Direct probing of acylperoxy radicals during ozonolysis of <i>α</i>-pinene: constraints on radical chemistry and production of highly oxygenated organic molecules

Acylperoxy radicals (RO2) are key intermediates in the atmospheric oxidation of organic compounds and different from the general alkyl RO2 radicals in reactivity. However, direct probing of the molecular identities and chemistry of acyl RO2 remains quite limited. Here, we report a combined experimental and kinetic modeling study of the composition and formation mechanisms of acyl RO2, as well as their contributions to the formation of highly oxygenated organic molecules (HOMs) during ozonolysis of α-pinene. We find that acyl RO2 radicals account for 67 %, 94 %, and 32 % of the highly oxygenated C7, C8, and C9 RO2, respectively, but only a few percent of C10 RO2. The formation pathway of acyl RO2 species depends on their oxygenation level. The highly oxygenated acyl RO2 (oxygen atom number ≥6) are mainly formed by the intramolecular aldehydic H shift (i.e., autoxidation) of RO2, while the less oxygenated acyl RO2 (oxygen atom number &lt;6) are basically derived from the C–C bond cleavage of alkoxy (RO) radicals containing an α-ketone group or the intramolecular H shift of RO containing an aldehyde group. The acyl-RO2-involved reactions explain 50 %–90 % of C7 and C8 closed-shell HOMs and 14 % of C10 HOMs, respectively. For C9 HOMs, this contribution can be up to 30 %–60 %. In addition, acyl RO2 contribute to 50 %–95 % of C14–C18 HOM dimer formation. Because of the generally fast reaction kinetics of acyl RO2, the acyl RO2 + alkyl RO2 reactions seem to outcompete the alkyl RO2 + alkyl RO2 pathways, thereby affecting the fate of alkyl RO2 and HOM formation. Our study sheds lights on the detailed formation pathways of the monoterpene-derived acyl RO2 and their contributions to HOM formation, which will help to understand the oxidation chemistry of monoterpenes and sources of low-volatility organic compounds capable of driving particle formation and growth in the atmosphere.