Water-soluble ions in atmospheric aerosols measured in Xi'an, China: Seasonal variations and sources

Daily PM_2.5 and water-soluble inorganic ions (Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺, Cl^-, NO₃^- and SO₄^2-) were collected in Xi'an (34.23°N, 108.88°E), China from March 2006 to March 2007. PM_2.5 was collected using battery-powered mini-volume samplers. And the ions were determined by ion chromatography from the measured aerosol mass. The annual average mass concentration of PM_2.5 was found to be 194.1±78.6μgm^-3, which exceeded substantially the international guidelines for health concerns. The seasonal average mass concentration of PM_2.5 was highest in winter (266.8μgm^-3) and lowest in summer (138.6μgm^-3). The three highest abundant ions were SO₄^2-, NO₃^-, and NH₄⁺, with average concentrations of 35.6±19.5μgm^-3, 16.4±10.1μgm^-3, and 11.4±6.8μgm^-3, which were accounted for 18.7%, 8.0%, and 5.7% of the PM_2.5 mass, respectively. The major ions were in the species of (NH₄)₂SO₄, NH₄HSO₄ and NH₄NO₃, and their concentrations were highest in winter, due to high coal combustion. The concentrations of Ca²⁺ were higher in spring than other seasons, due to the higher mineral dust concentrations. Ca²⁺ was strongly correlated with CO₃^2-, which was calculated as the difference in the measured cations minus anions. Ion balance calculations indicate that the PM_2.5 was acidic, and this result is consistent with the measurement of pH values. Sulfur oxidation ratio was higher in summer and autumn, which implies that the formation of secondary sulfate-rich particles is favored by warm and relatively moist weather. Nitrogen oxidation ratio was highest in autumn.