Kinetics of Different Substituted Phenolic Compounds’ Aqueous OH Oxidation in Atmosphere

Atmospheric aqueous-phase reactions have been recognized as an important source of secondary organic aerosols (SOAs). However, the unclear reaction kinetics and mechanics hinder the in-depth understanding of the SOA sources and formation processes. This study selected ten different substituted phenolic compounds (termed as PhCs) emitted from biomass burning as precursors, to investigate the kinetics using OH oxidation reactions under simulated sunlight. The factors influencing reaction rates were examined, and the contribution of reactive oxygen species (ROS) was evaluated through quenching and kinetic analysis experiments. The results showed that the pseudo-first-order rate constants (kobs) for the OH oxidation of phenolic compounds ranged from 1.03 × 10−4 to 7.85 × 10−4 s−1 under simulated sunlight irradiation with an initial H2O2 concentration of 3 mM. Precursors with electron-donating groups (-OH, -OCH3, -CH3, etc.) exhibited higher electrophilic radical reactivity due to the enhanced electron density of the benzene ring, leading to higher reaction rates than those with electron-withdrawing groups (-NO2, -CHO, -COOH). At pH 2, the second-order reaction rate (kPhCs, OH) was lower than at pH 5. However, the kobs did not show dependence on pH. The presence of O2 facilitated substituted phenols’ photodecay. Inorganic salts and transition metal ions exhibited varying effects on reaction rates. Specifically, NO3− and Cu2+ promoted kPhCs, OH, Cl− significantly enhanced the reaction at pH 2, while SO42− inhibited the reaction. The kPhCs, OH were determined to be in the range of 109~1010 L mol−1 s−1 via the bimolecular rate method, and a modest relationship with their oxidation potential was found. Additionally, multiple substituents can suppress the reactivity of phenolic compounds toward •OH based on Hammett plots. Quenching experiments revealed that •OH played a dominant role in phenolic compound degradation (exceeding 65%). Electron paramagnetic resonance confirmed the generation of singlet oxygen (1O2) in the system, and probe-based quantification further explored the concentrations of •OH and 1O2 in the system. Based on reaction rates and concentrations, the atmospheric aqueous-phase lifetimes of phenolic compounds were estimated, providing valuable insights for expanding atmospheric kinetic databases and understanding the chemical transformation and persistence of phenolic substances in the atmosphere.