Submicron aerosols （PMt） in Beijing were studied using an Aerodyne aerosol mass spectrometer （AMS） from January to Oc tober 2008. This paper presents seasonal variations of different chemical components （sulfate, nitrate, ammonium, chloride and organics） and size distributions of PM1. Results show that mass concentration of PMI was highest in summer, and lowest in autumn. Organics represented the dominant species in all seasons, accounting for 36%58% of PML, and their concentrations were highest in winter. Concentrations of inorganic components, sulfate, nitrate, and ammonium were highest in summer. Based on principal component analysis, organics were deconvolved and quantified as hydrocarbonlike and oxygenated organ ic aerosol （HOA and OOA, respectively）. HOA was highest in winter, accounting for about 70% of organics. However, OOA was highest in summer, and had lower values in autumn and winter. A similar diurnal pattern of various components was ob served, which is higher at nighttime and lower during daytime. HOA increased more dramatically than other species between 17：00 and 21：00 and peaked at noon, which could be related to cooking emissions. OOA, sulfate, nitrate, ammonium and chlo ride varied with the same trend. Their concentrations increased with solar radiation from 9：00 to 13：00, and declined with weakening solar radiation. Size distributions of all species showed apparent peaks in the range 500600 nm. Size distributions of organics were much broader than other species, particularly in autumn and winter. Distributions of sulfate, nitrate and am monium had similar patterns, broadening in winter. Contributions of different species were sizedependent; the finer the parti cle, the greater the contribution of organics. Organics represented more than 60% of particles smaller than 200 nm, contrib uting 50% to PM1 in winter. In spring and summer, HOA was the dominant organic fraction for particles smaller than 200 nm, while OOA contributed more to particles larger than 300 nm. In win
