Chemical composition of size-segregated aerosols in Lhasa city, Tibetan Plateau

Abstract

To reveal the chemical characteristics of size-segregated aerosols in the high-altitude city of Tibetan Plateau, eight-size aerosol samples were collected in Lhasa from March 2013 to February 2014. The annual mean of online PM2.5 was 25.0±16.0μgm−3, which was much lower than Asian cities but similar with some European cities. The annual mean concentrations of organic carbon (OC, 7.92μgm−3 in PM2.1 and 12.66μgm−3 in PM9.0) and elemental carbon (EC, 1.00μgm−3 in PM2.1 and 1.21μgm−3 in PM9.0) in Lhasa aerosols were considerably lower than those heavily polluted cities such as Beijing and Xi'an, China and Kathmandu, Nepal. Sulfate, NO3−, NH4+ and Ca2+ were 0.75±0.31, 0.82±0.35, 0.38±0.34 and 0.57±0.29μgm−3 in fine particles while in coarse particles they were 0.57±0.37, 0.73±0.23, 0.07±0.03 and 2.52±1.37μgm−3, respectively. Secondary water-soluble ions composed 35.8% of the total ionic components in fine particles according to the established electroneutrality, while in coarse particles they took up only 9.3%. Ca2+ (40.6%) was the major component of the coarse particles. For seasonality, the concentrations of OC, EC, SO42−, NH4+, K+, Ca2+, Mg2+, Cl− and Na+ presented higher values during late autumn and winter but were relatively lower in spring and summer. Nevertheless, NO3− was considerably higher in summer and autumn, presumably due to increased tourist-vehicle emissions. During winter and spring, [Ca2+]/[NO3−+SO42−] ratios in coarse particles showed higher values of 7.31 and 6.17, respectively, emphasizing the dust influence. [NO3−]/[SO42−] ratios in fine particles during spring, summer and autumn exceeding 1 indicated that the currently predominant vehicle exhaust makes a greater contribution to the aerosols. While more stationary sources such as coal and biomass burning existed in winter since the [NO3−]/[SO42−] ratio was less than 1. Different sources and formation processes lead to a bimodal size distribution (0–0.65μm in fine particles and 4.7–9.0μm in coarse particles) for all of the compounds except Na+.