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

Daily PM2.5 and water-soluble inorganic ions (Na+, NH4+, K+, Mg2+, Ca2+, Cl−, NO3− and SO42−) were collected in Xi'an (34.23°N, 108.88°E), China from March 2006 to March 2007. PM2.5 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 PM2.5 was found to be 194.1 ± 78.6 μg m− 3, which exceeded substantially the international guidelines for health concerns. The seasonal average mass concentration of PM2.5 was highest in winter (266.8 μg m− 3) and lowest in summer (138.6 μg m− 3). The three highest abundant ions were SO42−, NO3−, and NH4+, with average concentrations of 35.6 ± 19.5 μg m− 3, 16.4 ± 10.1 μg m− 3, and 11.4 ± 6.8 μg m− 3, which were accounted for 18.7%, 8.0%, and 5.7% of the PM2.5 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 CO32−, which was calculated as the difference in the measured cations minus anions. Ion balance calculations indicate that the PM2.5 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.

Research Highlights
► Coal combustion has heavy impact on the variations of water-soluble ions.
► PM2.5 in non-spring was acidic, while the aerosol was less acidic and possibly even alkaline in spring due to the buffering effects of mineral dust.
► Relationship between CO32- and Ca2+ implies that Ca2CO3 was the major form for the aerosol carbonate.