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

Abstract

The acetyl peroxyl radical (CH₃C(O)O₂) is a key intermediate in the atmospheric oxidation of volatile organic compounds (VOCs). In clean regions (low NO_x), reactions with the hydroperoxyl radical (HO₂) dominate the fate 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 using 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 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 and lower winter temperatures promotes the conversion 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 the reaction with the HO₂ radical to form the OH radical. 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. However, after conversion to the CH₃C(O)O₂ radical and subsequent reaction with the HO₂ radical, they 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, offering a basis for predicting its reactions under diverse atmospheric conditions and supporting atmospheric modeling and environmental management.