Rapid Iron-Mediated Aqueous-Phase Reactions of Organic Peroxides from Monoterpene-Derived Criegee Intermediates and Implications for Aerosol and Cloud Chemistry

Fenton-like reactions between organic peroxides and transition-metal ions in the atmospheric aqueous phase have profound impacts on the chemistry, composition, and health effects of aerosols. However, the kinetics, mechanisms, and key influencing factors of such reactions remain poorly understood. In this study, we synthesized a series of monoterpene-derived α-acyloxyalkyl hydroperoxides (AAHPs), an important class of organic peroxides formed from Criegee intermediates during the ozonolysis of alkenes, and investigated their Fenton-like reactions with iron ions in the aqueous phase. We found that the AAHPs are essentially chemically inert to Fe³⁺ but highly reactive toward Fe²⁺. The aqueous-phase reaction rate constant between AAHPs and Fe²⁺ (k^II_AAHP+Fe(II)) was determined to range between 11.0 ± 0.8 and 150.0 ± 3.3 M^-1 s^-1, depending positively on the solution pH (1-3), water content (50%-90%), and temperature (8-25 °C). Meanwhile, the k^II_AAHP+Fe(II) value is linearly correlated to the O/C ratio of AAHPs, which allows for the estimation of the Fenton-like reactivity of AAHPs based on their oxygenation level. In addition, the decomposition of AAHPs via Fenton-like reactions with Fe²⁺ predominantly yields alkoxy (RO) radicals with the production yield of OH radicals smaller than 16%. Similar to synthesized AAHPs, several abundant peroxides including the pinonic acid-derived AAHP exhibit high Fenton-like reactivity toward Fe²⁺ but low reactivity toward Fe³⁺ in dissolved α-pinene secondary organic aerosol. A quantitative analysis based on the measured kinetics suggests that Fenton-like reactions are important and even dominant drivers behind the transformation of AAHPs in the atmosphere, which would significantly affect atmospheric multiphase chemistry and aerosol health impacts.