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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 9 | Copyright
Atmos. Chem. Phys., 18, 6679-6689, 2018
https://doi.org/10.5194/acp-18-6679-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 09 May 2018

Research article | 09 May 2018

NO2-initiated multiphase oxidation of SO2 by O2 on CaCO3 particles

Ting Yu1,*, Defeng Zhao1,a,*, Xiaojuan Song1, and Tong Zhu1 Ting Yu et al.
  • 1BIC-ESAT and SKL-ESPC, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China
  • anow at: Department of Atmospheric and Oceanic Sciences & Institute of Atmospheric Sciences, Fudan University, Shanghai, 200438, China
  • *These authors contributed equally to this work.

Abstract. The reaction of SO2 with NO2 on the surface of aerosol particles has been suggested to be important in sulfate formation during severe air pollution episodes in China. However, we found that the direct oxidation of SO2 by NO2 was slow and might not be the main reason for sulfate formation in ambient air. In this study, we investigated the multiphase reaction of SO2 with an O2NO2 mixture on single CaCO3 particles using Micro-Raman spectroscopy. The reaction converted the CaCO3 particle to a Ca(NO3)2 droplet, with CaSO42H2O solid particles embedded in it, which constituted a significant fraction of the droplet volume at the end of the reaction. The reactive uptake coefficient of SO2 for sulfate formation was on the order of 10−5, which was higher than that for the multiphase reaction of SO2 directly with NO2 by 2–3 orders of magnitude. According to our observations and the literature, we found that in the multiphase reaction of SO2 with the O2NO2 mixture, O2 was the main oxidant of SO2 and was necessary for radical chain propagation. NO2 acted as the initiator of radical formation, but not as the main oxidant. The synergy of NO2 and O2 resulted in much faster sulfate formation than the sum of the reaction rates with NO2 and with O2 alone. We estimated that the multiphase oxidation of SO2 by O2 initiated by NO2 could be an important source of sulfate and a sink of SO2, based on the calculated lifetime of SO2 regarding the loss through the multiphase reaction versus the loss through the gas-phase reaction with OH radicals. Parameterization of the reactive uptake coefficient of the reaction observed in our laboratory for further model simulation is needed, as well as an integrated assessment based on field observations, laboratory study results, and model simulations to evaluate the importance of the reaction in ambient air during severe air pollution episodes, especially in China.

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The reaction of SO2 with NO2 on particles is proposed to be one major pathway of sulfate formation in the polluted atmosphere. We found that in the reaction of SO2 with NO2 on CaCO3 particles, presence of O2 enhanced the uptake rate of SO2 by 2–3 orders of magnitude compared with the reaction of SO2 directly with NO2. O2 was the main oxidant of SO2 and NO2 was the initializer of chain reactions. The multiphase oxidation of SO2 by NO2/O2 can be an important source of sulfate in the atmosphere.
The reaction of SO2 with NO2 on particles is proposed to be one major pathway of sulfate...
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