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Abstract. To develop mitigation strategies for reducing concentrations of both PM 2.5 and PM 10 , the origin of particulate matter (PM) needs to be established. An intensive, one-year measurement campaign from August 2007 to August 2008 was carried out to determine the composition of PM 10 and PM 2.5 at five locations in the Netherlands, aiming at reducing the uncertainties on the origin of PM. Generally, a considerable conformity in the chemical composition of PM 2.5 (and PM 10 ) is observed. From all constituents present in PM 2.5 , the secondary inorganic aerosol is the most dominant (42-48%), followed by the total carbonaceous matter (22-37%). Contributions from sea salt (maximum 8%), mineral dust and metals (maximum 5%) are relatively low. For the first time, a detailed overview of the composition of the coarse fraction can be presented. Compared to the fine fraction, contributions of sea salt, mineral dust and metals are larger resulting in a more balanced distribution between the various constituents. Through mass closure a considerable part of the PM mass could be defined (PM 2.5 : 80-94%). The chemical distribution on days with high PM levels shows a distinct increase in nitrate as well as in the unaccounted mass. Contributions of the other constituents remain equal or are lower (sea salt) when expressed in percentages. A correspondence between nitrate and the unaccounted mass is observed hinting at the presence of water on the filters. The contribution from natural sources in the Netherlands (at a rural station) was estimated to be 19 to 24% for PM 10 and 13 to 17% for PM 2.5 .

Abstract. Assessment of the effects of ozone depletion on biologically effective solar UV at ground level has been greatly advanced through the use of remote sensing data. Satellite data on atmospheric properties allow the construction of geographically distributed surface UV radiation maps based on radiative transfer calculations. In this respect, clouds play a dominant but rather complex role. We compared the reduction of daily UV doses due to clouds, as derived from satellite cloud data, with the reduction derived from routine ground-based measurements of global solar radiation (i.e., broadband total solar irradiances with wavelengths between 0.3 and 2.8 •m). An empirical relationship is used to link the reduction due to clouds of global solar radiation and UV radiation. The abundance of global solar radiation measurements (data from over 125 stations in 30 satellite grid cells) for the European region ensured a sound basis for the data analysis for the period considered (May, June, and July of 1990, 1991, and 1992). Approximately 6500 daily UV-reduction factors, defined as the ratio of daily UV doses calculated with and without clouds, were thus obtained applying both methods. The daily UV-reduction factors (and 10-day averaged UV reduction factors) from the two independent sources correlated well, with r 2 = 0.83 (r 2 = 0.89), and had a standard deviation of 0.06 (0.03). Over 90% of the satellite-derived results agreed within a range of +_0.14 (+_0.07) with the ground-based measurement-derived results. We evaluated sources of uncertainty related to spatial and temporal resolution, and optical properties, and estimated their consequences and range. Among these different sources the largest uncertainties are caused by the sampling error, i.e., grid-cell average versus station average, which is on average 0.10 for daily UV-reduction factors. Information on the atmospheric optical properties during the measurements may reduce the stated range of uncertainty from +_0.14 to _+0.07. The variation of the measurements from station to station is then the limiting factor. We concluded that the reduction of daily UV based on satellite-derived cloud cover and cloud optical thickness relates well with the UV reduction due to clouds derived from ground-based global solar radiation measurements. [e.g., Slaper et al., 1996, 1998]. 5069 Introduction Stratospheric ozone is the major atmospheric absorber of solar UV-B (280-315 nm). For this reason the decrease in ozone columns observed in the last few decades is expected to lead to increases in the UV

[1] Four different satellite-UV mapping methods are assessed by comparing them against ground-based measurements. The study includes most of the variability found in geographical, meteorological and atmospheric conditions. Three of the methods did not show any significant systematic bias, except during snow cover. The mean difference (bias) in daily doses for the Rijksinstituut voor Volksgezondheid en Milieu (RIVM) and Joint Research Centre (JRC) methods was found to be less than 10% with a RMS difference of the order of 30%. The Deutsches Zentrum für Luft-und Raumfahrt (DLR) method was assessed for a few selected months, and the accuracy was similar to the RIVM and JRC methods. It was additionally used to demonstrate how spatial averaging of high-resolution cloud data improves the estimation of UV daily doses. For the Institut d'Aéronomie Spatiale de Belgique (IASB) method the differences were somewhat higher, because of their original cloud algorithm. The mean difference in daily doses for IASB was about 30% or more, depending on the station, while the RMS difference was about 60%. The cloud algorithm of IASB has been replaced recently, and as a result the accuracy of the IASB method has improved. Evidence is found that further research and development should focus on the improvement of the cloud parameterization. Estimation of daily exposures is likely to be improved if additional time-resolved cloudiness information is available for the satellite-based methods. It is also demonstrated that further development work should be carried out on the treatment of albedo of snow-covered surfaces.

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