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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 18, issue 4 | Copyright
Atmos. Chem. Phys., 18, 2481-2493, 2018
https://doi.org/10.5194/acp-18-2481-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 19 Feb 2018

Research article | 19 Feb 2018

Multiphase oxidation of SO2 by NO2 on CaCO3 particles

Defeng Zhao1,*, Xiaojuan Song1,*, Tong Zhu1, Zefeng Zhang1, Yingjun Liu1, and Jing Shang1 Defeng Zhao et al.
  • 1BIC-ESAT and SKL-ESPC, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China
  • *These authors contributed equally to this work.

Abstract. Heterogeneous/multiphase oxidation of SO2 by NO2 on solid or aqueous particles is thought to be a potentially important source of sulfate in the atmosphere, for example, during heavily polluted episodes (haze), but the reaction mechanism and rate are uncertain. In this study, in order to assess the importance of the direct oxidation of SO2 by NO2 we investigated the heterogeneous/multiphase reaction of SO2 with NO2 on individual CaCO3 particles in N2 using Micro-Raman spectroscopy. In the SO2NO2H2ON2 gas mixture, the CaCO3 solid particle was first converted to the Ca(NO3)2 droplet by the reaction with NO2 and the deliquescence of Ca(NO3)2, and then NO2 oxidized SO2 in the Ca(NO3)2 droplet forming CaSO4, which appeared as needle-shaped crystals. Sulfate was mainly formed after the complete conversion of CaCO3 to Ca(NO3)2, that is, during the multiphase oxidation of SO2 by NO2. The precipitation of CaSO4 from the droplet solution promoted sulfate formation. The reactive uptake coefficient of SO2 for sulfate formation is on the order of 10−8, and RH enhanced the uptake coefficient. We estimate that the direct multiphase oxidation of SO2 by NO2 is not an important source of sulfate in the ambient atmosphere compared with the SO2 oxidation by OH in the gas phase and is not as important as other aqueous-phase pathways, such as the reactions of SO2 with H2O2, O3, and O2, with or without transition metals.

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The oxidation of SO2 directly by NO2 on solid/liquid particles is proposed to be a major pathway of particle sulfate formation in the polluted atmosphere. We found that the reaction of SO2 and NO2 on CaCO3 particles produced Ca(NO3)2 aqueous droplets, providing a site for the multiphase oxidation of SO2. The direct multiphase oxidation of SO2 by NO2 led to a reactive uptake coefficient of SO2 on the order of 10-8, which is unlikely to be an important source of sulfate in the ambient atmosphere.
The oxidation of SO2 directly by NO2 on solid/liquid particles is proposed to be a major pathway...
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