TOF-SIMS Characterization of Ammonium Single Particles within PM2.5

Fine particulate matter (PM2.5) is the primary atmospheric pollutant in the haze episodes. Water-soluble inorganic ions such as nitrate, sulfate, ammonium and chloride salt are generally considered to be the main components of PM2.5 during heavy atmospheric pollution period. Due to the unstable and decomposable characteristics of ammonium salts, the traditional single particle analysis technique is limited for the characterization. The specific combination of ammonium salts in atmospheric aerosols is still ambiguous and worthy of study. In this study, PM2.5 samples were collected using a medium volume sampler in a typical urban area of Beijing from December 1st to December 31th, 2015. Samples of typical heavy air pollution period were analyzed using time of flight-secondary ion mass spectrometry (TOF-SIMS). The ammonium salt in PM2.5 was characterized by high quality resolution mode and positive and negative ion imaging mode. The results showed that a variety of inorganic ions groups were accumulated on the surfaces of aerosol particles in the haze episodes of Beijing, more than ten kinds of water-soluble inorganic ions including ammonium were detected by TOF-SIMS. The single-particle mapping analysis of PM2.5 showed that the presence of ammonium bisulfate (NH₄HSO₄) single particles were observed in three groups of samples and two different scales (100 μm×100 μm, 40 μm×40 μm). Ammonium nitrate and ammonium chloride particulate matters were not detected. In addition, this paper also demonstrated the manual calibration method for positive and negative ion imaging deviation, and pointed out the characteristics, advantages and application potential of TOF-SIMS for the characterization of water-soluble inorganic components in PM2.5.