Single particle characterization of spring and summer aerosols in Beijing: Formation of composite sulfate of calcium and potassium

Scanning electron microscopy–energy dispersive X-ray analysis (SEM–EDX) was used for the analysis of 2500 single particles in five atmospheric aerosol samples collected during the spring and summer of 2000 in Beijing, China. Mineral dust appeared to be the dominant particles during an Asian dust episode, while in other circumstances mineral dust and S-containing particles constituted the major particle components. During anthropogenic pollution episodes in the summer, a large abundance of S-containing particles featured the atmospheric aerosol. Chemical and size distribution characteristics are discussed for Ca–S, K–S and Ca–K–S particle classes. Formation of Ca–K–S and other S-containing particle classes with high abundance was closely related to meteorological conditions such as relative humidity and cloud coverage. Simple and composite sulfate particles with an elongated crystalline morphology were detected which appear to be indicative of aqueous phase oxidation, such as in-cloud processing for sulfate formation pathway.