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

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