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

Acylperoxy radicals (inline-formulaRO₂) are key intermediates in the atmospheric oxidation of organic compounds and different from the general alkyl inline-formulaRO₂ radicals in reactivity. However, direct probing of the molecular identities and chemistry of acyl inline-formulaRO₂ remains quite limited. Here, we report a combined experimental and kinetic modeling study of the composition and formation mechanisms of acyl inline-formulaRO₂, as well as their contributions to the formation of highly oxygenated organic molecules (HOMs) during ozonolysis of inline-formulaα-pinene. We find that acyl inline-formulaRO₂ radicals account for 67 %, 94 %, and 32 % of the highly oxygenated inline-formulaC₇, inline-formulaC₈, and inline-formulaC₉inline-formulaRO₂, respectively, but only a few percent of inline-formulaC₁₀inline-formulaRO₂. The formation pathway of acyl inline-formulaRO₂ species depends on their oxygenation level. The highly oxygenated acyl inline-formulaRO₂ (oxygen atom number inline-formula≥6) are mainly formed by the intramolecular aldehydic H shift (i.e., autoxidation) of inline-formulaRO₂, while the less oxygenated acyl inline-formulaRO₂ (oxygen atom number inline-formula<6) are basically derived from the C–C bond cleavage of alkoxy (RO) radicals containing an inline-formulaα-ketone group or the intramolecular H shift of RO containing an aldehyde group. The acyl-inline-formulaRO₂-involved reactions explain 50 %–90 % of inline-formulaC₇ and inline-formulaC₈ closed-shell HOMs and 14 % of inline-formulaC₁₀ HOMs, respectively. For inline-formulaC₉ HOMs, this contribution can be up to 30 %–60 %. In addition, acyl inline-formulaRO₂ contribute to 50 %–95 % of inline-formulaC₁₄–inline-formulaC₁₈ HOM dimer formation. Because of the generally fast reaction kinetics of acyl inline-formulaRO₂, the acyl inline-formulaRO₂inline-formula+ alkyl inline-formulaRO₂ reactions seem to outcompete the alkyl inline-formulaRO₂inline-formula+ alkyl inline-formulaRO₂ pathways, thereby affecting the fate of alkyl inline-formulaRO₂ and HOM formation. Our study sheds lights on the detailed formation pathways of the monoterpene-derived acyl inline-formulaRO₂ 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.