Abstract: Aerosol samples were taken in Lin'an city from 2002-08-14 to 2002-08-24 in order to analyze the size distributions of aerosol mass and ionic components. The results indicated that SO₄^2- with size less than 2.1 μm accounted for 94% of the total mass in all size ranges. In fine particles (PM_2.1), SO₄^2- and NH₄⁺ had the largest mass concentrations and accounted for 89% of the total mass ofall analyzed ionic components. The Community Multiscale Air Quality (CMAQ) model was used to analyze the main factors affecting distribution of sulfate aerosol. The results indicated that the sulfate formation rate through in-cloud aqueousoxidation and wet deposition was 0.19 μg/(m³·h), which was 2.5 times the rateof sulfur gas condensation. Among the SO₂ aqueous oxidations, H₂O₂ had the largest reaction rate 5.26 μg/(m³·h), followed by O₂ 4.14 μg/(m³·h) and O₃ 1.56 μg/(m³·h). This proved that SO₄^2- in Lin'an was produced via the in-cloud aqueous oxidations of SO₂ by H₂O₂ and O₂ over Shanghai and the coastal region of Zhejiang. A sensitivity test was applied by shutting down SO₂ emissions in Shanghai, and it was found that SO₂ emitted from Shanghai accounted for 15% and 22% ofSO₂ and SO₄^2- in Lin'an, respectively.