Theoretical study of the reaction mechanisms, kinetics, and toxicity of acetyl peroxyl and hydroperoxyl radicals: Implications for atmospheric chemistry

Abstract

Acetyl peroxyl radical (CH3C(O)O2) is a key intermediate in the atmospheric oxidation of volatile organic compounds (VOCs). In clean regions (low NO), reactions with the hydroperoxyl radical (HO2) dominate the fate and depletion of the CH3C(O)O2 radical. However, the detailed atmospheric reaction mechanisms and kinetics of the CH3C(O)O2 and HO2 radicals are still not fully understood. Therefore, in this study, the reaction mechanism and kinetics were investigated by quantum chemical calculations and chemical kinetics methods, and the ecotoxicity of the reaction precursors and products was evaluated using structure-activity relationships. In the atmosphere, the CH3C(O)O2 radical primarily exists mainly as Cis-CH3C(O)O2 and Trans-CH3C(O)O2 isomers, both of which contribute to reactions with the HO2 radical. The reaction of the CH3C(O)O2 radical with the HO2 radical follows two distinct mechanisms: (i) a hydrogen transfer mechanism on the triplet state potential energy surface (PES) and (ii) an addition-decomposition mechanism on the singlet state PES. The predicted apparent rate constant is 9.44 × 10-12 cm3 molecule−1 s−1 at 290 K and 1 atm, which is in good agreement with the experimental data. Kinetic analyses indicate that the dominant reaction pathways and product distributions vary significantly with temperature. In forested regions, where the HO2 radical concentrations are typically high, the combination of elevated HO2 radical levels and lower temperatures during winter promotes the degradation of the CH3C(O)O2 radical into CH3COOH and O3. This process impacts both the acidity and the oxidative capacity of the atmosphere. However, during summer, higher temperatures extend the atmospheric lifetime of the CH3C(O)O2 radical, favoring their reaction with the HO₂ radical to form OH radicals. This may help explain the observed higher concentrations of the CH3C(O)O2 radical and the OH radical in tropical rainforests. Ecotoxicity results indicate that some of the precursors of the acetyl peroxyl radical are potentially ecotoxic, but they can react with the HO2 radical and subsequently degrade into environmentally friendly compounds. Overall, this study provides a comprehensive theoretical understanding of the atmospheric behavior and ecological impact of the CH3C(O)O2 radical, providing a basis for predicting its reactions under diverse atmospheric conditions and supporting atmospheric modeling and environmental management.