Insights into the Formation Mechanism of Reactive Oxygen Species in the Interface Reaction of SO 2 on Hematite.

The interplay between sulfur and iron holds significant importance in their atmospheric cycle, yet a complete understanding of their coupling mechanism remains elusive. This investigation delves comprehensively into the evolution of reactive oxygen species (ROS) during the interfacial reactions involving sulfur dioxide (SO 2 ) and iron oxides under varying relative humidity conditions. Notably, the direct activation of water by iron oxide was observed to generate a surface hydroxyl radical (•OH). In comparison, the aging of SO 2 was found to markedly augment the production of •OH radicals on the surface of α-Fe 2 O 3 under humid conditions. This augmentation was ascribed to the generation of superoxide radicals (•O 2 - ) stemming from the activation of O 2 through the Fe(II)/Fe(III) cycle and its combination with the H + ion to produce hydrogen peroxide (H 2 O 2 ) on the acidic surface. Moreover, the identification of moderate relative humidity as a pivotal factor in sustaining the surface acidity of iron oxide during SO 2 aging underscores its crucial role in the coupling of iron dissolution, ROS production, and SO 2 oxidation. Consequently, the interfacial reactions between SO 2 and iron oxides under humid conditions are elucidated as atmospheric processes that enhance oxidation capacity rather than deplete ROS. These revelations offer novel insights into the mechanisms underlying •OH radical generation and oxidative potential within atmospheric interfacial chemistry.