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A 4-year campaign from 2001 to 2004 monitoring PM_2.5 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 SO₄^2->Ca²⁺≫NO₃^->Cl⁻>NH₄⁺>Na⁺ in dust days, and of SO₄^2->NO₃^->NH₄⁺≫Cl⁻>Ca²⁺>K⁺ in haze days. Ions has been classified into three groups, “Na⁺, Mg²⁺, Ca²⁺”, “K⁺, SO₄^2-, 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 …