Ionic composition of TSP and PM_2.5 during dust storms and air pollution episodes at Xi'an, China

TSP and PM_2.5 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 SO₄^2-). were major aerosol components during the pollution episodes, but their concentrations were lower during DSs. NH₄⁺, K⁺, F^-, Cl^-, NO₃^-, and SO₄^2- were more abundant in PM_2.5 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 SO₄^2-. 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 (SO₄^2-/K⁺, NO₃^-/SO₄^2-, and Cl^-/K⁺) proved effective as indicators for different pollution episodes.