A recent study has analysed the physical and chemical characteristics of particulate matter (PM) from 60 sites across Europe. The results suggest there is no single ratio between PM2.5 and PM10 mass concentrations for all the sites, and that PM mass cannot be directly related to the concentration of particle numbers. Particulate matter can negatively affect human health, alter ecosystems, reduce visibility and influence climate change. PM10 mass concentrations have been monitored for two decades and PM2.5 measurements have been included more recently as a result of the European Directive1 on ambient air quality and cleaner air for Europe. This sets limit values for PM2.5 concentration by 2015 and the target for exposure reduction between 2010 and 2020. In order to ensure that legislation to reduce PM is effective, a thorough knowledge of the aerosol characteristics is needed in addition to monitoring PM mass concentrations. Conducted within the framework of the EU-funded COST initiative, the researchers categorised PM2.5 and PM10 mass concentrations, as well as the number of particles and aerosol compositions, according to whether the sampling sites were from remote rural backgrounds (where the distance from large pollution sources is greater than 50 km, as compared to rural backgrounds where the distance from large pollution sources must be larger than 10 km), near-city, urban, industrial or kerbside backgrounds from one of three regions: North-western, Southern or Central Europe. A wide range of PM10 concentrations (5-54 �g/m3 (10-6 of a gram per cubic metre)) was observed across Europe. Measurements were representative for a site during at least a season (ie minimum 6 weeks of continuous measurements). Although there were exceptions, PM10 concentrations generally increased from remote rural background through to kerbside sites in all three regions. A similar range was observed for PM2.5 concentrations (3-35 �g/m3), with general increases from remote rural to urban background sites in NW and Southern Europe. However, in Central Europe, PM2.5 levels were comparable in rural and urban sites and concentrations at kerbsides were not particularly higher compared with urban sites. Furthermore, the study suggests PM2.5 concentrations cannot be predicted from PM10 concentrations and there is not a clear relationship between the ratio of PM2.5 to PM10 values and the type and location of the site. In all three regions, particle numbers of PM2.5 generally increased from remote rural background to kerbside sites, excluding one kerbside site. However, at none of the sites did the total particle number of PM2.5 increase in direct proportion to the PM mass concentration. Most of the PM particles are ultrafine in size (diameter less than 100 nm (10-9 of a metre)) and the numbers of ultrafine particles does not generally increase with PM2.5 levels. Different processes are responsible for producing ultrafine particles which influence total particle numbers and for producing larger particles which control PM mass. On average, there is more organic matter in PM10 in Central Europe and more mineral dust in southern Europe. In addition, sea salt percentages are smaller in Central Europe and larger in NW Europe. Sulphate and nitrate contributions to PM10 do not vary much across the regions but generally decrease from rural to kerbside sites. Total carbon generally increases from rural to kerbside sites. The contribution of nitrates to PM10 and PM2.5 increases with PM mass concentrations in almost every urban site and some other locations. In order to limit the number of times the daily PM10 value is exceeded, the researchers suggest particular efforts should be made to reduce NOx (nitrogen oxides) emissions.