Hydroxyl radical regeneration in the photochemical oxidation of glyoxal: kinetics and mechanism of the HC(O)CO + O2 reaction

Glyoxal, HC(O)CHO, is an important trace component of the Earth's atmosphere, formed in biomass burning and in the photooxidation of volatile organic compounds (VOCs) like isoprene and aromatic hydrocarbons. The HC(O)CO free radical is the primary product of the glyoxal + OH reaction, and this study uses computational chemistry to show that the HC(O)CO radical can react with O-2 to regenerate the hydroxyl radical (OH) in the atmosphere. Master equation simulations indicate that the HC(O)C(O)O-2 peroxy radical adduct proceeds directly to CO2 + CO + OH in a chemically activated mechanism, with minor collisional deactivation of the relatively unstable HC(O)C(O)O-2 peroxy radical. The reaction of HC(O)CO with O-2 is found to be competitive with thermal decomposition to HCO + CO at tropospheric temperatures and pressures, accounting for ca. 40% or more of the total yield. The present process provides a new mechanism for OH regeneration in the troposphere, which involves the decomposition of unstable alpha-formylperoxy radicals.