Atmospheric aerosol pollution has attracted widespread attention in recent years because of its adverse effects on human health, visibility, and climate (Thalman et al., 2017; Davidson et al., 2005; Pöschl, 2005). In many developing countries, such as China and India, high concentrations of SO2, NOx, and volatile organic compounds (VOCs) coexist in the atmosphere (Zou et al., 2015; Liu et al., 2013; Yang et al., 2009) and result in “complex atmospheric pollution” (Yang et al., 2011) and heavy haze events. Sulfate was found to play important roles in the occurrence of these haze events (Zhang et al., 2011; Z. R. Liu et al., 2017) due to both its high mass concentration in fine particles (PM2.5) and its strong hygroscopicity. Rapid formation of sulfate was frequently observed in haze episodes in China, in which heterogeneous reactions played important roles (He et al., 2014; Zhang et al., 2006; Ma et al., 2018). However, the mechanism of the heterogeneous reaction process as well as its contribution to sulfate formation in complex atmospheric pollution remains uncertain (Yang et al., 2018; Ma et al., 2018; Wang et al., 2018; Yu and Jang, 2018). These uncertainties are considered to be the main reason for the inaccuracy of sulfate simulation in air quality models (Wang et al., 2014b; Zheng et al., 2015; Yu and Jang, 2018).
About 1000 to 3000 Tg of mineral aerosols are emitted into the atmosphere every year (Dentener et al., 1996; Shen et al., 2013; Jaoui et al., 2008) and provide abundant surface area for the heterogeneous oxidation of SO2. The heterogeneous uptake of SO2 can form bisulfite (HSO3-) or sulfite (SO32-) on γ-Al2O3 and sulfate (SO42-) on MgO (Goodman et al., 2001b). Similarly, SO2 can be converted into sulfite, bisulfite, or sulfate on mineral dust such as metal oxides (Zhang et al., 2006), calcite, and Chinese loess (Usher et al., 2002). The heterogeneous reaction of SO2 on mineral dust can be promoted by gaseous oxidants. For example, SO2 could be oxidized into sulfate by O3 on the surface of CaCO3 particles (Li et al., 2006; Zhang et al., 2018). Similar results were obtained when introducing H2O2 into the heterogeneous oxidation system (Capaldo et al., 1999; Jayne et al., 1990). NO2 can also promote the heterogeneous oxidation of SO2. In our previous studies, it was found that SO2 was oxidized to sulfate on γ-Al2O3 in the presence of NO2 and O2, while it was only converted to sulfite in the absence of them (Ma et al., 2008). Therefore, NO2 was proposed to act as a catalyst in the oxidation of SO2 by O2, in which the intermediates observed in the spectra, i.e., nitrogen tetroxide (N2O4), might play an important role (Ma et al., 2008). This synergistic effect between SO2 and NO2 was further observed on many other mineral oxides such as CaO, α-Fe2O3, ZnO, MgO, α-Al2O3, and TiO2 (Liu et al., 2012; Ma et al., 2017; Zhao et al., 2018; Yu et al., 2018). These effects were confirmed in smog chamber studies and field observations of heavy haze in China, and they were proposed to be an important reason for the rapid growth of sulfate in haze events (He et al., 2014; Ma et al., 2018; Wang et al., 2014a; Chu et al., 2016). Heterogeneous oxidation of SO2 may also be affected by the coexistence of organic compounds. Pre-adsorption of acetaldehyde (CH3CHO) was found to suppress the heterogeneous reaction of large amounts of SO2 on the surface of α-Fe2O3 (Zhao et al., 2015), while HCHO was proposed to react with SO32- and generate hydroxymethanesulfonate (HMS) in the northern China winter haze period (Moch et al., 2018; Song et al., 2019). Wu et al. (2013) found that the synergistic effects between formic acid (HCOOH) and SO2 in the heterogeneous reaction on hematite provide a new source of sulfate.
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UV illumination can affect both the properties of particles and heterogeneous reactions on them (Nanayakkara et al., 2012; Cwiertny et al., 2008; George et al., 2015). The photooxidation of SO2 in the presence of mineral dust may represent an important pathway for generating sulfate aerosols (Park et al., 2017; Yu and Jang, 2018). TiO2, an n-type semiconductor material, has been widely used for studying heterogeneous photochemical reactions (Chen et al., 2012). TiO2 can be excited by UV light (λ<387 nm), resulting in electrons and holes that can react with O2 and H2O and produce •O2- and •OH, respectively. These reactive oxygen species (ROS), primarily •O2- and •OH, can participate in the heterogeneous oxidation of SO2 on TiO2 (Chen et al., 2012). Shang et al. (2010) studied the heterogeneous reaction of SO2 on TiO2 particles using in situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) and observed that SO2 was oxidized to sulfate on TiO2 with UV illumination while remaining as sulfite under dark conditions. Our recent study showed that O2 and H2O have contrary roles in the photooxidation of SO2 on TiO2, where surface water exhibits a competition effect in the reaction of SO2 due to the occupation of surface OH (Ma et al., 2019). Besides H2O, the coexistence of organics may also suppress the formation of sulfate due to competition with SO2 for reactive oxygen species. For example, Du et al. (2000) studied the photocatalytic reaction of SO2 in the presence of heptane (C7H16) and found that the formation of sulfate was suppressed.
Despite these studies involving the heterogeneous oxidation of SO2 under various conditions, the effects of coexisting pollutants on the heterogeneous oxidation of SO2 under both dark and illuminated conditions need further investigation. Meanwhile, the interactions between organic and inorganic species in these heterogeneous processes at low concentrations are not fully understood. In this study, we focus on the effects of coexisting NO2 and propene (C3H6) on the heterogeneous oxidation of SO2 on TiO2 under both dark and illuminated conditions with in situ DRIFTS. In order to better study the effects of NO2 and C3H6 on the heterogeneous oxidation in a relatively complex oxidation system (with coexistence of multiple gases, in both dark and illuminated conditions), we chose TiO2 due to the fact that it is a semiconductor material and a well-known photocatalyst. TiO2 has been widely reported to be present in airborne particulate matter (PM) (Chen et al., 2012). Although TiO2 represents only a relatively small portion of the mass of PM and is less abundant than CaO, Fe2O3, or MgO, the TiO2 particles are expected to provide important surfaces for heterogeneous photocatalysis of atmospheric gases due to their high photocatalytic activity, especially with the growing application of TiO2 in human activities (Chen et al., 2012). Propene is selected as a representative VOC since it is the most abundant alkene compound in the atmosphere, and it coexists with NOx in vehicle exhaust emissions (Wang et al., 2016a). Propene is widely used as an accelerator in photochemical reactions in some smog chamber studies (Jang and Kamens, 2001; Song et al., 2007). The relatively simple oxidation products and well understood oxidation mechanism of propene are also helpful in explaining our experimental results. Propene is selected also due to the high vapor pressure of its oxidation products, which normally do not generate condensed organic aerosol (Odum et al., 1996). However, we must point out that the heterogeneous reactivity depends greatly on the properties of the mineral oxides, such as acid-base nature or redox properties (Tang et al., 2016; Yang et al., 2016, 2019), while different VOCs may also have quite different heterogeneous and photochemical reactivity. Investigating these processes on different mineral dust and authentic dust particles with different types of VOCs is needed in future studies. Rather than UV lights, a xenon light is used in this study to better simulate the solar ultraviolet radiation on the earth’s surface. Generally, our study could be helpful for gaining a better understanding of the heterogeneous formation of sulfate under complex air pollution conditions, in which abundant SO2, NOx, VOCs, and mineral dust coexist in the atmosphere.
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