Atmospheric particulate characterization by ToF-SIMS in an urban site in Beijing

The chemical composition of atmospheric particulate is greatly different in air pollution in different periods or regions, which affects human health to various degrees. This work aims to establish a strategy to distinguish the aerosol samples of different air quality levels and figure out their major differences using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Considering archived meteorological data and environmental data, the samples from five days were selected for analysis, representing five air quality levels: I, good; II, moderate; III, lightly polluted; IV, moderately polluted; and V, heavily polluted. ToF-SIMS results showed that the surface chemical composition of PM_2.5 was complicated during haze. A large amount of inorganic salts and organic substances derived from volatile organic compounds (VOCs) were detected. Principal component analysis (PCA) was used to search for the species that have large difference under various pollution conditions. PCA showed that worse air quality is usually associated with higher proportion of organic substances on the aerosol with lower proportion of inorganic salts. The organic substances such as saturated hydrocarbons, unsaturated hydrocarbons, aromatic hydrocarbons, alcohols and carbonyls, and nitrogen-containing organics or heterocyclic compounds contributed greatly to the difference of these samples. These species were assigned and their possible sources were discussed. Our results suggested that the exhaust emissions from motor vehicles have large impact on the growth of atmospheric aerosol in urban areas of Beijing.