[Size Distributions of Water-soluble Components in Ambient Aerosol of Beijing]

A micro-orifice uniform deposit impactor (MOUDI-122) was used to collect ambient aerosol at an urban site in Beijing in both winter and summer from 2016 to 2017. The water-soluble components, including ions and water-soluble organic carbon (WSOC) were analyzed. The characteristics of concentrations and size distributions for water-soluble components under different seasons and pollution conditions were determined. The results showed that NH₄⁺, NO₃^-, SO₄^2-, and K⁺ in both seasons and Cl^- in winter mainly distributed in the accumulation mode, and Mg²⁺ and Ca²⁺ primarily distributed in the coarse mode. The secondary ions were still the main components of PM_2.5 in Beijing. The concentrations of SO₄^2- were higher in summer, whereas those of NO₃^-, K⁺, and Cl^- were higher in winter. Mg²⁺ and Ca²⁺ had lower correlations with other main components of aerosols, indicating their independent sources. The average size distributions and concentration levels of NO₃^- and SO₄^2- exhibited apparent differences between daytime and nighttime in summer. During polluted periods, the concentrations of secondary ions increased in both the accumulation and coarse modes but decreased in the Aitken mode. As pollution levels increased in winter, the mass median diameters of secondary ions in the droplet mode also increased. The WSOC concentration and particle size distribution under accumulation mode in summer were significantly larger than those in winter. The distribution peaks of WSOC in accumulation mode were higher in summer than those in winter. The WSOC in particles of 0.056-0.32 μm were relatively stable under different pollution levels. However, the WSOC concentration in particles larger than 0.32 μm during polluted periods was evidently higher than that during clean periods.