Ionic composition of TSP and PM2.5 during dust storms and air pollution episodes at Xi'an, China

TSP and PM2.5 samples were collected at Xi'an, China during dust storms (DSs) and several types of pollution events, including haze, biomass burning, and firework displays. Aerosol mass concentrations were up to 2 times higher during the particulate matter (PM) events than on normal days (NDs), and all types of PM led to decreased visibility. Water-soluble ions (Na+, NH4+, K+, Mg2+, Ca2+, F−, Cl−, NO3−, and SO42−). were major aerosol components during the pollution episodes, but their concentrations were lower during DSs. NH4+, K+, F−, Cl−, NO3−, and SO42− were more abundant in PM2.5 than TSP but the opposite was true for Mg2+ and Ca2+. PM collected on hazy days was enriched with secondary species (NH4+, NO3−, and SO42) while PM from straw combustion showed high K+ and Cl−. Firework displays caused increases in K+ and also enrichments of NO3− relative to SO42−. During DSs, the concentrations of secondary aerosol components were low, but Ca2+ was abundant. Ion balance calculations indicate that PM from haze and straw combustion was acidic while the DSs samples were alkaline and the fireworks' PM was close to neutral. Ion ratios (SO42−/K+, NO3−/SO42−, and Cl−/K+) proved effective as indicators for different pollution episodes.