Evidence of Formation of Bicyclic Species in the Early Stages of Atmospheric Benzene Oxidation

Abstract

The oxidation of aromatic compounds in the troposphere substantially contributes to the formation of O(3) and secondary aerosol on a regional scale. Nevertheless, the initial stages of aromatic oxidation remain poorly understood. In this work, we present a quantitative analysis of previous experimental measurements relevant to atmospheric benzene oxidation. Using results from G3X(MP2), G3X(MP2)-RAD, CASSCF, and CASPT2 electronic structure theory, we have performed master equation (ME) calculations examining the kinetics of the benzene-OH adduct in the presence of O(2). Our results show the system to be complicated, with four isomers that may be formed following O(2) addition giving rise to multiple decay time scales of the benzene-OH adduct. We have examined the available experimental data in line with our findings and performed a sensitivity analysis of the agreement between the experimental and calculated kinetics with respect to uncertainties in the calculated stationary point energies. Our mechanism gives a phenol yield of 0.55 to 0.65, with the remainder giving a cis bridged bicyclic peroxy radical. Under atmospheric conditions, the epoxide yield is small. Distinct from the TST approaches and free energy surfaces available in previous studies, analysis of our ME results shows that several of the reactions occurring in this system are not at the high-pressure limit in the atmosphere.