Kinetics of the chemically activated HSO5 radical under atmospheric conditions – a master-equation study

Abstract

A detailed theoretical analysis of the HOSO₂ + O₂ reaction is performed, paying special attention to the kinetics of the intermediate HSO₅ radical. The possible formation of the monohydrated adduct, HSO₅·H₂O, in the presence of water vapor is examined. For the binding energy of the most stable isomer at T = 0 K, a value of D₀(HOSO₄–H₂O) = 51.7 kJ mol⁻¹ was obtained at the CCSD(T)/CBS level of theory; other energies were adopted from a recently published high-level quantum chemical study of our laboratory. Molecular geometries and vibrational frequencies of the reactants, intermediates, and products were obtained from B3LYP/cc-pVTZ calculations. Rate coefficients and lifetimes of the HSO₅ intermediate were calculated by solving a master equation with specific rate coefficients from statistical rate theory. The master equation is extended by a bimolecular sink term, which accounts for the HSO₅ + H₂O reaction. The relation between thermal and chemical activation in this reaction system is examined. The results show that the lifetime of the HSO₅ intermediate under atmospheric conditions is too short for a bimolecular reaction with water to become important. Relative yields of HSO₅·H₂O well below 1% were obtained, ruling out this reaction pathway as a relevant source for aerosols.