Abstract
Abstract. The heterogeneous reactions of SO2 in the presence of NO2 and C3H6 on TiO2 were investigated with the aid of in situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) under dark conditions or with UV–Vis irradiation. Sulfate formation with or without the coexistence of NO2 and/or C3H6 was analyzed with ion chromatography (IC). Under dark conditions, SO2 reacting alone resulted in sulfite formation on TiO2, while the presence of parts per billion (ppb) levels of NO2 promoted the oxidation of SO2 to sulfate. The presence of C3H6 had little effect on sulfate formation in the heterogeneous reaction of SO2 but suppressed sulfate formation in the heterogeneous reaction of SO2 and NO2. UV–Vis irradiation could significantly enhance the heterogeneous oxidation of SO2 on TiO2, leading to copious generation of sulfate, while the coexistence of NO2 and/or C3H6 significantly suppressed sulfate formation in experiments with UV–Vis lights. Step-by-step exposure experiments indicated that C3H6 mainly competes for reactive oxygen species (ROS), while NO2 competes with SO2 for both surface active sites and ROS. Meanwhile, the coexistence of NO2 with C3H6 further resulted in less sulfate formation compared to introducing either one of them separately to the SO2–TiO2 reaction system. The results of this study highlighted the complex heterogeneous reaction processes that take place due to the ubiquitous interactions between organic and inorganic species and the need to consider the influence of coexisting volatile organic compounds (VOCs) and other inorganic gases in the heterogeneous oxidation kinetics of SO2.
Highlights
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)
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 diffuse reflectance infrared fourier transform spectroscopy (DRIFTS)
In situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) spectra were recorded on a Nicolet Nexus 670 Fourier transform infrared spectroscope (FTIR) equipped with a mercury cadmium telluride (MCT) detector, scanning from 4000 to 650 cm−1 at a resolution of 4 cm−1 for 100 scans
Summary
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 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 ob-
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