Unveiling the Role of Chirality in the Oxidation of Monoterpenes

Monoterpene oxidation is a major contributor to the formation and growth of a secondary organic aerosol (SOA) in the atmosphere. Although most monoterpenes naturally exist in two enantiomeric forms, the impact of their molecular chirality on SOA formation has been neglected, in part due to the hypothesis that a precursor's chirality will not impact gaseous oxidation processes when considering achiral oxidants (e.g., O₃, OH radicals). Within this study, we systematically investigated the gas-phase oxidation of the enantiomers of two common monoterpenes, (±)-α-pinene and (±)-limonene, at atmospherically relevant concentrations. We demonstrate greater formation (15 ± 0.3% and 19 ± 0.2%) of oxidized organic molecules (OOMs) for (+)-α-pinene and (+)-limonene, respectively, compared to their (-) enantiomers. We propose these observations are due to differences in key intermediate steps within the oxidation process that correlate with RO₂ lifetime and bimolecular reactions. For example, oxidation of (+)-limonene promotes the formation of alkoxy radicals, which predominately undergo isomerization and subsequent radical propagation, leading to enhanced OOM production. In contrast, (-)-limonene oxidation favors the scission of the carbon skeleton, producing more volatile products. The observed differences in product yields between the enantiomer populations are expected to have atmospheric repercussions on SOA formation due to shifting enantiomer compositions from vegetation stress events (e.g., drought due to climate change) and the use of volatile chemical products.