Atmospheric particle number size distributions determined over 1.5 years at a central European site were statistically analyzed in terms of their relation to time of day, season, meteorology, and synoptic-scale air masses. All size distributions were decomposed into lognormal particle modes corresponding to the accumulation, Aitken, aged nucleation, and nucleation modes. The concentration of nucleation mode particles (30 nm) lacked such diurnal behavior, and proved to be indicative of different synoptic-scale air mass types. Over 70% of the time, air masses of Atlantic origin and maritime character prevailed, showing obvious signs of anthropogenic influence most of the time (accumulation mode: 500 cm-3; Aitken mode: 2300 cm-3). During a limited period of time (10%), however, continentally aged air with significantly enhanced concentrations of aerosol was observed (accumulation mode: 1200 cm-3; Aitken mode: 3300 cm-3). These air masses were advected from source regions in Russia, and eastern, southeastern, and central Europe, mainly under anticyclonic and high-pressure influence. The analysis provides a refined picture of the behavior of the particle number size distribution and provides parameterizations that are representative for a variety of air masses in Europe and thus suitable for future climate modeling applications. ¿ 2001 American Geophysical Union