Direct aqueous photochemistry of methylglyoxal and its effect on sulfate formation.

In recent years, among many oxidation pathways studied for atmospheric sulfate formation, the aqueous phase oxidation pathways of HO and organic hydroperoxides (ROOHs) have attracted great scientific attention. Higher concentrations of HO and ubiquitous ROOHs have been observed in atmospheric aqueous phase environments (cloud water, fog droplets, etc.). However, there are still some gaps in the study of their aqueous phase generation and their influences on sulfate formation. In this study, the aqueous phase photochemical reaction of methylglyoxal, a ubiquitous organic substance in the atmospheric aqueous phase, was studied under UV irradiation, and the generation of HO and ROOHs in this system was investigated. It is found for the first time that the aqueous phase photolysis of methylglyoxal not only produces HO but also produces ROOHs, and UV light and O are necessary for the formation of HO and ROOHs. Based on the experimental results, the possible mechanism of aqueous phase photochemistry of methylglyoxal and the generation of HO and ROOHs were proposed. The effect of aqueous phase photolysis of methylglyoxal on sulfate formation under different conditions was also investigated. The results show that the aqueous phase photolysis of methylglyoxal significantly promoted SO oxidation and sulfate formation, in which SO oxidation was realized by the generated HO, ROOHs and •OH radicals, and the importance of the formed ROOHs cannot be ignored. These results fill some gaps in the field of aqueous phase HO and ROOHs production, and to a certain extent confirm the important roles of the aqueous phase photolysis of methylglyoxal and the formed HO and ROOHs in the production of sulfate. The study reveals the new sources of HO and ROOHs, and provides a new insight into the heterogeneous aqueous phase oxidation pathways and mechanisms of SO in cloud and fog droplets and haze particles.