In this study, an aerosol chemical speciation monitor (ACSM) and various collocated instruments are used to observe and analyze the chemical compositions, sources and extinction characteristics of submicron aerosol (PM1; aerodynamic diameter < 1 µm) in Beijing from July to September 2012. The results show that the average mass concentration of the PM1 for the entire observation period is 53.8 µg m−3, accounting for 70–85% on average of the PM2.5, and the average mass concentration of the non-refractory submicron aerosol (NR-PM1) declines monthly from July to September as the fraction of organic aerosol (OA) in it increases. During clean days, OA forms the largest mass fraction of the PM1, and the fraction of inorganics shows a significant increasing trend as pollutants accumulate. The effects of meteorology on PM pollution and aerosol processing are also explored. In particular, the SOR increases significantly during periods of elevated relative humidity (RH), suggesting that SO2 is more efficiently converted to SO42− during pollution episodes via aqueous-phase oxidation than gas-phase oxidation. In addition, the effect of wind speed is significantly weaker on primary species (PPM) than secondary species (SPM). Furthermore, the mass concentration of the SPM (or organics) is more sensitive than that of the PPM (or inorganics) to changes in wind speed. The proportion of oxygenated OA (OOA) is significantly higher than that of hydrocarbon-like OA (HOA) in the OA, and as the proportion of OA in the PM1 increases, the mass fraction of OOA in the OA gradually decreases. Moreover, the aerosol acidity in Beijing is essentially neutral during the observation period. The total extinction coefficient of the particulate matter (PM) correlates well with the mass concentration of 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 is weaker between the OA and the extinction coefficient (r2 = 0.56) than between the inorganic aerosol and the extinction coefficient (r2 = 0.86).