Atmospheric reactions of HO₂ radicals

Hydroperoxy radicals, HO₂, are central to atmospheric chemisty. In regions of the troposphere impacted by nitrogen oxides (NO_x = NO + NO₂) they are involved in ozone formation. On the other hand, in the remote troposphere (e.g., the marine bounday layer) or in the stratosphere they play an important role in ozone depletion. This talk concerns new studies of some reactions of HO₂ which are relevant under low-NO_x conditions. A laser flash photolysis system with tunable diode laser detection has been used to measure the rate coefficient of the reaction HO₂ + HO₂ → H₂O₂ + O₂. The HO₂ radicals were produced by 351 nm laser flash photolysis of Cl₂-CH₃OH-O₂ mixtures, and detected by long path laser absorption at 1,506 nm in the overtone band of the O-H stretch. The rate coefficient has been measured as a function of temperature, CH₃OH and H₂O concentration at approximately 30 torr total pressure. A negative temperature dependence has been found for the rate coefficient, in good agreement with earlier studies, but in contrast with a more recent measurement. The mechansims of some reactions of HO₂ with carbonyl compounds (e.g., acetaldehyde, acetone) and peroxy radicals derived from carbonyl compounds have been investigated in an environmental chamber using FTIR detection. New results will be presented on the formation of OH radicals from reactions of HO₂ with substituted peroxy radicals.