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In this paper, density-functional theory (DFT) was employed to investigate the reaction mechanism of SO 2 and NO 2 and the important role of intermediate N 2 O 4 formation. In addition, the effect of pure particulate surface (TiO 2) and particulate surface containing other atmospheric components (H 2 O, NH 3 and SO 4 2−) on the conversion of SO 2 and NO 2 to sulfate also was analyzed. The detailed information that intermediate N 2 O 4 acts as the oxidant was demonstrated in this oxidation reaction of SO 2 and NO 2 (as a rate-determining step). The pure particulate surface (TiO 2) has hardly effect on the oxidation process of SO 2 and N 2 O 4. Whereas, different amounts of H 2 O and NH 3 molecules as well as SO 4 2−, contained on the particulate surface, can effectively reduce the activation energy of oxidation step. And, the optimal process is that SO 2 is oxidized by cis-ONONO 2 with the energy barrier of 4.54 kcal/mol when one NH 3 molecule and one H 2 O molecule are contained on the TiO 2 (101) surface. When more H 2 O molecules are contained on particulate surface, SO 2 tends to form HSO 3 − first. However, HSO 3 − is more difficult to be further oxidized by asy-ONONO 2 than SO 2. This study gains more insight into the contribution of SO 2 and NO 2 to haze and the potential impact of atmospheric constituents (including H 2 O/NH 3 /TiO 2 and SO 4 2−/TiO 2) on the formation sulfate. Image 1 • Asy-ONONO 2 acted as oxidant is easily contained on the particulate surface. • The oxidation of SO 2 and asy-ONONO 2 is easy than HSO 3 −. • The oxidation step of SO 2 and N 2 O 4 is the rate-determining step. • Pure particulate surface has hardly effect on the rate-determining step. • Particulate containing H 2 O, NH 3 and SO 4 2− promote the reaction of SO 2 and N 2 O 4. [ABSTRACT FROM AUTHOR]