Ring-cleavage Mechanisms of Toluene in Aqueous Photooxidation

Low water-soluble organic compounds (LWOCs) exist ubiquitously in atmospheric aqueous phases including clouds, fog, and wet aerosols. Recent research has shown that LWOCs may be involved in aqueous-phase reactions dramatically both in the bulk and at the air-water interface of aqueous phases. However, the role of water in these reactions is still unclear. This study investigated the aqueous-phase photooxidation of toluene by OH radicals as a case to explore the influence of water on reaction mechanisms of LWOCs. The ring-cleavage pathway through the C7 bicyclic peroxy radical (BPR) dominates the aqueous-phase oxidation of toluene, similar to that in the gas-phase oxidation of toluene. However, the detailed ring-cleavage pathway changes significantly in the presence of water leading to the declined formation of -dicarbonyls and the enhanced formation of formic acid and acetic acid as primary ring-cleavage products. We suggest this is mainly due to the influence of water on the radical chemistry in the toluene oxidation process. Most importantly, water helps the formation of BPR with different structures from that in the gas-phase reaction. In the aqueous phase, the self-reaction of BPR is also favored over its reaction with NO to produce ring-cleavage products due to the water cage effect. In addition to BPR, the fate of other radicals is also influenced by water, such as acyl radicals which are generated from aqueous-phase reactions of BPR. They can rapidly hydrolysis and go through reactions different from that in the gas phase. With the suggested ring-cleavage mechanisms from this study, the box model can simulate the aqueous-phase product distributions of toluene better than that with the classical ring-cleavage mechanisms. Given the significant influence of water on reaction mechanisms, the importance of aqueous-phase reactions of LWOCs are underestimated before and more relevant studies are essential.