We have a comprehensive characterization of the sources, variations and processes of submicron aerosols in Beijing in winter using HR-AMS and GC/MS measurements. The primary sources including traffic, cooking, biomass burning and coal combustion emissions, and secondary components were separated and quantified with PMF. Our results elucidated the important roles of primary emissions, particularly coal combustion, and aqueous-phase processing in the formation of severe air pollution in winter.