NO 2-initiated multiphase oxidation of SO 2 by O 2 on CaCO 3 particles

Yu, Ting; Zhao, Defeng; Song, Xiaojuan; Zhu, Tong

The reaction of inline-formulaSO2 with inline-formulaNO2 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 inline-formulaSO2 by inline-formulaNO2 was slow and might not be the main reason for sulfate formation in ambient air. In this study, we investigated the multiphase reaction of inline-formulaSO2 with an inline-formulaO2inline-formula∕inline-formulaNO2 mixture on single inline-formulaCaCO3 particles using Micro-Raman spectroscopy. The reaction converted the inline-formulaCaCO3 particle to a inline-formulaCa(NO3)2 droplet, with inline-formulaCaSO4inline-formulaRadical⚫inline-formula2H2O 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 inline-formulaSO2 for sulfate formation was on the order of 10inline-formula−5, which was higher than that for the multiphase reaction of inline-formulaSO2 directly with inline-formulaNO2 by 2–3 orders of magnitude. According to our observations and the literature, we found that in the multiphase reaction of inline-formulaSO2 with the inline-formulaO2inline-formula∕inline-formulaNO2 mixture, inline-formulaO2 was the main oxidant of inline-formulaSO2 and was necessary for radical chain propagation. inline-formulaNO2 acted as the initiator of radical formation, but not as the main oxidant. The synergy of inline-formulaNO2 and inline-formulaO2 resulted in much faster sulfate formation than the sum of the reaction rates with inline-formulaNO2 and with inline-formulaO2 alone. We estimated that the multiphase oxidation of inline-formulaSO2 by inline-formulaO2 initiated by inline-formulaNO2 could be an important source of sulfate and a sink of inline-formulaSO2, based on the calculated lifetime of inline-formulaSO2 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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Yu, Ting / Zhao, Defeng / Song, Xiaojuan / et al: NO2-initiated multiphase oxidation of SO2 by O2 on CaCO3 particles. 2018. Copernicus Publications.

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