Monoterpene oxidation pathways initiated by acyl peroxy radical addition

Abstract. Monoterpenes are released into the atmosphere in significant quantities, where they undergo various oxidation reactions. Despite extensive studies in this area, there are still gaps that need to be addressed to fully understand the oxidation processes occurring in the atmosphere. Recent findings suggest that reactions between alkenes and acyl peroxy radicals can be competitive under atmospheric conditions. In this study, we investigate the oxidation reactions of seven monoterpenes with acyl peroxy radicals and the subsequent diverse reactions, including accretion, alkyl radical rearrangements, H-shift, and β-scissions reactions. The accretion reaction leads to the release of excess energy, which is sufficient to open small secondary rings in the alkyl radical structures. This reaction is most significant for sabinene (39 %) and α-thujene (20 %). A competing reaction is O₂ addition, which the majority of alkyl radicals undergo, subsequently leading to the formation of peroxy radicals. These then react further, forming alkoxy radicals that can subsequently undergo β-scission reactions. Our calculations show that for β-pinene, camphene, and sabinene, β-scission rearrangements result in radicals capable of undergoing further propagation of the oxidative chain, while for limonene, α-pinene and α-thujene, scissions leading to closed-shell products that terminate the oxidative chain are preferred. The results indicate that if reactions of monoterpenes with acyl peroxy radicals are indeed competitive under atmospheric conditions, their oxidation would lead to more oxygenated compounds with a higher molar mass, potentially contributing to secondary organic aerosol yields. Moreover, this study highlights the significance of stereochemistry in controlling the oxidation of monoterpenes initiated by acyl peroxy radicals.