Mechanism of gas-phase ozonolysis of sabinene in the atmosphere

Abstract

Sabinene is one of the monoterpenes of biogenic origin in the atmosphere. Ozonolysis is one of the important oxidation removal reactions for sabinene in the atmosphere during the daytime, leading to the formation of secondary organic aerosols. In this study, we investigate the mechanism of gas-phase ozonolysis of sabinene using quantum chemistry and kinetic calculations. The reaction starts with the formation of four primary ozonides (POZs), which decompose to primary product channels CH₂OO + sabinaketone and CH₂O + two Criegee intermediates (CI-1 and CI-2) with branching ratios of 17%, 45%, and 38%, respectively, at 298 K and 760 Torr. Calculations showed that the stabilized CI-1 would undergo a rapid intramolecular H-shift to a vinyl hydroperoxide (VHP) at a rate of ∼2700 s⁻¹ followed by rapid decomposition to an OH radical and a vinoxy-type radical (VTR) and CI-2 would slowly isomerize to dioxirane at a rate of 0.97 s⁻¹. In the atmosphere, CI-2 would instead react with water and the water dimer, forming α-hydroxyalkyl hydroperoxides (αHAHPs), which would decompose to sabinaketone and H₂O₂via heterogeneous processes. The reaction of CI-2 with SO₂ would also be significant in a dry and cold atmosphere. The yield of sabinaketone of 47%, from primary POZ decomposition and secondary reactions of αHAHPs, agrees with the previously measured values of 35–50%. The OH radical, formed from CI-1, could reach 44%, agreeing with the previously reported value of (33 ± 6)%. Further reaction of the VTR radical would form highly-oxidized multifunctional products containing carboxylic and/or carbonyl groups which might contribute substantially to SOA formation.