Effects of NO<sub>2</sub> and C<sub>3</sub>H<sub>6</sub> on the heterogeneous oxidation of SO<sub>2</sub> on TiO<sub>2</sub> in the presence or absence of UV irradiation

Abstract

<p><strong>Abstract.</strong> The heterogeneous reactions of SO<sub>2</sub> in the presence of NO<sub>2</sub> and C<sub>3</sub>H<sub>6</sub> on TiO<sub>2</sub> were investigated with the aid of <i>in situ</i> DRIFTS under dark conditions or with UV irradiation. Sulfate formation with or without the coexistence of NO<sub>2</sub> and/or C<sub>3</sub>H<sub>6</sub> was analyzed with IC. Under dark conditions, SO<sub>2</sub> reacting alone resulted in sulfite formation on TiO<sub>2</sub>, while the presence of ppb levels of NO<sub>2</sub> promoted the oxidation of SO<sub>2</sub> to sulfate. The presence of C<sub>3</sub>H<sub>6</sub> had little effect on sulfate formation in the heterogeneous reaction of SO<sub>2</sub> but suppressed sulfate formation in the heterogeneous reaction of SO<sub>2</sub> and NO<sub>2</sub>. UV irradiation could significantly enhance the heterogeneous oxidation of SO<sub>2</sub> on TiO<sub>2</sub>, leading to a copious generation of sulfate, while the coexistence of NO<sub>2</sub> and/or C<sub>3</sub>H<sub>6</sub> significantly suppressed sulfate formation in experiments with UV lights. Step-by-step exposure experiments indicated that C<sub>3</sub>H<sub>6</sub> mainly competes for reactive oxygen species (ROS), while NO<sub>2</sub> competes with SO<sub>2</sub> for both surface active sites and ROS. Meanwhile, the coexistence of NO<sub>2</sub> with C<sub>3</sub>H<sub>6</sub> further resulted in less sulfate formation compared to introducing either one of them separately to the SO<sub>2</sub>-TiO<sub>2</sub> 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 requirement to consider the influence of coexisting VOCs and other inorganic gases in the heterogeneous oxidation kinetics of SO<sub>2</sub>.</p>

Origin and Transformation of Ambient VOCs during a Dust-to-Haze Episode in Northwest China

“…In addition, during this transition period, much lower ratios of T/B and X/E were observed in comparison with those in other periods (as mentioned in the part 3.1.2). We propose the possibility that windblown dust which include sustainable TiO2 can influence the atmospheric photochemistry of VOCs, which would accelerate the oxidation of ambient VOCs (Chu et al 2019;Nie et al, 2014).…”

“…In detail, content of Fe increased from 19.3 µg m -3 in clear days to 40.8 µg m -3 in dust pollution days, and the content of Ti increased from 0.92 to 2.98 µg m -3 . Hence, huge increase of the Ti and Fe concentrations in particulate phase during the period of dust pollution days could possibly promote the gas-solid photochemical reaction of the ambient VOCs, which would reasonably ascribe the relative low level and aged composition of ambient VOCs in this period (Chu et al, 2019;He et al, 2014;Song et al, 2019).…”

“…Co-existence heterogeneous photochemical reactions of SO2, NO2 and VOCs on the surface of mineral dust were investigated in recent years. Both synergistic and suppress effects of VOCs on the formation of sulfate were found, which indicated the competition of reactive oxygen species and active sites between VOCs and inorganic gas pollutants (Chu et al, 2019;Song et al, 2019). In addition, oxidized products, like formate and acetate species, were observed in the co-existence reaction, which highlight the possibility of further oxidation of VOCs on the mineral dust (He et al, 2014).…”

“…Solid-gas heterogeneous reactions would cause the transformation of gaseous pollutants and change the property of particles (Zhang et al, 2000;Zhang et al, 2003;He et al, 2014). Recently, the oxidation of organic and inorganic gas on particles surface through the transitional-metal-catalyzed chain reaction was frequently found to play important roles on the transformation of ambient gas pollutants (Chu et al, 2019). Mineral dust is the most important sources of the transitionalmetal, like Iron (Fe) and titanium (Ti), in the natural environment (Chen et al, 2012).…”