It has been suggested that multiphase or heterogeneous oxidation of SO2 potentially plays an important role in sulfate formation in the atmosphere (Seinfeld and Pandis, 2006). During the severe pollution episodes that occur frequently in China, high sulfate concentrations cannot be explained by the gas-phase oxidation of SO2 and its well known aqueous chemistry (Zheng et al., 2015a; Cheng et al., 2016), highlighting the role of underappreciated heterogeneous oxidation or multiphase pathways.
Recently, the multiphase oxidation of SO2 by NO2 has been introduced in air quality model simulations to explain the discrepancy between the modeled and observed sulfate concentration during severe pollution episodes in China (Cheng et al., 2016; Gao et al., 2016; Wang et al., 2016; Xue et al., 2016), despite the uncertainties in the kinetic parameters for SO2 oxidation and in the pH value of aerosol particles in China (Wang et al., 2016; Cheng et al., 2016; Liu et al., 2017; Guo et al., 2017). However, according to our recently published results (Zhao et al., 2018), the direct oxidation of SO2 by NO2 could not contribute significantly to sulfate formation in the atmosphere because the reactive uptake coefficient of SO2 for sulfate formation due to direct oxidation by NO2 is very low (∼ 10−8).
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Although the contribution of the direct oxidation of SO2 by NO2 to sulfate formation is not significant, NO2 may be involved in other oxidation pathways of SO2. It has been reported that the reaction of NO2 with SO32- and HSO3- in the bulk aqueous phase can form the SO3•- radical, which can further react with O2 and produce a series of radicals that oxidize S(IV) species (Littlejohn et al., 1993). The reaction pathway may result in a fast SO2 oxidation due to the potential synergy of NO2 and O2.
Despite such a reaction mechanism for SO2 oxidation being proposed, its role in SO2 oxidation in the ambient atmosphere is not well established. Most previous studies have focused on the direct reaction of SO2 with NO2, including the determination of its rate constant (Lee and Schwartz, 1983; Clifton et al., 1988; Shen and Rochelle, 1998; Spindler et al., 2003; Nash, 1979; Huie and Neta, 1986). According to the reaction products and their reported yields (Lee and Schwartz, 1983; Clifton et al., 1988), the overall reaction equations of the direct reaction of SO2 with NO2 are as follows:
and the reactions are proposed to proceed via NO2-S(IV) adduct complexes (Clifton et al., 1988).
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Additionally, NO2-S(IV) adduct complexes may decompose as follows (Spindler et al., 2003):
However, studies of the oxidation rate of SO2 at the O2 concentrations relevant to the ambient atmosphere and the potential influence of the synergy of NO2 and O2 on the oxidation rate are very limited (Turšič et al., 2001; He et al., 2014), except for a few studies which investigated SO2 oxidation in the presence of NO2 as well as O2 (Littlejohn et al., 1993; Shen and Rochelle, 1998; Santachiara et al., 1990). Moreover, previous studies have mainly focused on the reaction in bulk solution and only few studies have investigated the oxidation of SO2 by NO2 on aerosol particles (Santachiara et al., 1990, 1993). On aerosol particles, water activity, pH, ionic strength, the presence of other compounds or ions, and the role of the particle surface are different from dilute bulk solution and may affect the reaction process and reaction rate. Therefore, further studies of the multiphase reaction of SO2 with O2 ∕ NO2 mixtures on aerosol particles are required to determine the kinetic parameters and the mechanism of the reaction.
In this study, we investigated the multiphase reaction of SO2 with O2 in the presence of NO2 on CaCO3 particles. We quantified the reactive uptake coefficient of SO2 due to the reaction with an O2 ∕ NO2 ∕ H2O mixture. Based on our observations and the existing literature, we further discussed the reaction mechanism. Furthermore, we estimated the role of the multiphase oxidation of SO2 by O2 in the presence of NO2 in the atmosphere.
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