To investigate the secondary formation and pollution sources of atmospheric particles in urban Beijing,PM2.5 and its chemical components were collected and determined by URG-9000D ambient ion monitor (AIM) from March 2016 to January 2017.Among water-soluble ions (WSls),NO3-,SO42-and NH4+ (SNA) had the largest proportion (77.8％) with the total concentration of 23.8 μg/m3.Moreover,as fine particle pollution worsened,the NO3-,SO42-and NH4+ concentrations increased basically,which revealed that secondary aerosols were the main cause of particle pollution in Beijing.Furthermore,the particle neutralization ratio (1.1),the ammonia to sulfate molar ratio (3.4) and the nitrate to sulfate molar ratio (2.2) showed that secondary aerosols are under ammonium-rich conditions with the main chemical forms of NH4NO3 and (NH4)2SO4,and vehicle emission could be the main anthropogenic source of secondary aerosols in Beijing.Source analysis further indicated that secondary aerosols,solid fuel combustion,dust and marine aerosol were the principal pollution sources of PM2.5,accounting for about 46.1％,22.4％ and 13.0％,respectively,and Inner Mongolia and Hebei Provinces could be considered as the main potential sources of PM2.5 in urban Beijing.In addition,secondary formation process was closely related with gaseous precursor emission amounts (SO2,NO2,NH3 and HONO),atmospheric ozone concentration (O3),meteorological conditions (temperature and relative humidity) and particle components.Sensitive analysis of the thermodynamic equilibrium model (ISORROPIA Ⅱ) revealed that controlling total nitrate (TN) is the effective measure to mitigate fine particle pollution in Beijing.