Spectroscopic and kinetic investigation of α-hydroxyalkylperoxyl radicals in the aqueous phase

Organic peroxyl radicals (RO₂˙) are crucial intermediates in atmospheric oxidation processes, driving the degradation of volatile organic compounds (VOCs), radical cycling, and secondary product formation. In aqueous environments such as cloud droplets and aerosol water, α-hydroxyalkylperoxyl radicals can significantly influence hydroperoxyl radical (HO₂˙) production and, hence, multiphase oxidation pathways. In the present study, the UV spectra of hydroxymethyl, hydroxyethyl, 1-hydroxypropyl, and 2-hydroxylpropyl peroxyl radicals were measured and the corresponding molar extinction coefficient (ε) was determined to enable the quantification and characterization of RO₂˙ in the aqueous phase. The kinetics of RO₂˙ decomposition was investigated by determining both second-order recombination rate constants (k_2nd) and first-order unimolecular decay rate constants (k_1st). Obtained k_2nd fall within the range of 10⁷-10⁸ L mol^-1 s^-1 and decrease with increasing carbon chain length, reflecting the structure-dependent effects. k_1st were derived through a model fitting approach and validated against the experimental data: k_1st,_RO2(MeOH) = 606 ± 59 s^-1, k_1st,_RO2(EtOH) = 154 ± 3 s^-1, k_1st,_RO2(1-PrOH) = 79 ± 9 s^-1, and k_1st,_RO2(2-PrOH) = 197 ± 7 s^-1. In addition, the concentration-dependent measurements were conducted to investigate the rate constants of RO₂˙ + H₂O₂ reactions. Corresponding rate constants were determined to be (1.9 ± 0.4) × 10⁴ L mol^-1 s^-1 for hydroxyethyl and (2.2 ± 0.6) × 10⁴ L mol^-1 s^-1 for 2-hydroxylpropyl peroxyl radical. Calculated atmospheric lifetimes are in the millisecond range and reveal a fast conversion to HO₂˙. The HO₂˙ elimination and subsequent radical recycling impacts the oxidative capacity in cloud and aerosol water.