Particle size distribution in the range 0.3<D(p)<or=20 microm, has been analysed from August 2002 to July 2006 at the GAW Station of Mt. Cimone (44.10 N, 10.42 E; 2165 m asl) in the northern Italian Apennines. The seasonal aerosol number size distribution, characterized by a bimodal shape, showed a behaviour typical for background conditions, characterized by highest values in summer and lowest in winter. The seasonal and diurnal variations of the larger accumulation mode (0.3<D(p)<or=1 microm average values: 26.15 cm(-3)) and the coarse mode (1<Dp<or=20 microm, average value: 0.17 cm(-3)) particle number concentrations (N 0.3-1 and N 1-20, respectively) exhibited a seasonal cycle with the highest values in spring-summer and the lowest value in autumn-winter. Except in winter, N 0.3-1 showed a clear diurnal variation with high values during day-time. N 1-20 showed a less marked diurnal variation (but with higher variability), suggesting the influence of non-continuous sources of coarse particle (i.e. Saharan dust events). Since July 2005, continuous measurement of black carbon (BC) concentrations was also available at the measurement site. On average low BC concentrations were recorded (average value: 0.28 microg m(-3)) even if a few events of high concentrations were recorded both in warm and cold season. Apart from wet scavenging processes which strongly affected aerosol concentrations, combined analysis of N 0.3-1, BC, meteorological parameters and air mass back-trajectories, suggests that the transport of polluted air masses from the lower troposphere (by local, regional or long-range transport) represents an important mechanism favouring N 0.3-1 and BC increases at Mt. Cimone. In particular, a trajectory statistical analysis for the period July 2005-July 2006 allowed the identification of the main source regions of BC and N 0.3-1 for Mt. Cimone: north Italy, west Europe and east Europe.