Reactive species-mediated phototransformation of triphenyl phosphate in cloud water: Kinetics, pathways and implications

Cloud water serves as crucial media for the transformation of organic compounds in the atmosphere, primarily through the oxidation of various reactive species. Triphenyl phosphate (TPHP) is one of the major organophosphate esters (OPEs) and is widely used in plasticizers and flame retardants, which has attracted extensive attention due to its toxicity and frequent occurrence in the atmosphere. However, its transformation pathway after partition into cloud water, especially mediated by reactive species, has not been reported. In this study, the kinetics, pathways, and atmospheric implications of TPHP transformation by hydroxyl radical (^•OH), sulfate radical (SO₄^•-), and triplet excited states of organic matters (³C∗) were investigated by using hydrogen peroxide (H₂O₂), persulfate (PDS), and 2-benzoylbenzoic acid (2-BBA) as precursors, respectively. Second-order rate constants of TPHP with these species were determined as k_TPHP,_•_OH = (7.20 ± 0.48) × 10⁹ M^-1 s^-1, [Formula: see text] = (5.99 ± 1.20) × 10⁹ M^-1 s^-1, k_TPHP,3C*=(1.73±1.50)×10⁹M^-1s^-1. Intermediate product analysis revealed the degradation pathway of TPHP, and short-chain carboxylic acids and phosphate ions were generated through the efficient oxidation of ^•OH. The contributions of ^•OH, SO₄^•-, and ³C∗ to the TPHP transformation in three typical cloud water environments (urban, remote, and marine) were evaluated by the pseudo-first-order rate. With the assessed TPHP lifetime of 94 min in urban areas, this study highlighted the dominant transformation process of organic contaminants in the atmospheric aqueous phase.