Mechanism of Gas-Phase Ozonolysis of β-Myrcene in the Atmosphere.

Abstract

β-Myrcene is one of the major monoterpenes in the atmosphere. Ozonolysis is one of the oxidation removal processes for myrcene in the atmosphere during the daytime. In this study, the mechanism of myrcene ozonolysis is studied using high-level quantum chemistry calculations and kinetic calculations. The reaction starts with the formation of primary ozonides (POZs), which decompose to three primary product channels as 4-vinyl-4-pentenal (4V4P) + (CH3)2OO and two Criegee intermediates ( anti- and syn-My-CIs) + acetone with branching ratios of 0.73, 0.23, and 0.04, respectively. The Criegee intermediates (CIs) are formed with high excitation, and they could either isomerize promptly or be thermalized to stabilized CIs (sCIs) at fractions of 0.39, 0.23, and 0.25 for (CH3)2COO*, anti-My-CI*, and syn-My-CI*. For stabilized My-CIs, syn-My-CI would undergo unimolecular H-migration to a vinyl hydroperoxide (VHP), which decomposes readily to OH radical and a vinoxy-type radical, while anti-My-CI would most likely react with water vapor or cyclize to a bicyclic compound when relative humidity is low. Reaction of My-CIs with SO2 is important only for conditions of low temperatures (<273 K) and low relative humidity. The predicted yields of 0.73 and 0.27 for 4V4P and acetone agree reasonably with previous experimental measurements.