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This paper synthesizes data on aerosol (particulate matter, PM) physical characteristics, which were obtained in European aerosol research activities at free- troposphere, natural, rural, near-city, urban, and kerbside sites over the past decade. It covers only two sites in the semi-arid Mediterranean area, and lacks data from Eastern Europe. The data include PM10 and/or PM2.5 mass concentrations, and aerosol particle size distributions. Such data sets are more comprehensive than those currently provided by air quality monitoring networks (e.g. EMEP, EUROAIRNET). Data available from 31 sites in Europe (called "The Network") were reviewed. They were processed and plotted to allow comparisons in spite of differences in the sampling and analytical techniques used in various studies. A number of conclusions are drawn as follows:

Background annual average PM10 and PM2.5 mass concentrations for continental Europe are 7.0+/-4.1 and 4.8+/-2.4 mug m(-3), respectively.

The EU 2005 annual average PM10 standard of 40 mug m(-3) is exceeded at a few sites in The Network. At all near city, urban and kerbside sites, the EU 2010 annual average PM10 standard of 20 mug m(-3), as well as the US-EPA annual average PM2.5 standard of 15 mug m(-3) are exceeded. In certain regions, PM10 and PM2.5 in cities are strongly affected by the regional aerosol background.

There is no "universal" (i.e. valid for all sites) ratio between PM2.5 and PM10 mass concentrations, although fairly constant ratios do exist at individual sites. There is no universal correlation between PM mass concentration on the one hand, and total particle number concentration on the other hand, although a ’baseline’ ratio between number and mass is found for sites not affected by local emissions. This paper is the first part of two companion papers of which the second part describes chemical characteristics. (C) 2004 Elsevier Ltd. All rights reserved.