Unpacking the diversity of monoterpene oxidation pathways via nitrooxy–alkyl radical ring-opening reactions and nitrooxy–alkoxyl radical bond scissions

Abstract

Terrestrial vegetation emits vast quantities of monoterpenes to the atmosphere. These compounds, once oxidized, can contribute to the formation and growth of secondary organic aerosol (SOA) particles. However, studies report widely different SOA yields from atmospheric oxidation of different monoterpenes, despite their structural similarities. The NO₃-radical-initiated oxidation of $\alpha$-pinene for instance, leads to minimal SOA yields, whereas with $\Delta$-carene a high SOA yield is observed. A previous study indicated that their oxidation mechanisms diverge after formation of a nitrooxy–alkoxyl radical intermediate, whose C–C bond scission reactions can either lead to early termination of the oxidative chain, thus limiting the yield of condensable vapors, or further propagate it, leading to low-volatility products. In this study, we employ computational methods to investigate these reactions in the NO₃-radical oxidation of five other monoterpenes: limonene, sabinene, $\beta$-pinene, $\alpha$-thujene and camphene. Additionally, we explore the possibility of rearrangement via ring-opening of the nitrooxy-alkyl radical adducts produced immediately after NO₃ radical attack. Our calculations predict that alkyl radical rearrangement is dominant over O₂-addition for sabinene, minor but competitive for $\alpha$-thujene and $\beta$-pinene, and negligible for camphene. These rearrangements can induce further propagation of the oxidative chain, and thus higher SOA yields. Concerning alkoxyl radical C–C bond scissions, our results indicate that endocyclic nitrate species (derived from limonene and $\alpha$-thujene) react preferentially via the channel leading to oxidative chain termination, whereas exocyclic nitrate species (sabinene, $\beta$-pinene, and camphene) react preferentially via channels leading to further propagation.