Synergistic Reaction of SO2 with NO2 in Presence of H2O and NH3: A Potential Source of Sulfate Aerosol.

Zehua Wang,Guochun Lv,Xiaomin Sun,Ning Wang,Zhiqiang Li,Chenxi Zhang

doi:10.3390/ijms20153746

Zehua Wang, Guochun Lv + Show 4 more

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https://doi.org/10.3390/ijms20153746

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Abstract

Effect of H2O and NH3 on the synergistic oxidation reaction of SO2 and NO2 is investigated by theoretical calculation using the molecule system SO2-2NO2-nH2O (n = 0, 1, 2, 3) and SO2-2NO2-nH2O-mNH3 (n = 0, 1, 2; m = 1, 2). Calculated results show that SO2 is oxidized to SO3 by N2O4 intermediate. The additional H2O in the systems can reduce the energy barrier of oxidation step. The increasing number of H2O molecules in the systems enhances the effect and promotes the production of HONO. When the proportion of H2O to NH3 is 1:1, with NH3 included in the system, the energy barrier is lower than two pure H2O molecules in the oxidation step. The present study indicates that the H2O and NH3 have thermodynamic effects on promoting the oxidation reaction of SO2 and NO2, and NH3 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 SO2-2NO2-H2O. It is obvious that the production of HONO is energetically favorable. A new reaction mechanism about SO2 oxidation in the atmosphere is proposed, which can provide guidance for the further study of aerosol surface reactions.

Highlights

Sulfur dioxide (SO2), a major air pollutant, is released into the atmosphere via the combustion of sulfur containing fuels and industrial production [1,2,3], volcanic eruptions, and decomposition of sulfides in nature
The IM2 is produced by TS2 with the energy barrier and reaction energy of 52.45 kcal/mol and −5.77 kcal/mol, respectively
The results indicate that HONO cannot be formed in the absence of H2O molecules

Summary

Introduction

Sulfur dioxide (SO2), a major air pollutant, is released into the atmosphere via the combustion of sulfur containing fuels and industrial production [1,2,3], volcanic eruptions, and decomposition of sulfides in nature. It is significant to investigate the SO2 oxidation mechanism in the atmosphere. Researchers have found that a considerable amount of PM2.5 can be formed in the atmosphere, and sulfate is the significant component of the fine particulate matter [13,14,15,16,17,18]. Widespread formation pathways (gas-phase oxidation and in-cloud oxidation) of sulfate cannot account for the phenomena of high content of sulfate. Some researchers have investigated that the oxidation reaction of SO2 by NO2 is proposed as a missing key pathway to form sulfate in a special condition [19,20,21,22,23,24]. Wang et al have considered that NO2 could effectively react with SO2 in the presence of H2O and NH3 during the severe haze period [25]: SO2(g) + 2NO2(g) + 2H2O(aq) → 2H+(aq) + SO42−(aq) + 2HONO(g)

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