The size distribution and chemical composition of ambient aerosol particles (PM I) were measured using an Aerodyne aerosol mass spectrometer (AMS) at the Preila air pollution research station on 3-15 September, 2006. The major observed components of aerosol particles were sulfate and organic matter with a smaller amount of nitrate and ammonium. Large contribution of organic matter was established in all air masses, however it reached 60% of the total aerosol particle mass in the North Atlantic marine air masses. The analysis of size distribution spectra enabled us to explain the origin of aerosol chemical components. In relatively clear North Atlantic air mass, mainly two modes were registered both for sulfate and organic matter - one in the submicron range, the other in the supermicron range. The diameter of sulfate-containing particles in the accumulation mode differed from that of organic-containing particles; they were about 270 and 170 nm, respectively. This difference showed different sources or transformation mechanisms of sulfate and organic compounds. In polluted air masses, the anthropogenic origin of both components was dominant, thus diameters in accumulation mode became equal and were about 400 nm, showing a dominant secondary production mechanism.