This study presents the new aerosol assimilation system, developed at the European Centre for Medium‐Range Weather Forecasts, for the Global and regional Earth‐system Monitoring using Satellite and in‐situ data (GEMS) project. The aerosol modeling and analysis system is fully integrated in the operational four‐dimensional assimilation apparatus. Its purpose is to produce aerosol forecasts and reanalyses of aerosol fields using optical depth data from satellite sensors. This paper is the second of a series which describes the GEMS aerosol effort. It focuses on the theoretical architecture and practical implementation of the aerosol assimilation system. It also provides a discussion of the background errors and observations errors for the aerosol fields, and presents a subset of results from the 2‐year reanalysis which has been run for 2003 and 2004 using data from the Moderate Resolution Imaging Spectroradiometer on the Aqua and Terra satellites. Independent data sets are used to show that despite some compromises that have been made for feasibility reasons in regards to the choice of control variable and error characteristics, the analysis is very skillful in drawing to the observations and in improving the forecasts of aerosol optical depth.

The aerosol light absorption coefficient is an essential parameter involved in atmospheric radiation budget calculations. The Aethalometer (AE) has the great advantage of measuring the aerosol light absorption coefficient at several wavelengths, but the derived absorption coefficients are systematically too high when compared to reference methods. Up to now, four different correction algorithms of the AE absorption coefficients have been proposed by several authors. A new correction scheme based on these previously published methods has been developed, which accounts for the optical properties of the aerosol particles embedded in the filter. All the corrections have been tested on six datasets representing different aerosol types and loadings and include multi-wavelength AE and white-light AE. All the corrections have also been evaluated through comparison with a Multi-Angle Absorption Photometer (MAAP) for four datasets lasting between 6 months and five years. The modification of the wavelength dependence by the different corrections is analyzed in detail. The performances and the limits of all AE corrections are determined and recommendations are given

Abstract. Sulphuric acid and organic vapours have been identified as the key components in the ubiquitous secondary new particle formation in the atmosphere. In order to assess their relative contribution and spatial variability, we analysed altogether 36 new particle formation events observed at four European measurement sites during EUCAARI campaigns in [2007][2008][2009]. We tested models of several different nucleation mechanisms coupling the formation rate of neutral particles (J ) with the concentration of sulphuric acid ([H 2 SO 4 ]) or low-volatility organic vapours ([org]) condensing on sub-4 nm particles, or with a combination of both concentrations. Furthermore, we determined the related nucleation coefficients connecting the neutral nucleation rate J with the vapour concentrations in each mechanism. The main goal of the study was to identify the mechanism of new particle formation and subsequent growth that minimizes the difference between the modelled and measured nucleation rates. At three out of four measurement sites -Hyytiälä (Finland), Melpitz (Germany) and San Pietro Capofiume (Italy) -the nucleation rate was closely connected to squared sulphuric acid concentration, whereas in Hohenpeissenberg (Germany) the low-volatility organic vapours were observed Correspondence to: P. Paasonen (pauli.paasonen@helsinki.fi) to be dominant. However, the nucleation rate at the sulphuric acid dominant sites could not be described with sulphuric acid concentration and a single value of the nucleation coefficient, as K in J =K [H 2 SO 4 ] 2 , but the median coefficients for different sites varied over an order of magnitude. This inter-site variation was substantially smaller when the heteromolecular homogenous nucleation between H 2 SO 4 and organic vapours was assumed to take place in addition to homogenous nucleation of H 2 SO 4 alone, i.e., In general, our results suggest that organic vapours do play a role, not only in the condensational growth of the particles, but also in the nucleation process, with a site-specific degree.

Abstract.Two years of harmonized aerosol number size distribution data from 24 European field monitoring sites have been analysed. The results give a comprehensive overview of the European near surface aerosol particle number concentrations Correspondence to: A. Asmi (ari.asmi@helsinki.fi) and number size distributions between 30 and 500 nm of dry particle diameter. Spatial and temporal distribution of aerosols in the particle sizes most important for climate applications are presented. We also analyse the annual, weekly and diurnal cycles of the aerosol number concentrations, provide log-normal fitting parameters for median number size distributions, and give guidance notes for data users. Emphasis is placed on the usability of results within the aerosol modelling community.Published by Copernicus Publications on behalf of the European Geosciences Union. We also show that the aerosol number concentrations of Aitken and accumulation mode particles (with 100 nm dry diameter as a cut-off between modes) are related, although there is significant variation in the ratios of the modal number concentrations. Different aerosol and station types are distinguished from this data and this methodology has potential for further categorization of stations aerosol number size distribution types.The European submicron aerosol was divided into characteristic types: Central European aerosol, characterized by single mode median size distributions, unimodal number concentration histograms and low variability in CCN-sized aerosol number concentrations; Nordic aerosol with low number concentrations, although showing pronounced seasonal variation of especially Aitken mode particles; Mountain sites (altitude over 1000 m a.s.l.) with a strong seasonal cycle in aerosol number concentrations, high variability, and very low median number concentrations. Southern and Western European regions had fewer stations, which decreases the regional coverage of these results. Aerosol number concentrations over the Britain and Ireland had very high variance and there are indications of mixed air masses from several source regions; the Mediterranean aerosol exhibit high seasonality, and a strong accumulation mode in the summer. The greatest concentrations were observed at the Ispra station in Northern Italy with high accumulation mode number concentrations in the winter. The aerosol number concentrations at the Arctic station Zeppelin in Ny-Ålesund in Svalbard have also a strong seasonal cycle, with greater concentrations of accumulation mode particles in winter, and dominating summer Aitken mode indicating more recently formed particles. Observed particles did not show any statistically significant regional work-week or weekday related variation in number concentrations studied.Analysis products are made for open-access to the research community, available in a freely accessible internet site. The results give to the modelling community a reliable, easy-touse and freely available comparison dataset of aerosol size distributions.

Abstract. Volcanic emissions from the Eyjafjallajökull vol-

Long-term observation of the trace gases NOa, ozone, and nitrate radicals (NO3) at a rural site in the Baltic Sea (near Kap Arkona, island Riigen) using differential optical absorption spectroscopy (DOAS) are presented here. The time series of these species cover more than 14 months of data. Additionally, meteorological data were taken from the Deutscher Wetterdienst station at Kap Arkona. The average nighttime NO3 concentration was determined at 7.8 ppt. Maximum lifetimes of more than 5000 s were observed, but average NO3 lifetimes are close to 250 s. As nitrate radicals are expected to play an important role in the nonphotochemical conversion of NOx to HN03, long-term observations offer a key for investigation of possible sink mechanisms of NO3 molecules mediated by both homogeneous and heterogeneous reactions. Analysis of the data shows that the NO3 lifetime is inversely proportional to the NOa concentration, thus suggesting that the removal of N20s is actually the dominating NO3 loss process. The mean N205 concentration from this long-term measurement was calculated at 1 ppb with an N20s lifetime between 1000 s and 20000 s covering 90 • of the data. Platt et al., [1980] and Nozon et al., [1980], nitrate radicals are being recognized as key intermediates in a growing list of important chemical processes in the atmosphere (e.g., see Figure 1). It became clear that the NO3 radical plays an important role in the nonphotochemical conversion of NOx to HNOa via heterogeneous hydrolysis of N205 formed in the NOa-NO2combination [Platt et al., 1979; Ehhalt et al., 1982; Richards, 1983; Heikes and Thompson, 1983; Calvert and Stockwell, 1983; Jones et al. 1983; Canttell et al., 1985; Platt et al., 1985,' Russel et al., 1985; 1986; Parrish et al., 1993; Dentenet and Crutzen, 1993]. In addition, NO3 is a strong oxidizing agent and initiates the nighttime degradation of many volatile organic com-Copyright 1996 by the Aanerican Geophysical Union. Paper number 96JD01549. 0148-0227/96/96JD-01549509.00 pounds (VOC), including olefins, terpenes, isoprene, dimethylsulphide (DMS), aromatic hydrocarbons, or organic sulphur compounds [Winer et al., 198•; Atkinson, 1990; Wayne et al., 1991]. Also, the formation of significant concentrations of peroxy radicals (HO2 and higher RO2) during nighttime can be initiated by NO3 reactions, even OH can be produced [Platt et al., 1990; LeBras et al., 1993]. Owing to the addition of NO3 to the C-C double bond of alkenes, and via formation of peroxiacetylnitrate-(PAN) type species a contribution to the formation of organic nitrates is assumed to occur during nighttime [Roberts, 1990; Canttell et al., 1986; Atkinson, 1990; Ridley et al., 1992]. Furthermore, liberation of highly reactive Br-and Cl-containing species from sea salt is possible. In particular, formation of BrNO2 and C1NO2 by reaction of N205 with NaBr or NaC1, ro,qpect, ive!y, cont, a.inod in sea salt aerosol particles is assumed to be of interest [Finla•lson-Pitts et al., 1989; 1990; Behnke et al., 1991; 1993]. Many more details of t...

At three successive days at the end of January 2000 the Deutsches Zentrum für Luft‐ und Raumfahrt (DLR) airborne lidar Ozone Lidar Experiment explored mountain‐wave‐induced polar stratospheric clouds above the Scandinavian mountain ridge. Global analyses and mesoscale modeling are applied to explain their complex internal structure and their day‐to‐day variability. Depending on the synoptical‐scale meteorological conditions, stratospheric temperature anomalies of different amplitude and horizontal extent are generated by the upward propagating mountain waves. Short‐term excitation of about 6 hours resulted in localized stratospheric temperature anomalies directly above the mountain ridge as for 25 January 2000. In this case, the elevation of the observed clouds differed not much from the synoptic‐scale clouds upstream above the Norwegian Sea. On the other hand, long‐lasting flow past the Scandinavian mountain ridge formed huge 400‐km horizontally extending stratospheric ice clouds in altitudes as much as 5 km above the elevation of the upstream clouds just 1 day later. Inertia gravity waves with horizontal wavelengths of about 350 km are responsible for their formation. For the first time a predicted temperature minimum far downstream of the mountains could be proofed by the observation of an isolated stratospheric ice cloud above Finland. The observed particles are classified in terms of their measured optical properties such as backscatter ratio and depolarization. In all cases, mountain waves generated ice clouds. In contrast to the nitric acid trihydrate tail of the ice cloud on 25 January the same classification results in a tail of liquid supercooled ternary solutions droplets 1 day later. The particle structure downstream of the mountains is very complex and needs detailed microphyical modeling and interpretation.

Abstract. The implementation and application of a newly developed coupled system combining ECMWF's integrated forecast system (IFS) with global chemical transport models (CTMs) is presented. The main objective of the coupled system is to enable the IFS to simulate key chemical species without the necessity to invert the complex source and sink processes such as chemical reactions, emission and deposition. Thus satellite observations of atmospheric composition can be assimilated into the IFS using its 4D-VAR algorithm. In the coupled system, the IFS simulates only the transport of chemical species. The coupled CTM provides to the IFS the concentration tendencies due to emission injection, deposition and chemical conversion. The CTMs maintain their own transport schemes and are fed with meteorological data at hourly resolution from the IFS. The CTM used in the coupled system can be either MOZART-3, TM5 or MOCAGE. The coupling is achieved via the special-purpose software OA-SIS4. The scientific integrity of the coupled system is proven by analysing the difference between stand-alone CTM simulations and the tracer fields in the coupled IFS. The IFS concentration fields match the CTM fields for about 48 h with the biggest differences occurring in the planetary boundary layer (PBL). The coupled system is a good test bed for processoriented comparison of the coupled CTM. As an example, the vertical structure of chemical conversion and emission injection is studied for a ten day period over Central Europe for the three CTMs.