The variation of characteristics and formation mechanisms of aerosols in dust, haze, and clear days in Beijing

A 4-year campaign from 2001 to 2004 monitoring PM2.5 and TSP in the spring season in urban Beijing, China was performed to study the variation of characteristics and the different formation mechanisms of aerosols in dust, haze, and clear days. A total of 315 aerosol samples were collected and used in this study. The aerosols were more basic in dust days and more acidic in haze days. The ions presented in the order of SO42->Ca2+≫NO3->Cl−>NH4+>Na+ in dust days, and of SO42->NO3->NH4+≫Cl−>Ca2+>K+ in haze days. Ions has been classified into three groups, “Na+, Mg2+, Ca2+”, “K+, SO42-, Cl−”, and “NO3-, NH4+”, representing crust, pollution-crust, and pollution species, respectively. Crust and pollution ions were the main ion fractions in dust and haze days, respectively. The variation of Ca2+/Al showed that the increase of dust in dust and haze days was from soil and construction, respectively. “CaCO3, CaSO4, and (NH4)2SO4” and “(NH4)2SO4, NH4NO3, and Ca(NO3)2” were the major species in dust and haze days, respectively. The formation of CaSO4 on airborne soil particles and the formation of (NH4)2SO4 and NH4NO3 were the predominant pathways of sulfate and nitrate formations in dust and haze days, respectively. Sulfate might be mainly formed through heterogeneous reactions in the aqueous surface layer on the pre-existing particles, while nitrate mainly through homogeneous gas-phase reactions in the spring season in Beijing. The formation of sulfate and nitrate was accelerated in dust and haze days.