Size Distributions and Seasonal Variations of Water-Soluble Inorganic Particulate Matter at a Suburban Site in Nanjing, China

Abstract

Size-resolved aerosol mass concentrations and concentrations of inorganic water-soluble ions (WSIs) were measured in four seasons, from December 22, 2016 to November 28, 2017, in Nanjing, China. Water-soluble ions were the main components of the atmospheric particles in Nanjing. The WSIs mainly comprised fine particles, especially in the 0.5–1-μm size range. Sulfate (SO42−), nitrate (NO3−), and ammonium (NH4+) (SNA) were the major WSI components. The secondary transformation products of the SNA WSIs were mainly distributed in the fine component of the total WSIs, while the coarse component contained a large amount of soluble mineral dust, comprising calcium (Ca2+), sodium (Na+), and magnesium (Mg2+). The mass concentrations of the total WSIs showed clear seasonal variations, with much higher concentrations in autumn and winter than in spring and summer. The mass concentrations of the WSIs increased significantly during pollution episodes, especially in the fine particle size component. The increases in the mass concentrations of WSIs on polluted days were mainly driven by the accumulation of secondary ions (i.e., SNA) in the fine component, while the contribution of the primary ions (Ca2+, Na+, and Mg2+) was relatively small. Ion charge-balance analysis showed that, for the inorganic WSIs, the charge concentration of the cations was greater than that of the anions in all seasons, with this phenomenon being strongest in spring and weakest in winter and strongest on nonpolluted days and weakest on heavily polluted days.