Water-soluble part of the aerosol in the dust storm season—evidence of the mixing between mineral and pollution aerosols

Six dust episodes were observed in Beijing in 2002. Both TSP (Total Suspended Particulate, particle size smaller than 100 μm) and PM2.5 (particle size smaller than 2.5 μm) aerosol samples in these episodes were collected and their characteristics of water-soluble part were elaborated in demonstrating the mixing of mineral aerosol with pollution aerosol in the long-range transport of Asia aerosols with various sources and different paths. The dust storm peaked on 20 March, in which the highest concentrations of TSP and PM2.5 were 10.9 and 1.4 mg m−3, respectively. The mass fraction of water-soluble part generally decreased with the increase of dust intensity. SO42− contributed 38–70% to the total anions and Ca2+ contributed 37–80% to the total cations, indicating that SO42− and Ca2+ were the most abundant anion and cation, respectively. The major ions of the water-soluble parts could be classified into three groups, i.e., the crust ions (Ca2+, Na+, and Mg2+), the pollution-crust ions (SO42−, Cl−, and K+), and the pollution ions (NO3−, NH4+, NO2−, and F−). Crust ions and pollution ions were the main ion fractions in super dust and non-dust days, respectively, whereas the pollution-crust ions were the main ion fractions in both dust days of various dust intensity and non-dust days, which demonstrated clearly that the mixing between mineral and pollution aerosols was ubiquitous during the dust seasons (even in the super dust storm days) although it was more obvious in those normal and weak dust episodes. The main chemical species of the water-soluble part of the aerosols were CaCO3 in the super dust storm, CaSO4 in the normal and the weak dust events, and NH4NO3 in the non-dust event days. The secondary transformation of sulfate and nitrate occurred on dust particles both during and after dust days provided the strong evidence of the mixing between mineral and pollution aerosols during the long-range transport of dust.