Acylperoxy radicals during ozonolysis of α-pinene: composition, formation mechanism, and contribution to the production of highly oxygenated organic molecules

Abstract. Acylperoxy radicals (RO₂) are key intermediates in 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 modelling 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 < 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 C_14–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.