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

摘要

The reaction of SO with NO 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 SO by NO was slow and might not be the main reason for sulfate formation in ambient air. In this study, we investigated the multiphase reaction of SO with NO on single CaCO particles in synthetic air, i.e., in the presence of O, using Micro-Raman spectroscopy. The reaction converted the CaCO particle to the Ca(NO) droplet containing CaSO × 2HO 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 O, 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 SO for sulfate formation was on the order of 10, which was two to three orders of magnitude higher than that in the absence of O. According to the difference between the reactive uptake coefficient of SO in the absence of O and that in the presence of O, we found that in the multiphase reaction of SO with NO in synthetic air, O was the main oxidant of SO and necessary for radical chain propagation. NO acted as the initializer of the radical formation but not the main oxidant. Such synergy of NO and O resulted in much faster sulfate formation than when either of them was absent. We estimated that the multiphase oxidation of SO by O in the presence of NO can be an important source of sulfate and sink of SO based on the calculated lifetime of SO 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.