Evidence for hydroperoxide formation through atmospheric aqueous photochemistry of α-keto acids

Hydroperoxides (HPs) are potent oxidants that exert a substantial impact on the atmospheric oxidizing capacity. However, a fundamental understanding of the in situ formation of HPs in atmospheric waters remains incomplete and controversial. Here, we report direct observations of HP (hydrogen peroxide, alkyl HP, and peroxycarboxylic acid) formation with quantum yields 10-4 - 10-3, through photochemistry of α-keto acids [α-KAs; glyoxylic acid, pyruvic acid (PA), and 2-ketobutyric acid]. These quantum yields are comparable to reported HP formation rates in cloud and fog waters, suggesting that α-KA photochemistry could contribute 5 to 15% of the observed atmospheric hydrogen peroxide in aqueous phases, based on typical α-KA concentrations (0.1 to 10 μM). The model results showed that the photochemistry of excited triplet 3PA* contributes ~40 and ~33%, respectively, to the formation of hydrogen peroxide and peroxyacetic acid at pH 2.5. Given the recurrent detection of α-KAs in organic-rich aerosols, this photochemistry represents a notable in situ source of HPs.