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. The reaction of SO2 with NO2 on the surface of aerosolparticles has been suggested to be important in sulfate formation duringsevere air pollution episodes in China. However, we found that the directoxidation of SO2 by NO2 was slow and might not be the mainreason for sulfate formation in ambient air. In this study, we investigatedthe multiphase reaction of SO2 with an O2 ∕ NO2 mixtureon single CaCO3 particles using Micro-Raman spectroscopy. Thereaction converted the CaCO3 particle to a Ca(NO3)2 droplet, with CaSO4 ⚫ 2H2O solid particlesembedded in it, which constituted a significant fraction of the dropletvolume at the end of the reaction. The reactive uptake coefficient of SO2 for sulfate formation was on the order of 10 −5 , which washigher than that for the multiphase reaction of SO2 directly with NO2 by 2–3 orders of magnitude. According to our observations andthe literature, we found that in the multiphase reaction of SO2 withthe 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. Thesynergy of NO2 and O2 resulted in much faster sulfateformation 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 ofsulfate and a sink of SO2 , based on the calculated lifetime of SO2 regarding the loss through the multiphase reaction versus theloss through the gas-phase reaction with OH radicals. Parameterization of thereactive uptake coefficient of the reaction observed in our laboratory forfurther model simulation is needed, as well as an integrated assessment basedon field observations, laboratory study results, and model simulations toevaluate the importance of the reaction in ambient air during severe airpollution episodes, especially in China.

atmospheric chemistry and physics

NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;-initiated multiphase oxidation of SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; by O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; on CaCO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; particles