Synergistic Reaction of SO<sub>2</sub> with NO<sub>2</sub> in Presence of H<sub>2</sub>O and NH<sub>3</sub>: A Potential Source of Sulfate Aerosol
2019
Zehua Wang | Chenxi Zhang | Guochun Lv | Xiaomin Sun | Ning Wang | Zhiqiang Li
Effect of H<sub>2</sub>O and NH<sub>3</sub> on the synergistic oxidation reaction of SO<sub>2</sub> and NO<sub>2</sub> is investigated by theoretical calculation using the molecule system SO<sub>2</sub>-2NO<sub>2</sub>-nH<sub>2</sub>O (n = 0, 1, 2, 3) and SO<sub>2</sub>-2NO<sub>2</sub>-nH<sub>2</sub>O-mNH<sub>3</sub> (n = 0, 1, 2; m = 1, 2). Calculated results show that SO<sub>2</sub> is oxidized to SO<sub>3</sub> by N<sub>2</sub>O<sub>4</sub> intermediate. The additional H<sub>2</sub>O in the systems can reduce the energy barrier of oxidation step. The increasing number of H<sub>2</sub>O molecules in the systems enhances the effect and promotes the production of HONO. When the proportion of H<sub>2</sub>O to NH<sub>3</sub> is 1:1, with NH<sub>3</sub> included in the system, the energy barrier is lower than two pure H<sub>2</sub>O molecules in the oxidation step. The present study indicates that the H<sub>2</sub>O and NH<sub>3</sub> have thermodynamic effects on promoting the oxidation reaction of SO<sub>2</sub> and NO<sub>2</sub>, and NH<sub>3</sub> has a more significant role in stabilizing product complexes. In these hydrolysis reactions, nethermost barrier energy (0.29 kcal/mol) can be found in the system SO<sub>2</sub>-2NO<sub>2</sub>-H<sub>2</sub>O. It is obvious that the production of HONO is energetically favorable. A new reaction mechanism about SO<sub>2</sub> oxidation in the atmosphere is proposed, which can provide guidance for the further study of aerosol surface reactions.
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