Aqueous photochemistry of brown carbon: H2O2 formation, enhanced SO2 oxidation, and the real role of excited triplets

Field observations highlight high H 2 O 2 level and enhanced sulfate production during haze episodes in China, but so far, the known H 2 O 2 sources and traditional secondary sulfate formation mechanisms cannot elucidate these phenomena. In recent years, the atmospheric photosensitized multiphase oxidation of SO 2 to trigger sulfate production has attracted great attention. However, few reports on the contributions of triplet excited states and its secondary reactive oxygen species (ROS) formed in photosensitization reaction to sulfate formation. In this study, the aqueous photochemistry of water-soluble humic-like substances (HULIS) proxy was further investigated. It was found that hydroxyl radicals and peroxyl radicals were produced after 313 nm UV irradiation, accompanied by H 2 O 2 production. The effect of HULIS photosensitization on SO 2 oxidation was also investigated. The results showed that the triplet excited states of HULIS ( 3 HULIS∗) cannot directly and effectively oxidize SO 2 to form sulfate, but the formation of sulfate was realized by the oxidation of ROS generated by the secondary reaction of 3 HULIS∗ in the presence of O 2 and hydrogen atom donors such as HULIS itself. These findings clarify the misunderstood role of the 3 HULIS∗ in the photosensitized oxidation of SO 2 , and the study helps better understand atmospheric photosensitization processes.