Multiphase Reaction of SO₂ with NO₂ on CaCO₃ Particles. 2. NO₂-initialized Oxidation of SO₂ by O₂

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 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 NO2 on single CaCO3 particles in synthetic air, i.e., in the presence of O2, using Micro-Raman spectroscopy. The reaction converted the CaCO3 particle to the Ca(NO3)2 droplet containing CaSO4 × 2H2O solid particles embedded in it, which constituted a large fraction of the droplet volume at the end of the reaction. Compared with the reaction in the absence of O2, the morphology of the particle during the reaction in synthetic air was significantly different and the amount of sulfate formed at the end of the experiment was much higher. The reactive uptake coefficient of SO2 for sulfate formation was on the order of 10−5, which was two to three orders of magnitude higher than that in the absence of O2. According to the difference between the reactive uptake coefficient of SO2 in the absence of O2 and that in the presence of O2, we found that in the multiphase reaction of SO2 with NO2 in synthetic air, O2 was the main oxidant of SO2 and necessary for radical chain propagation. NO2 acted as the initializer of the radical formation but not the main oxidant. Such synergy of NO2 and O2 resulted in much faster sulfate formation than when either of them was absent. We estimated that the multiphase oxidation of SO2 by O2 in the presence of NO2 can be an important source of sulfate and sink of SO2 based on the calculated lifetime of SO2 regarding the loss by the multiphase reaction versus the lifetime regarding the loss by the gas phase reaction with OH radical. Parameterizing 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 observation, laboratory study results, and model simulation to evaluate the importance of the reaction in ambient air during the severe air pollution period, especially in China.