Seasonal variations and size distributions of water-soluble ions of atmospheric particulate matter at Shigatse, Tibetan Plateau

Size-segregated atmospheric particulate matter (PM) samples were collected from July 2012 to September 2013 at Shigatse, high-altitude (3836 m above sea level) site on the south Tibetan Plateau (TP); objectives were to determine the characteristics and size distribution of water-soluble ions (WSIs). Eight major WSIs (Na+, K+, Mg2+, Ca2+, NH4+, Cl−, SO42−, and NO3−) were detected by ion chromatography. The total concentrations of WSIs were 6370 ± 1916 ng m−3 in dry season (October - December, January - April), and 5261 ± 769 ng/m3 during wet phase (May - September). The contribution of K+ (130 ng m−3), Cl− (2035 ng m−3), SO42− (1176 ng m−3), and NO3− (706 ng m−3)−were significantly enhanced in dry season, and that of Na+ (455 ng m−3), Mg2+(65.4 ng m−3), Ca2+(1034 ng m−3), and NH4+ (1948 ng m−3) were significantly enhanced during wet phase. Mg2+ and Ca2+ were concentrated in the coarse mode, and the other ions concentrated in fine mode and coarse mode during two seasons. The correlation coefficients between K+ and NH4+, Cl−, SO42− and NO3− were 0.58 (P < 0.01), 0.40 (P < 0.05), 0.82 (P < 0.01) and 0.69 (P < 0.01), indicating their dominant contribution from biomass burning in dry season. The significant correlation between NH4+ and HCO3− which were calculated by ion balance (r = 0.89, P < 0.01), suggesting the source from nitrogen fertilizers during wet phase.