in the tropical belt. In the SH, agreement between AIRS and MOPITT CO burdens is better for the larger CO TC in austral winter and worse in austral summer when CO TC are smaller. Before July 2008, all variations in retrieved CO burden can be explained by changes in fire emissions. After July 2008, global and tropical CO burdens decreased until October before recovering by the beginning of 2009. The NH CO burden also decreased but reached a minimum in January 2009 before starting to recover. The decrease in tropical CO burdens is explained by lower than usual fire emissions in South America and Indonesia. This decrease in tropical emissions also accounts for most of the change in the global CO burden. However, no such diminution of NH biomass burning is indicated by GFED2. Thus, the CO burden decrease in the NH could result from a combination of lower fossil fuel emissions during the global economic recession and transport of CO-poor air from the tropics. More extensive modeling will be required to fully resolve this issue.
Do biomass burning aerosols intensify drought in equatorial Asia during El Niño?
Authors: Tosca M. G. , Randerson J. T. , Zender C. S. , Flanner M. G. , Rasch P. J.
Published: April 2010

Abstract. During El Niño years, fires in tropical forests and peatlands in equatorial Asia create large regional smoke clouds. We characterized the sensitivity of these clouds to regional drought, and we investigated their effects on climate by using an atmospheric general circulation model. Satellite observations during 2000–2006 indicated that El Niño-induced regional drought led to increases in fire emissions and, consequently, increases in aerosol optical depths over Sumatra, Borneo and the surrounding ocean. Next, we used the Community Atmosphere Model (CAM) to investigate how climate responded to this forcing. We conducted two 30 year simulations in which monthly fire emissions were prescribed for either a high (El Niño, 1997) or low (La Niña, 2000) fire year using a satellite-derived time series of fire emissions. Our simulations included the direct and semi-direct effects of aerosols on the radiation budget within the model. We assessed the radiative and climate effects of anthropogenic fire by analyzing the differences between the high and low fire simulations. Fire aerosols reduced net shortwave radiation at the surface during August–October by 19.1±12.9 W m−2 (10%) in a region that encompassed most of Sumatra and Borneo (90° E–120° E, 5° S–5° N). The reductions in net shortwave radiation cooled sea surface temperatures (SSTs) and land surface temperatures by 0.5±0.3 and 0.4±0.2 °C during these months. Tropospheric heating from black carbon (BC) absorption averaged 20.5±9.3 W m−2 and was balanced by a reduction in latent heating. The combination of decreased SSTs and increased atmospheric heating reduced regional precipitation by 0.9±0.6 mm d−1 (10%). The vulnerability of ecosystems to fire was enhanced because the decreases in precipitation exceeded those for evapotranspiration. Together, the satellite and modeling results imply a possible positive feedback loop in which anthropogenic burning in the region intensifies drought stress during El Niño.
Turbulence associated with mountain waves over Northern Scandinavia – a case study using the ESRAD VHF radar and the WRF mesoscale model
Authors: Kirkwood S. , Mihalikova M. , Rao T. N. , Satheesan K.
Published: April 2010

Abstract. We use measurements by the 52 MHz wind-profiling radar ESRAD, situated near Kiruna in Arctic Sweden, and simulations using the Advanced Research and Weather Forecasting model, WRF, to study vertical winds and turbulence in the troposphere in mountain-wave conditions on 23, 24 and 25 January 2003. We find that WRF can accurately match the vertical wind signatures at the radar site when the spatial resolution for the simulations is 1 km. The horizontal and vertical wavelengths of the dominating mountain-waves are ~10–20 km and the amplitudes in vertical wind 1–2 m/s. Turbulence below 5500 m height, is seen by ESRAD about 40% of the time. This is a much higher rate than WRF predictions for conditions of Richardson number (Ri) <1 but similar to WRF predictions of Ri<2. WRF predicts that air crossing the 100 km wide model domain centred on ESRAD has a ~10% chance of encountering convective instabilities (Ri<0) somewhere along the path. The cause of low Ri is a combination of wind-shear at synoptic upper-level fronts and perturbations in static stability due to the mountain-waves. Comparison with radiosondes suggests that WRF underestimates wind-shear and the occurrence of thin layers with very low static stability, so that vertical mixing by turbulence associated with mountain waves may be significantly more than suggested by the model.
The impact of dust on sulfate aerosol, CN and CCN during an East Asian dust storm
Authors: Manktelow P. T. , Carslaw K. S. , Mann G. W. , Spracklen D. V.
Published: April 2010

Abstract. A global model of aerosol microphysics is used to simulate a large East Asian dust storm during the ACE-Asia experiment. We use the model together with size resolved measurements of aerosol number concentration and composition to examine how dust modified the production of sulfate aerosol and the particle size distribution in East Asian outflow. Simulated size distributions and mass concentrations of dust, sub- and super-micron sulfate agree well with observations from the C-130 aircraft. Modeled mass concentrations of fine sulfate (Dp<1.3 μm) decrease by ~10% due to uptake of sulfur species onto super-micron dust. We estimate that dust enhanced the mass concentration of coarse sulfate (Dp>1.0 μm) by more than an order of magnitude, but total sulfate concentrations increase by less than 2% because decreases in fine sulfate have a compensating effect. Our analysis shows that the sulfate associated with dust can be explained largely by the uptake of H2SO4 rather than reaction of SO2 on the dust surface, which we assume is suppressed once the particles are coated in sulfate. We suggest that many previous model investigations significantly overestimated SO2 oxidation on East Asian dust, possibly due to the neglect of surface saturation effects. We extend previous model experiments by examining how dust modified existing particle concentrations in Asian outflow. Total particle concentrations (condensation nuclei, CN) modeled in the dust-pollution plume are reduced by up to 20%, but we predict that dust led to less than 10% depletion in particles large enough to act as cloud condensation nuclei (CCN). Our analysis suggests that E. Asian dust storms have only a minor impact on sulfate particles present at climate-relevant sizes.
Kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB): the influence of interfacial transport and bulk diffusion on the oxidation of oleic acid by ozone
Authors: Shiraiwa M. , Pfrang C. , Pöschl U.
Published: April 2010

Abstract. We present a novel kinetic multi-layer model that explicitly resolves mass transport and chemical reaction at the surface and in the bulk of aerosol particles (KM-SUB). The model is based on the PRA framework of gas-particle interactions (Pöschl-Rudich-Ammann, 2007), and it includes reversible adsorption, surface reactions and surface-bulk exchange as well as bulk diffusion and reaction. Unlike earlier models, KM-SUB does not require simplifying assumptions about steady-state conditions and radial mixing. The temporal evolution and concentration profiles of volatile and non-volatile species at the gas-particle interface and in the particle bulk can be modeled along with surface concentrations and gas uptake coefficients. In this study we explore and exemplify the effects of bulk diffusion on the rate of reactive gas uptake for a simple reference system, the ozonolysis of oleic acid particles, in comparison to experimental data and earlier model studies. We demonstrate how KM-SUB can be used to interpret and analyze experimental data from laboratory studies, and how the results can be extrapolated to atmospheric conditions. In particular, we show how interfacial and bulk transport, i.e., surface accommodation, bulk accommodation and bulk diffusion, influence the kinetics of the chemical reaction. Sensitivity studies suggest that in fine air particulate matter oleic acid and compounds with similar reactivity against ozone (carbon-carbon double bonds) can reach chemical lifetimes of many hours only if they are embedded in a (semi-)solid matrix with very low diffusion coefficients (≤10−10 cm2 s−1). Depending on the complexity of the investigated system, unlimited numbers of volatile and non-volatile species and chemical reactions can be flexibly added and treated with KM-SUB. We propose and intend to pursue the application of KM-SUB as a basis for the development of a detailed master mechanism of aerosol chemistry as well as for the derivation of simplified but realistic parameterizations for large-scale atmospheric and climate models.
Sulfur cycle and sulfate radiative forcing simulated from a coupled global climate-chemistry model
Authors: Tsai I.-C. , Chen J.-P. , Lin P.-Y. , Wang W.-C. , Isaksen I. S. A.
Published: April 2010

Abstract. The sulfur cycle and radiative effects of sulfate aerosol on climate are studied with a Global tropospheric Climate-Chemistry Model in which chemistry, radiation and dynamics are fully coupled. Production and removal mechanisms of sulfate are analyzed for the conditions of natural and anthropogenic sulfur emissions. Results show that the 1985 anthropogenic emission tripled the global SO2 and sulfate loadings from its natural value of 0.16 and 0.10 Tg S, respectively. Under natural conditions, the fraction of sulfate produced in-cloud is 74%; whereas with anthropogenic emissions, the fraction of in-cloud sulfate production slightly increased to 76%. Lifetimes of SO2 and sulfate under polluted conditions are estimated to be 1.7 and 2.0 days, respectively. The tripling of sulfate results in a direct radiative forcing of −0.43 W m−2 (clear-sky) or −0.24 W m−2 (all-sky), and a significant first indirect forcing of −1.85 W m−2, leading to a mean global cooling of about 0.1 K. Regional forcing and responses are significantly stronger than the global values. The first indirect forcing is sensitive to the relationship between aerosol concentration and cloud droplet number concentration which requires further investigation. Two aspects of chemistry-climate interaction are addressed. Firstly, the coupling effects lead to a slight decrease of 1% in global sulfate loading for both the cases of natural and anthropogenic added sulfur emissions. Secondly, only the indirect effect of sulfate aerosols yields significantly stronger signals in changes of near surface temperature and sulfate loading than changes due to intrinsic climate variability, while other responses to the indirect effect and all responses to the direct effect are below noise level.
Factors influencing the contribution of ion-induced nucleation in a boreal forest, Finland
Authors: Gagné S. , Nieminen T. , Kurtén T. , Manninen H. E. , Petäjä T. , Laakso L. , Kerminen V.-M. , Boy M. , Kulmala M.
Published: April 2010

Abstract. We present the longest series of measurements so far (2 years and 7 months) made with an Ion-DMPS at the SMEAR II measurement station in Hyytiälä, Southern Finland. We show that the classification into overcharged (implying some participation of ion-induced nucleation) and undercharged (implying no or very little participation of ion-induced nucleation) days, based on Ion-DMPS measurements, agrees with the fraction of ion-induced nucleation based on NAIS measurements. Those classes are based on the ratio of ambient charged particle to steady-state charged particle concentration, known as the charging state. We analyzed the influence of different parameters on the contribution of ion-induced nucleation to the total particle formation rate. We found that the fraction of ion-induced nucleation is typically higher on warmer, drier and sunnier days compared to colder days with less solar radiation and a higher relative humidity. Also, we observed that bigger concentrations of new particles were produced on days with a smaller fraction of ion-induced nucleation. Moreover, sulfuric acid saturation ratios were smaller for days with a bigger fraction of ion-induced nucleation. Finally, we propose explanations on how these different parameters could influence neutral and ion-induced nucleation, and show that the different mechanisms seem to take place at the same time during an event. For example, we propose that these observed differences could be due to high temperature and low vapors' saturation ratios (water and sulfuric acid) increasing the height of the energy barrier a particle has to reach before it can grow and thus limiting neutral nucleation.
Hydroxyl radicals in the tropical troposphere over the Suriname rainforest: airborne measurements
Authors: Martinez M. , Harder H. , Kubistin D. , Rudolf M. , Bozem H. , Eerdekens G. , Fischer H. , Klüpfel T. , Gurk C. , Königstedt R. , Parchatka U. , Schiller C. L. , Stickler A. , Williams J. , Lelieveld J.
Published: April 2010

Abstract. Direct measurements of OH and HO2 over a tropical rainforest were made for the first time during the GABRIEL campaign in October 2005, deploying the custom-built HORUS instrument (HydrOxyl Radical measurement Unit based on fluorescence Spectroscopy), adapted to fly in a Learjet wingpod. Biogenic hydrocarbon emissions were expected to strongly reduce the OH and HO2 mixing ratios as the air is transported from the ocean over the forest. However, surprisingly high mixing ratios of both OH and HO2 were encountered in the boundary layer over the rainforest. The HORUS instrumentation and calibration methods are described in detail and the measurement results obtained are discussed. The extensive dataset collected during GABRIEL, including measurements of many other trace gases and photolysis frequencies, has been used to quantify the main sources and sinks of OH. Comparison of these measurement-derived formation and loss rates of OH indicates strong previously overlooked recycling of OH in the boundary layer over the tropical rainforest, occurring in chorus with isoprene emission.
Radiosonde stratospheric temperatures at Dumont d'Urville (Antarctica): trends and link with polar stratospheric clouds
Authors: David C. , Keckhut P. , Armetta A. , Jumelet J. , Snels M. , Marchand M. , Bekki S.
Published: April 2010

Abstract. Temperature profiles measurements are performed daily (00:00 UT) in Dumont d'Urville (66°40' S, 140°01' E) by Météo-France, using standard radiosondes, since the International Geophysical Year in 1957. Yet, due to a 16 years data gap between 1963 and 1978, the entire dataset is only used for a qualitative overview. Only the most recent series, between 1979 and 2008, is used to investigate the inter-annual stratospheric temperatures variability. Over Dumont d'Urville, at the edge of the vortex, the annual mean temperature cooling of about 1 K/decade at 20 km is the result of the cooling trends between 0.5 and 1.4 K/decade, in summer and autumn and a warming of about 1.1 K/decade in spring. These values are consistent with values obtained using data from inner vortex stations, but with smaller amplitude. No statistically significant trend is detected in winter. We propose here the first attempt to link stratospheric temperature trends to Polar Stratospheric Cloud (PSC) trends in Antarctica based on the only continuous 20 years database of PSC lidar detection. Despite the absence of mean temperature trend during winter, the occurrence of temperatures below the NAT threshold between 1989 and 2008 reveals a significant trend of about +6%/decade. The PSCs occurrences frequency exhibits a concomitant trend of about +3%/decade, although not statistically significant. Yet, this is consistent with results obtained in the Northern Hemisphere. Such a possible positive trend in PSC occurrence has to be further explored to be confirmed or invalidated. If confirmed, this PSC trend is likely to have strong impacts, both on ozone recovery and climate evolution in Antarctica. The study also reveals the importance of trends on extreme temperatures, and not only on mean temperatures.
Impact of dust on tropospheric chemistry over polluted regions: a case study of the Beijing megacity
Authors: Zhu S. , Butler T. , Sander R. , Ma J. , Lawrence M. G.
Published: April 2010

Abstract. The box model MECCA (Module Efficiently Calculating the Chemistry of the Atmosphere) is extended by incorporating detailed heterogeneous chemistry occurring on mineral aerosol surfaces. The model is used to investigate the impact of dust on tropospheric photochemistry, when the dust is transported to a polluted region, focusing on the example of Beijing. The impacts of dust via heterogeneous removal of gases are analyzed for different hypothetical transport rates, which are described by four different exchange rate coefficients Kt in the model. Along with the dust, airmasses with trace gas levels characteristic for regions upwind of Beijing are transported with the same rate (Kt). Substantial impacts are found for many gases, including Ox (O3+O(3P)), NOx (NO+NO2) and OH. The Ox daily average mixing ratio decreases due to heterogeneous reactions on dust. The change ranges from −2.5 to −18.4 nmol mol−1, and is larger for faster mixing with upwind air masses (i.e. greater Kt). This translates into a large relative change in Ox, ranging from −44% to −55%, depending on Kt. By assuming an artificial 50% decrease of all photolysis rates, the impacts of dust via perturbation of the photolysis rates in the polluted region are also estimated. Furthermore, the uncertainties in the results due to the uncertainties in the uptake coefficients are evaluated. It is found that for all gases which are heterogeneously removed, the self-removal results in the largest uncertainty (e.g. −49% for O3, −76% for NO2, −47% for HNO3, −92% for HCHO, −64% for CH3OH and −93% for SO2). The heterogeneous removal of NO2 is found to be particularly important, because it results in significant levels of uncertainty not only for itself, but also for OH (340%) and HO2 (365%). Moreover, the heterogeneous removal rates of HCHO and O3 also have farther-reaching effects on the OH concentration (resulting in changes of −55% and 45%, respectively), and the heterogeneous removal of HCHO results in an uncertainty of −38% in the HO2 concentration. The limitations of MECCA due to its missing oxidation mechanism for aromatics and other higher VOC species has also been considered, and shown to be potentially important in the quantitative results, though not likely to change the qualitative results of this study.
Effects of relative humidity on aerosol light scattering in the Arctic
Authors: Zieger P. , Fierz-Schmidhauser R. , Gysel M. , Ström J. , Henne S. , Yttri K. E. , Baltensperger U. , Weingartner E.
Published: April 2010

Abstract. Aerosol particles experience hygroscopic growth in the ambient atmosphere. Their optical properties – especially the aerosol light scattering – are therefore strongly dependent on the ambient relative humidity (RH). In-situ light scattering measurements of long-term observations are usually performed under dry conditions (RH>30–40%). The knowledge of this RH effect is of eminent importance for climate forcing calculations or for the comparison of remote sensing with in-situ measurements. This study combines measurements and model calculations to describe the RH effect on aerosol light scattering for the first time for aerosol particles present in summer and fall in the high Arctic. For this purpose, a field campaign was carried out from July to October 2008 at the Zeppelin station in Ny-Ålesund, Svalbard. The aerosol light scattering coefficient σsp(λ) was measured at three distinct wavelengths (λ=450, 550, and 700 nm) at dry and at various, predefined RH conditions between 20% and 95% with a recently developed humidified nephelometer (WetNeph) and with a second nephelometer measuring at dry conditions with an average RH<10% (DryNeph). In addition, the aerosol size distribution and the aerosol absorption coefficient were measured. The scattering enhancement factor f(RH, λ) is the key parameter to describe the RH effect on σsp(λ) and is defined as the RH dependent σsp(RH, λ) divided by the corresponding dry σsp(RHdry, λ). During our campaign the average f(RH=85%, λ=550 nm) was 3.24±0.63 (mean ± standard deviation), and no clear wavelength dependence of f(RH, λ) was observed. This means that the ambient scattering coefficients at RH=85% were on average about three times higher than the dry measured in-situ scattering coefficients. The RH dependency of the recorded f(RH, λ) can be well described by an empirical one-parameter equation. We used a simplified method to retrieve an apparent hygroscopic growth factor g(RH), defined as the aerosol particle diameter at a certain RH divided by the dry diameter, using the WetNeph, the DryNeph, the aerosol size distribution measurements and Mie theory. With this approach we found, on average, g(RH=85%) values to be 1.61±0.12 (mean±standard deviation). No clear seasonal shift of f(RH, λ) was observed during the 3-month period, while aerosol properties (size and chemical composition) clearly changed with time. While the beginning of the campaign was mainly characterized by smaller and less hygroscopic particles, the end was dominated by larger and more hygroscopic particles. This suggests that compensating effects of hygroscopicity and size determined the temporal stability of f(RH, λ). During sea salt influenced periods, distinct deliquescence transitions were observed. At the end we present a method on how to transfer the dry in-situ measured aerosol scattering coefficients to ambient values for the aerosol measured during summer and fall at this location.
Volcanic ash as fertiliser for the surface ocean
Authors: Langmann B. , Zakšek K. , Hort M. , Duggen S.
Published: April 2010

Abstract. Iron is a key limiting micro-nutrient for marine primary productivity. It can be supplied to the ocean by atmospheric dust deposition. Volcanic ash deposition into the ocean represents another external and so far largely neglected source of iron. This study demonstrates strong evidence for natural fertilisation in the iron-limited oceanic area of the NE Pacific, induced by volcanic ash from the eruption of Kasatochi volcano in August 2008. Atmospheric and oceanic conditions were favourable to generate a massive phytoplankton bloom in the NE Pacific Ocean which for the first time strongly suggests a connection between oceanic iron-fertilisation and volcanic ash supply.
Vertically-resolved particle size distribution within and above the mixing layer over the Milan metropolitan area
Authors: Ferrero L. , Perrone M. G. , Petraccone S. , Sangiorgi G. , Ferrini B. S. , Lo Porto C. , Lazzati Z. , Cocchi D. , Bruno F. , Greco F. , Riccio A. , Bolzacchini E.
Published: April 2010

Abstract. Vertical aerosol profiles were directly measured over the city of Milan during three years (2005–2008) of field campaigns. An optical particle counter, a portable meteorological station and a miniaturized cascade impactor were deployed on a tethered balloon. More than 300 vertical profiles were measured, both in winter and summer, mainly in conditions of clear, dry skies. The mixing height was determined from the observed vertical aerosol concentration gradient, and from potential temperature and relative humidity profiles. Results show that inter-consistent mixing heights can be retrieved highlighting good correlations between particle dispersion in the atmosphere and meteorological parameters. Mixing height growth speed was calculated for both winter and summer showing the low potential atmospheric dispersion in winter. Aerosol number size distribution and chemical composition profiles allowed us to investigate particle behaviour along height. Aerosol measurements showed changes in size distribution according to mixing height. Coarse particle profiles (dp>1.6 μm) were distributed differently than the fine ones (dp<1.6 μm) were, at different heights of the mixing layer. The sedimentation process influenced the coarse particle profiles, and led to a reduction in mean particle diameter for those particles observed by comparing data above the mixing height with ground data (−14.9±0.6% in winter and −10.7±1.0% in summer). Conversely, the mean particle diameter of fine particles increased above the mixing height under stable atmospheric conditions; the average increase, observed by comparing data above the mixing height with ground data, was +2.1±0.1% in winter and +3.9±0.3% in summer. A hierarchical statistical model was created to describe the changes in the size distribution of fine particles along height. The proposed model can be used to estimate the typical vertical profile characterising launches within pre-specified groups starting from: aerosol size and meteorological conditions measured at ground-level, and a mixing height estimation. The average increase of fine particle diameter, estimated on the basis of the model, was +1.9±0.5% in winter and +6.1±1.2% in summer, in keeping with experimental findings.
Influence of vortex dynamics and atmospheric turbulence on the early evolution of a contrail
Authors: Paugam R. , Paoli R. , Cariolle D.
Published: April 2010

Abstract. This study describes three-dimensional numerical simulations of the evolution of an aircraft contrail during the first 30 min following the emission of exhausts. The wake is modeled as a vortex pair descending in a stratified atmosphere where turbulent fluctuations are sustained in the late dissipation regime. The focus of the study is laid on the interactions between vortex dynamics, atmospheric turbulence and contrail microphysics, and their role in determining the growth and the distribution of ice crystals. The atmospheric turbulence is synthesized using a methodology developed to force anisotropic turbulent fluctuations. The results show the feasibility of three-dimensional simulations of the early development of a contrail in supersaturated conditions before its transition into a contrail-cirrus. %(when radiative heating and sedimentation are no more negligible). It is shown that in case of strongly supersaturated and shear-free atmosphere the optical depth is maintained as the contrail spreads by turbulent diffusion in the late dissipation regime.
Correlations of mesospheric winds with subtle motion of the Arctic polar vortex
Authors: Bhattacharya Y. , Gerrard A. J.
Published: April 2010

Abstract. This paper investigates the relationship between high latitude upper mesospheric winds and the state of the stratospheric polar vortex in the absence of major sudden stratospheric warmings. A ground based Michelson Interferometer stationed at Resolute Bay (74°43' N, 94°58' W) in the Canadian High Arctic is used to measure mesopause region neutral winds using the hydroxyl (OH) Meinel-band airglow emission (central altitude of ~85 km). These observed winds are compared to analysis winds in the upper stratosphere during November and December of 1995 and 1996; years characterized as cold, stable polar vortex periods. Correlation of mesopause wind speeds with those from the upper stratosphere is found to be significant for the 1996 season when the polar vortex is subtly displaced off its initial location by a strong Aleutian High. These mesopause winds are observed to lead stratospheric winds by approximately two days with increasing (decreasing) mesospheric winds predictive of decreasing (increasing) stratospheric winds. No statistically significant correlations are found for the 1995 season when there is no such displacement of the polar vortex.
Advective mixing in a nondivergent barotropic hurricane model
Authors: Rutherford B. , Dangelmayr G. , Persing J. , Schubert W. H. , Montgomery M. T.
Published: April 2010

Abstract. This paper studies Lagrangian mixing in a two-dimensional barotropic model for hurricane-like vortices. Since such flows show high shearing in the radial direction, particle separation across shear-lines is diagnosed through a Lagrangian field, referred to as R-field, that measures trajectory separation orthogonal to the Lagrangian velocity. The shear-lines are identified with the level-contours of another Lagrangian field, referred to as S-field, that measures the average shear-strength along a trajectory. Other fields used for model diagnostics are the Lagrangian field of finite-time Lyapunov exponents (FTLE-field), the Eulerian Q-field, and the angular velocity field. Because of the high shearing, the FTLE-field is not a suitable indicator for advective mixing, and in particular does not exhibit ridges marking the location of finite-time stable and unstable manifolds. The FTLE-field is similar in structure to the radial derivative of the angular velocity. In contrast, persisting ridges and valleys can be clearly recognized in the R-field, and their propagation speed indicates that transport across shear-lines is caused by Rossby waves. A radial mixing rate derived from the R-field gives a time-dependent measure of flux across the shear-lines. On the other hand, a measured mixing rate across the shear-lines, which counts trajectory crossings, confirms the results from the R-field mixing rate, and shows high mixing in the eyewall region after the formation of a polygonal eyewall, which continues until the vortex breaks down. The location of the R-field ridges elucidates the role of radial mixing for the interaction and breakdown of the mesovortices shown by the model.
Forecasted deep stratospheric intrusions over Central Europe: case studies and climatologies
Authors: Trickl T. , Feldmann H. , Kanter H.-J. , Scheel H.-E. , Sprenger M. , Stohl A. , Wernli H.
Published: April 2010

Abstract. Based on daily predictions of stratospheric air intrusions, obtained from trajectory calculations by ETH Zürich with wind fields from ECMWF forecasts, a high number of measurements with the ozone lidar at IMK-IFU (Garmisch-Partenkirchen, Germany) were carried out in 2001. The lidar measurements show a large variety of rather different cases reflecting the full complexity of intrusion episodes that is not visible in classical case studies. In part, tropopause folds could be fully captured. The frequency of intrusion cases forecasted and verified by vertical sounding or in the in-situ data recorded at the nearby Zugspitze summit (2962 m a.s.l.) exceed that in previous work by more than a factor of two. Three cases mapped with the lidar were selected to validate the results for the corresponding time periods extracted from a one-year run with the new hemispheric version of the chemistry-transport model EURAD. Due to the high spatial resolution chosen for these simulations the agreement with the lidar measurements is satisfactory. The Zugspitze ozone data from 1978 to 2004 were recently filtered by applying different criteria for stratospheric air, based on the 7Be and humidity measurements. Here, by using the daily model forecasts during the time period 2001–2005, we examine three criteria and determine how well they represent the stratospheric air intrusions reaching the mountain site. Seasonal cycles for the period 2001–2005 were derived for the forecasts as well as the intrusion frequency per month for the forecasted intrusions and each of the criteria, distinguishing eight different characteristic transport pathways. In most cases a winter maximum and a summer minimum was obtained, but in the case of cyclonic arrival of intrusions starting over Greenland a late-spring maximum is seen. Two of the filtering criteria examined, based on combining a relative-humidity (RH) threshold of 60% with either a 7Be threshold of 5.5 mBq m−3 or the requirement for RH≤30% within ±6 h, rather reliably predict periods of deep intrusions reaching the Zugspitze station. An "or" combination of both these criteria yields slightly more cases and covers 77.9% of the intrusions identified. The lack of observations in the complementary 22.1% are mostly explained by overpasses. In this way the long-term trend of stratospheric ozone observed at this site as well as the corresponding ozone budget may be derived on the basis of measurements only. This effort will be the subject of a subsequent publication.
Comparison of in situ and columnar aerosol spectral measurements during TexAQS-GoMACCS 2006: testing parameterizations for estimating aerosol fine mode properties
Authors: Atkinson D. B. , Massoli P. , O'Neill N. T. , Quinn P. K. , Brooks S. D. , Lefer B.
Published: April 2010

Abstract. During the 2006 Texas Air Quality Study and Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS-GoMACCS 2006), the optical, chemical and microphysical properties of atmospheric aerosols were measured on multiple mobile platforms and at ground based stations. In situ measurements of the aerosol light extinction coefficient (σep) were performed by two multi-wavelength cavity ring-down (CRD) instruments, one located on board the NOAA R/V Ronald H. Brown (RHB) and the other located at the University of Houston, Moody Tower (UHMT). An AERONET sunphotometer was also located at the UHMT to measure the columnar aerosol optical depth (AOD). The σep data were used to extract the extinction Ångström exponent (åep), a measure of the wavelength dependence of σep. There was general agreement between the åep (and to a lesser degree σep) measurements by the two spatially separated CRD instruments during multi-day periods, suggesting a regional scale consistency of the sampled aerosols. Two spectral models are applied to the σep and AOD data to extract the fine mode fraction of extinction (η) and the fine mode effective radius (Reff,f). These two parameters are robust measures of the fine mode contribution to total extinction and the fine mode size distribution, respectively. The results of the analysis are compared to Reff,f values extracted using AERONET V2 retrievals and calculated from in situ particle size measurements on the RHB and at UHMT. During a time period when fine mode aerosols dominated the extinction over a large area extending from Houston/Galveston Bay and out into the Gulf of Mexico, the various methods for obtaining Reff,f agree qualitatively (showing the same temporal trend) and quantitatively (pooled standard deviation = 28 nm).