OH-initiated atmospheric degradation of hydroxyalkyl hydroperoxides: mechanism, kinetics, and structure-activity relationship

Hydroxyalkyl hydroperoxides (HHPs), formed in the reactions of Criegee intermediates (CIs) with water vapor, play essential roles in the formation of secondary organic aerosol (SOA) under atmospheric conditions. However, the transformation mechanisms for the OH-initiated oxidation of HHPs remain incompletely understood. Herein, the quantum chemical and kinetics modeling methods are applied to explore the mechanisms of the OH-initiated oxidation of the distinct HHPs (HOCH2OOH, HOCH(CH3)OOH, and HOC(CH3)(2)OOH) formed from the reactions of CH2OO, anti-CH3CHOO, and (CH3)(2)COO with water vapor. The calculations show that the dominant pathway is H-abstraction from the -OOH group in the initiation reactions of the OH radical with HOCH2OOH and HOC(CH3)(2)OOH. H-abstraction from the -CH group is competitive with that from the -OOH group in the reaction of the OH radical with HOCH(CH3)OOH. The barrier of H-abstraction from the -OOH group slightly increases when the number of methyl groups increase. In pristine environments, the self-reaction of the RO2 radical initially produces a tetroxide intermediate via oxygen-to-oxygen coupling, and then it decomposes into propagation and termination products through asymmetric two-step O-O bond scission, in which the rate-limiting step is the first O-O bond cleavage. The barrier height of the reactions of distinct RO2 radicals with the HO2 radical is not affected by the number of methyl substitutions. In urban environments, the reaction with O-2 to form formic acid and the HO2 radical is the dominant removal pathway for the HOCH2O radical formed from the reaction of the HOCH2OO radical with NO. The beta-site C-C bond scission is the dominant pathway in the dissociation of the HOCH(CH3)O and HOC(CH3)(2)O radicals formed from the reactions of NO with HOCH(CH3)OO and HOC(CH3)(2)OO radicals. These new findings deepen our understanding of the photochemical oxidation of hydroperoxides under realistic atmospheric conditions.