Differential mobility particle sizer (DMPS) aerosol concentrations (N 13−800) were collected over a one-year-period (2004) at an urban background site in Barcelona, North-Eastern Spain. Quantitative contributions to particle number concentrations of the nucleation (33–39 %), Aitken (39–49 %) and accumulation mode (18–22 %) were esti-mated. We examined the source and time variability of at-mospheric aerosol particles by using both K-means cluster-ing and Positive Matrix Factorization (PMF) analysis. Per-forming clustering analysis on hourly size distributions, nine K-means DMPS clusters were identified and, by directional association, diurnal variation and relationship to meteorolog-ical and pollution variables, four typical aerosol size distribu-tion scenarios were identified: traffic (69 % of the time), dilu-tion (15 % of the time), summer background conditions (4 % of the time) and regional pollution (12 % of the time). Ac-cording to the results of PMF, vehicle exhausts are estimated to contribute at least to 62–66 % of the total particle number concentration, with a slightly higher proportion distributed towards the nucleation mode (34 %) relative to the Aitken mode (28–32 %). Photochemically induced nucleation parti-cles make only a small contribution to the total particle num-ber concentration (2–3 % of the total), although only parti-cles larger than 13 nm were considered in this study. Overall the combination of the two statistical methods is successful at separating components and quantifying relative contribu-tions to the particle number population.