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

Acylperoxy radicals (RO.sub.2) are key intermediates in the atmospheric oxidation of organic compounds and different from the general alkyl RO.sub.2 radicals in reactivity. However, direct probing of the molecular identities and chemistry of acyl RO.sub.2 remains quite limited. Here, we report a combined experimental and kinetic modeling study of the composition and formation mechanisms of acyl RO.sub.2, as well as their contributions to the formation of highly oxygenated organic molecules (HOMs) during ozonolysis of α-pinene. We find that acyl RO.sub.2 radicals account for 67 %, 94 %, and 32 % of the highly oxygenated C.sub.7, C.sub.8, and C.sub.9 RO.sub.2, respectively, but only a few percent of C.sub.10 RO.sub.2 . The formation pathway of acyl RO.sub.2 species depends on their oxygenation level. The highly oxygenated acyl RO.sub.2 (oxygen atom number â¥6) are mainly formed by the intramolecular aldehydic H shift (i.e., autoxidation) of RO.sub.2, while the less oxygenated acyl RO.sub.2 (oxygen atom number