Ultrafine particles have attracted interest as perhaps the most dangerous fraction of atmospheric PM. This study focuses on the characterization of ultrafine particulate matter (PM_0.1) mass concentrations and their chemical composition during a summer and winter period in Europe.</p> <p>Predicted levels of PM_0.1 varied substantially, both in space and in time. The average predicted PM_0.1mass concentration was 0.6 &mu;g m^-3 in the summer, higher than the 0.3 &mu;g m^-3 predicted in the winter period. PM_0.1 chemical composition exhibited significant seasonality. In summer, PM_0.1 was mostly comprised of secondary inorganic matter (38 % sulfate and 13 % ammonium) and organics (9 % primary and 32 % secondary). During the winter, the fraction of secondary inorganic matter increased, with sulfate contributing 47 % and ammonium 19 %, on average. Primary organic matter contribution also increased from 9 % in summer to 23 % in winter, while secondary organic matter decreased significantly to 6 % on average during winter.</p> <p>During summertime, the model performance at 12 sites for daily average ultrafine particle volume (PV_0.1) concentrations was considered good, with normalized mean error (NME) equal to 46 % and normalized mean bias (NMB) equal to 15 %. For the winter period, the corresponding values for daily average levels were -27 % for NMB and 64 % for NME, indicating an average model performance.</p> <p>Correlations between PM_0.1 and the currently regulated PM_2.5were generally low. Better correlations were observed in cases where the primary component of PM_0.1 was significant. This suggests that there are significant differences between the dominant sources and processes of PM_0.1and PM_2.5.