Size-fractioned atmospheric aerosol particles were collected during a typical heavy air pollution event in Beijing.The organic and inorganic components on the surfaces of the samples were analyzed using time-of-flight secondary ion mass spectrometry (TOF-SIMS).The variation characteristics of the surface chemical composition and influencing factors were studied,and the possible sources of these chemical compositions were identified through principal component analysis.The results showed that inorganic components such as crustal elements and sulfate,and organic components such as aliphatic hydrocarbons and oxygen-containing organic groups were present.Some surface components,such as polycyclic aromatic hydrocarbons,heavy metals and fluorides may exert adverse effects on human health.The species and relative percentages of the chemical components varied with particle size,dial and pollution progress.During a heavy pollution event,the species and relative percentages of secondary components such as oxygen-containing organic groups and sulfurous compounds increased,indicating that particles aged during this event.The surface chemical composition of the aerosol particles was affected mainly by emissions from coal combustion and motor vehicles.In addition,air pollution,meteorological factors,and air mass transport also exerted a significant effect on the surface chemical composition of aerosol particles.