The formation mechanism of highly oxygenated organic molecules produced by toluene in the urban atmosphere

Abstract

Highly oxygenated organic molecules (HOMs) with low volatility from atmospheric toluene oxidation play an important role in new particle formation and contribute significantly to secondary organic aerosols in cities. In this work, quantum chemical calculations were performed to study the formation mechanism of HOM monomers and dimers from toluene oxidation initiated by OH radicals. Under 0.4 ppbv NO and 40 pptv HO2 radical conditions, the formed initial mono- and multi–OH–substituted organic peroxyl radicals (RO2) first undergo intramolecular H-shift and cyclization reactions to promote autoxidation, which is followed by bimolecular reactions of highly oxidized RO2 with NO and HO2 radical, resulting in the formation of HOM monomers. It implies that high NO concentration hardly inhibits the formation of RO2 with a low O:C ratio in the aforementioned processes, but mainly intercepts the formation of RO2 with a high O:C ratio. Except for the self- and cross-reaction of RO2, the addition reactions of RO2 and acyl peroxy radicals (RC(O)O2) with toluene-derived alkenes (RO2 + alkene and RC(O)O2 + alkene reactions) have sufficiently low potential barriers, which are non-ignorable pathways for the HOM dimers formation. The present work expands our knowledge of the mechanism of HOMs formation from the oxidation of atmospheric aromatic hydrocarbons.