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

• Main Authors: T. Yu, D. Zhao, X. Song, T. Zhu
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-05-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/18/6679/2018/acp-18-6679-2018.pdf

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 O₂ ∕ NO₂ mixture on single CaCO₃ particles using Micro-Raman spectroscopy. The reaction converted the CaCO₃ particle to a Ca(NO₃)₂ droplet, with CaSO₄ ⚫ 2H₂O 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 O₂ ∕ NO₂ 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.