Photosensitized Reactions in Atmospheric Water: Triplet State Models Molecules, Reactivity, and Impact on the Carboxylic Acids and Amino Acids Fate

Abstract

Recent studies have demonstrated that new reactions can be initiated by the irradiation of organic compounds in the atmospheric aqueous phase. In this medium, these photosensitized reactions represent an important, still understudied pathway affecting the composition of tropospheric organic matter. In this work, we investigated the photoreactivity of vanillic acid (VA) and 2-benzoylbenzoic acid (A2BB), as relevant proxies for sources of triplet excited states (VA* and A2BB*). Experiments conducted under various pH conditions revealed different reactivities toward molecular oxygen which is an important source of singlet oxygen with rate constants $k_{triplet,3_{O_2}}^{″}$ ranging from 6.35 × 10⁸ to 4.31 × 10⁹ M^–1 s^–1 for VA* and A2BB*. The reactivity with short-chain carboxylic acids and amino acids, which are regularly found in the atmospheric aqueous phase, was also studied. Rate constants of these reactions varied from less than 10⁷ to 7.1 × 10⁹ M^–1 s^–1, suggesting that the reactivity with triplet excited states has to be considered important in some cases, in competition with that of the hydroxyl radical. In addition, the transformation of carboxylic acids and amino acids induced by triplet excited states was investigated, providing insights into the reactivity of these states and future consideration of the transformation pathways into atmospheric chemistry models.