In this study, aerosol chemical speciation monitor (ACSM) and various collocated instruments are used to observe and analyze the chemical compositions, source and extinction characteristics of submicron aerosol (PM1, aerodynamic diameter less than 1 μm) in Beijing from July to September, 2012. The results show that the average mass concentration of PM1 during the whole observation period is 53.8μg m-3, accounting for 70-85% of PM2.5 on average. From July to September, the average mass concentration of non-refractory submicron aerosol (NR-PM1) declines monthly with the increasing fraction of OA in NR-PM1. The organics aerosol (OA) contributes the major mass fraction of PM1 during the cleaning days, and the fraction of inorganics shows a significant increasing trend with the accumulation of pollutants. The effects of meteorology on PM pollution and aerosol processing are also explored. In particular, SOR increase significantly at elevated relative humidity (RH) periods which suggested that the conversion of SO2 to SO42- in pollution episodes is more effective through the aqueous-phase oxidation of SO2 instead of the gas-phase oxidation. In addition, the effect of wind speed on the primary species (PPM) is significantly weaker than that on the secondary species (SPM). In addition, the mass concentration of SPM (or organics) is more sensitive to wind speed changes, compared with PPM (or inorganics). The proportion of oxygenated OA (OOA) in OA is significantly higher than that of hydrocarbon-like OA (HOA), and as the proportion of OA in PM1 increases, the mass fraction of OOA in OA decreases gradually. Moreover, the particulate matter (PM) in Beijing shows essentially neutral during the observation period. The total extinction coefficient of PM tracks well with the PM1 (r2= 0.72), and the extinction efficiency of the secondary particulate matter (SPM) (r2= 0.92) is significantly higher than that of the primary particulate matter (PPM) (r2= 0.58). Meanwhile, the correlation between OA and extinction coefficient (r2 = 0.56) is weaker than that between inorganics and extinction coefficient (r2 = 0.86).