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Many of the next generation of global climate models will include aerosol schemes which explicitly simulate the microphysical processes that determine the particle size distribution. These models enable aerosol optical properties and cloud condensation nuclei (CCN) concentrations to be determined by fundamental aerosol processes, which should lead to a more physically based simulation of aerosol direct and indirect radiative forcings. This study examines the global variation in particle size distribution simulated by 12 global aerosol microphysics models to quantify model diversity and to identify any common biases against observations. Evaluation against size distribution measurements from a new European network of aerosol supersites shows that the mean model agrees quite well with the observations at many sites on the annual mean, but there are some seasonal biases common to many sites. In particular, at many of these European sites, the accumulation mode number concentration is biased low during winter and Aitken mode concentrations tend to be overestimated in winter and underestimated in summer. At high northern latitudes, the models strongly underpredict Aitken and accumulation particle concentrations compared to the measurements, consistent with previous studies that have highlighted the poor performance of global aerosol models in the Arctic. In the marine boundary layer, the models capture the observed meridional variation in the size distribution, which is dominated by the Aitken mode at high latitudes, with an increasing concentration of accumulation particles with decreasing latitude. Considering vertical profiles, the models reproduce the observed peak in total particle concentrations in the upper troposphere due to new particle formation, although modelled peak concentrations tend to be biased high over Europe. Overall, the multimodel-mean data set simulates the global variation of the particle size distribution with a good degree of skill, suggesting t, AuthorCount:52

Cluster analysis of particle number size distributions from background sites across Europe is presented. This generated a total of nine clusters of particle size distributions which could be further combined into two main groups, namely: a south-to-north category (four clusters) and a west-to-east category (five clusters). The first group was identified as most frequently being detected inside and around northern Germany and neighbouring countries, showing clear evidence of local afternoon nucleation and growth events that could be linked to movement of air masses from south to north arriving ultimately at the Arctic contributing to Arctic haze. The second group of particle size spectra proved to have narrower size distributions and collectively showed a dependence of modal diameter upon the longitude of the site (west to east) at which they were most frequently detected. These clusters indicated regional nucleation (at the coastal sites) growing to larger modes further inland. The apparent growth rate of the modal diameter was around 0.6-0.9 nm h(-1). Four specific air mass back-trajectories were successively taken as case studies to examine in real time the evolution of aerosol size distributions across Europe. While aerosol growth processes can be observed as aerosol traverses Europe, the processes are often obscured by the addition of aerosol by emissions en route. This study revealed that some of the 24 stations exhibit more complex behaviour than others, especially when impacted by local sources or a variety of different air masses. Overall, the aerosol size distribution clustering analysis greatly simplifies the complex data set and allows a description of aerosol aging processes, which reflects the longer-term average development of particle number size distributions as air masses advect across Europe., AuthorCount:39

The atmospheric concentration of elemental carbon (EC) in Europe during the six-year period 2005-2010 has been simulated with the EMEP MSC-W model. The model bias compared to EC measurements was less than 20% for most of the examined sites. The model results suggest that fossil fuel combustion is the dominant source of EC in most of Europe but that there are important contributions also from residential wood burning during the cold seasons and, during certain episodes, also from open biomass burning (wildfires and agricultural fires). The modelled contributions from open biomass fires to ground level concentrations of EC were small at the sites included in the present study, <3% of the long-term average of EC in PM10. The modelling of this EC source is subject to many uncertainties, and it was likely underestimated for some episodes. EC measurements and modelled EC were also compared to optical measurements of black carbon (BC). The relationships between EC and BC (as given by mass absorption cross section, MAC, values) differed widely between the sites, and the correlation between observed EC and BC is sometimes poor, making it difficult to compare results using the two techniques and limiting the comparability of BC measurements to model EC results. A new bottom-up emission inventory for carbonaceous aerosol from residential wood combustion has been applied. For some countries the new inventory has substantially different EC emissions compared to earlier estimates. For northern Europe the most significant changes are much lower emissions in Norway and higher emissions in neighbouring Sweden and Finland. For Norway and Sweden, comparisons to source-apportionment data from winter campaigns indicate that the new inventory may improve model-calculated EC from wood burning. Finally, three different model setups were tested with variable atmospheric lifetimes of EC in order to evaluate the model sensitivity to the assumptions regarding hygroscopicity and atmospheric

Reactive nitrogen (Nr) compounds have different fates in the atmosphere due to differences in the governing processes of physical transport, deposition and chemical transformation. Nr compounds addressed here include reduced nitrogen (NHx: ammonia (NH3) and its reaction product ammonium (NH4+)), oxidized nitrogen (NOy: nitrogen monoxide (NO) + nitrogen dioxide (NO2) and their reaction products) as well as organic nitrogen compounds (organic N). Pollution abatement strategies need to take into account the differences in the governing processes of these compounds when assessing their impact on ecosystem services, biodiversity, human health and climate. NOx (NO &plus; NO2) emitted from traffic affects human health in urban areas where the presence of buildings increases the residence time in streets. In urban areas this leads to enhanced exposure of the population to NOx concentrations. NOx emissions generally have little impact on nearby ecosystems because of the small dry deposition rates of NOx. These compounds need to be converted into nitric acid (HNO3) before removal through deposition is efficient. HNO3 sticks quickly to any surface and is thereby either dry deposited or incorporated into aerosols as nitrate (NO3−). In contrast to NOx compounds, NH3 has potentially high impacts on ecosystems near the main agricultural sources of NH3 because of its large ground-level concentrations along with large dry deposition rates. Aerosol phase NH4&plus; and NO3− contribute significantly to background PM2.5 and PM10 (mass of aerosols with an aerodynamic diameter of less than 2.5 and 10 μm, respectively) with an impact on radiation balance as well as potentially on human health. Little is known quantitatively and qualitatively about organic N in the atmosphere, other than that it contributes a significant fraction of wet-deposited N, and is present in both gaseous and particulate forms. Further studies are needed to characterise the sources, air chemistry and removal rates o

The REgents PARk and Tower Environmental Experiment (REPARTEE) comprised two campaigns in London in October 2006 and October/November 2007. The experiment design involved measurements at a heavily trafficked roadside site, two urban background sites and an elevated site at 160–190 m above ground on the BT Tower, supplemented in the second campaign by Doppler lidar measurements of atmospheric vertical structure. A wide range of measurements of airborne particle physical metrics and chemical composition were made as well as measurements of a considerable range of gas phase species and the fluxes of both particulate and gas phase substances. Significant findings include (a) demonstration of the evaporation of traffic-generated nanoparticles during both horizontal and vertical atmospheric transport; (b) generation of a large base of information on the fluxes of nanoparticles, accumulation mode particles and specific chemical components of the aerosol and a range of gas phase species, as well as the elucidation of key processes and comparison with emissions inventories; (c) quantification of vertical gradients in selected aerosol and trace gas species which has demonstrated the important role of regional transport in influencing concentrations of sulphate, nitrate and secondary organic compounds within the atmosphere of London; (d) generation of new data on the atmospheric structure and turbulence above London, including the estimation of mixed layer depths; (e) provision of new data on trace gas dispersion in the urban atmosphere through the release of purposeful tracers; (f) the determination of spatial differences in aerosol particle size distributions and their interpretation in terms of sources and physico-chemical transformations; (g) studies of the nocturnal oxidation of nitrogen oxides and of the diurnal behaviour of nitrate aerosol in the urban atmosphere, and (h) new information on the chemical composition and source apportionment of particulate matter size fra

Two years of harmonized aerosol number size distribution data from 24 = ropean field monitoring sites have been analysed. The results give a = mprehensive overview of the European near surface aerosol particle = mber concentrations and number size distributions between 30 and 500 = of dry particle diameter. Spatial and temporal distribution of = rosols in the particle sizes most important for climate applications = e presented. We also analyse the annual, weekly and diurnal cycles of = e aerosol number concentrations, provide log-normal fitting parameters = r median number size distributions, and give guidance notes for data = ers. Emphasis is placed on the usability of results within the aerosol = delling community., authorCount :37

This paper synthesizes data on aerosol (particulate matter, PM) physical and chemical characteristics, which were obtained over the past decade in aerosol research and monitoring activities at more than 60 natural background, rural, near-city, urban, and kerbside sites across Europe. The data include simultaneously measured PM10 and/or PM2.5 mass on the one hand, and aerosol particle number concentrations or PM chemistry on the other hand. The aerosol data presented in our previous works (Van Dingenen et al., 2004; Putaud et al., 2004) were updated and merged to those collected in the framework of the EU supported European Cooperation in the field of Scientific and Technical action COST633 (Particulate matter: Properties related to health effects). A number of conclusions from our previous studies were confirmed. There is no single ratio between PM2.5 and PM10 mass concentrations valid for all sites, although fairly constant ratios ranging from 0.5 to 0.9 are observed at most individual sites. There is no general correlation between PM mass and particle number concentrations, although particle number concentrations increase with PM2.5 levels at most sites. The main constituents of both PM10 and PM2.5 are generally organic matter, sulfate and nitrate. Mineral dust can also be a major constituent of PM10 at kerbside sites and in Southern Europe. There is a clear decreasing gradient in SO42- and NO3- contribution to PM10 when moving from rural to urban to kerbside sites. In contrast, the total carbon/PM10 ratio increases from rural to kerbside sites. Some new conclusions were also drawn from this work: the ratio between ultrafine particle and total particle number concentration decreases with PM2.5 concentration at all sites but one, and significant gradients in PM chemistry are observed when moving from Northwestern, to Southern to Central Europe. Compiling an even larger number of data sets would have further increased the significance of our conclusions, but collect, authorCount :39

We synthesised observations of total particle number (CN) concentration from 36 sites around the world. We found that annual mean CN concentrations are typically 300-2000 cm(-3) in the marine boundary layer and free troposphere (FT) and 1000-10 000 cm(-3) in the continental boundary layer (BL). Many sites exhibit pronounced seasonality with summer time concentrations a factor of 2-10 greater than wintertime concentrations. We used these CN observations to evaluate primary and secondary sources of particle number in a global aerosol microphysics model. We found that emissions of primary particles can reasonably reproduce the spatial pattern of observed CN concentration (R-2=0.46) but fail to explain the observed seasonal cycle (R-2=0.1). The modeled CN concentration in the FT was biased low (normalised mean bias, NMB=-88%) unless a secondary source of particles was included, for example from binary homogeneous nucleation of sulfuric acid and water (NMB=-25%). Simulated CN concentrations in the continental BL were also biased low (NMB=-74%) unless the number emission of anthropogenic primary particles was increased or a mechanism that results in particle formation in the BL was included. We ran a number of simulations where we included an empirical BL nucleation mechanism either using the activation-type mechanism (nucleation rate, J, proportional to gas-phase sulfuric acid concentration to the power one) or kinetic-type mechanism (J proportional to sulfuric acid to the power two) with a range of nucleation coefficients. We found that the seasonal CN cycle observed at continental BL sites was better simulated by BL particle formation (R-2=0.3) than by increasing the number emission from primary anthropogenic sources (R-2=0.18). The nucleation constants that resulted in best overall match between model and observed CN concentrations were consistent with values derived in previous studies from detailed case studies at individual sites. In our model, kinetic and activati, authorCount :34

Nitrate makes a significant contribution to the concentration of PM2.5 and PM10 with important implications for human health and regional climate forcing. This is particularly true for NW Europe, where large emissions of NH3 and NOx combine with comparably low temperatures and high relative humidities to create conditions that favour the production of NH4NO3. Despite its importance, most atmospheric chemistry models are still not modelling NH4NO3 very well, indicating that dynamics are still not represented correctly. In addition, due to its changing equilibrium, there are processes which operated at a sub-grid scale and are therefore difficult to simulate. In this presentation we review new evidence on nitrate dynamics in urban environments, drawing on data from recent urban field studies, mainly, but not exclusively, in the UK. This new look is enabled through the use of new measurement technology such as Aerosol Mass Spectrometry (AMS), urban eddy-covariance flux measurements of aerosol chemical compounds and long-term nitrate measurements. We quantify the relative contribution NH4NO3 to the UK aerosol, reviewing the existing UK AMS database and data from UK and European denuder/filter-pack networks. Paired long-term measurements of aerosol concentrations in and outside of two UK urban areas (London & Edinburgh; Tang et al., 2008), have provided information on the urban NO3- increment. The measurements indicate that, on average: NO3-(urban) = 1.13 × NO3-(rural) + 0.58 μg m-3, with somewhat larger increments during the winter months than during summer. Measurements of the size-distributions of sub-micron non-refractory NO3- by aerosol mass spectrometry frequently show periods of a NO3- size mode in the range 100 to 300 nm, in addition to the accumulation mode at 300 to 800 nm. This suggests that NO3- is formed by condensation on the combustion mode which is prevalent near traffic sources. The fine NO3- mode is sporadically observed, and appears to correlate with c

Concentrations of dissolved iron (DFe) and Fe-binding ligands were determined in the tropical Northeast Atlantic Ocean (12-30°N, 21-29°W) as part of the UK-SOLAS (Surface Ocean Lower Atmosphere Study) cruise Poseidon 332 (P332) in January-February 2006. The surface water DFe concentrations varied between 0.1 and 0.4 nM with an average of 0.22 ± 0.05 nM (n = 159). The surface water concentrations of total Fe-binding ligands varied between 0.82 and 1.46 nM with an average of 1.11 ± 0.14 nM (n = 33). The concentration of uncomplexed Fe-binding ligands varied between 0.64 and 1.35 nM with an average of 0.90 ± 0.14 nM (n = 33). Thus, on average 81 of the total Fe-binding ligand concentration was uncomplexed. The average logarithmic conditional stability constant of the pool of Fe-binding ligands was 22.85 ± 0.38 with respect to Fe 3+ (n = 33). A transect (12°N, 26°W to 16°N, 25.3°W) was sailed during a small Saharan dust event and repeated a week later. Following the dust event, the concentration of DFe increased from 0.20 ± 0.026 nM (n = 125) to 0.25 ± 0.028 (n = 17) and the concentration of free Fe-binding ligands decreased from 1.15 ± 0.15 (n = 4) to 0.89 ± 0.10 (n = 4) nM. Furthermore, the logarithmic stability constants of the Fe-binding ligands south of the Cape Verde islands were distinctively lower than north of the islands. The absence of a change in the logarithmic stability constant after the dust event south of the Cape Verde islands suggests that there was no significant atmospheric input of new Fe-binding ligands during this dust event.