An AMS was employed to measure the mass and size distributions of PM1 at an elevated site. Features of PM1 at four seasons, during different kinds of episodes including NPF, polluted, PBL, LFT and in-cloud, were discussed. The characterizations of PM1 at seven clusters of air masses were also analyzed. BBOA, CCOA and oxidized organic aerosols were resolved by AMS-PMF (positive matrix function). Almost half of OA were oxidized, and BBOA is 34% of OA in summer; CCOA is 22% of OA in winter as well.