Characteristics of water soluble ionic species in fine particles from a high altitude site on the northern boundary of Tibetan Plateau: Mixture of mineral dust and anthropogenic aerosol.

Abstract: A year-long field study on the seasonal characteristics of fine particulate matter (PM2.5) was conducted at the Qilian Shan Station of Glaciology and Ecologic Environment (QSS), a remote site on the northeast edge of the Tibetan Plateau. The PM2.5 samples were collected weekly using a low volume (16.7L/min) sampler, and then analyzed by ion chromatography for the water soluble ionic species (WSIs). The annual average mass loading of PM2.5, retrieved from co-located measurement on aerosol size distribution, was 9.5±5.4μgm−3 with WSIs accounting for 39±2%. The WSIs were dominated by SO4 2− (39%), CO3 2− (19%), Ca2+ (16%), NO3 − (10%), and NH4 + (6%), suggesting important contributions from both anthropogenic aerosol and mineral dust. The mass loading of total WSIs showed a seasonal variation with higher concentrations in spring (6.3μgm−3) and summer (5.1μgm−3) and lower concentrations in winter (2.2μgm−3) and fall (1.7μgm−3). A linear regression of ammonium versus sulfate by equivalent concentration showed a slope of 0.51, suggesting an excess of acids. The excess sulfuric and nitric acids likely reacted with mineral dust, as evidenced by the tight correlation between [Ca2+ +NH4 +] and [SO4 2− +NO3 −]. The oxidation ratios of nitrogen and sulfur estimated based on the datasets of SO2 and NO2 from the closest air quality station at Jiayuguan (~150km from sampling site) showed that the gas-particle partitioning of nitrogen on dust was more efficient than that of sulfur. The size distribution of SO4 2− showed a dominant accumulation mode, with a mode diameter (D p) ranging from 0.18 to 0.56μm. In comparison, NO3 − and Ca2+ showed a prominent coarse mode with D p ranging from 1 to 10μm. The results of air mass trajectory cluster and potential source contribution function analysis demonstrated that the source regions of the mineral dust were located in the arid areas of northwest China while the anthropogenic aerosol was likely from urban areas in the low elevation areas to the east of QSS. The effects of meteorological parameters on the particle mass loading were also evaluated. [Copyright &y& Elsevier]