Multiphase oxidation of SO 2 by NO 2 on CaCO 3 particles
Heterogeneous/multiphase oxidation of SO 2 by NO 2 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 SO 2 by NO 2 we investigated the heterogeneous/multiphase reaction of SO 2 with NO 2 on individual CaCO 3 particles in N 2 using Micro-Raman spectroscopy. In the SO 2 ∕ NO 2 ∕ H 2O ∕ N 2 gas mixture, the CaCO 3 solid particle was first converted to the Ca(NO 3) 2 droplet by the reaction with NO 2 and the deliquescence of Ca(NO 3) 2, and then NO 2 oxidized SO 2 in the Ca(NO 3) 2 droplet forming CaSO 4, which appeared as needle-shaped crystals. Sulfate was mainly formed after the complete conversion of CaCO 3 to Ca(NO 3) 2, that is, during the multiphase oxidation of SO 2 by NO 2. The precipitation of CaSO 4 from the droplet solution promoted sulfate formation. The reactive uptake coefficient of SO 2 for sulfate formation is on the order of 10 −8, and RH enhanced the uptake coefficient. We estimate that the direct multiphase oxidation of SO 2 by NO 2 is not an important source of sulfate in the ambient atmosphere compared with the SO 2 oxidation by OH in the gas phase and is not as important as other aqueous-phase pathways, such as the reactions of SO 2 with H 2O 2, O 3, and O 2, with or without transition metals.