Aqueous Photochemistry of Pyruvic Acid: H2O2 Formation and Enhanced SO2-to-Sulfate Conversion

Abstract Secondary organic aerosols (SOAs) influence atmospheric chemistry and climate, yet the contributions of small-molecular organic acids in SOAs to atmospheric oxidation remain unclear. Pyruvic acid (PA), a ubiquitous secondary ketoacid in both gas and aqueous phases, has been widely studied for aqueous photolysis, with most research focusing on SOA formation and influencing factors. However, their roles in atmospheric H2O2 production and sulfate aerosol formation have received limited attention. Here, we investigate aqueous PA photochemistry and its influence on SO2 oxidation. In the presence of O2, ultraviolet (UV) light, visible light, and sunlight all initiated PA photolysis and produced reactive oxygen species (ROS), including H2O2. The generated H2O2 was directly linked to enhanced aqueous SO2 oxidation and sulfate production in our experimental systems. Control experiments further show that the triplet excited state 3PA* cannot make a dominant contribution to sulfate formation, as it cannot directly and effectively oxidize S(IV) species. Instead, SO2 oxidation is mainly driven by the ROS produced by the secondary reactions of 3PA* in the presence of O2 and PA. These findings reveal that the aqueous PA photochemistry can produce H2O2 and enhance sulfate formation, suggesting a possible atmospheric oxidizing contribution warranting further investigation in cloud, fog, and aerosol systems.