Abstract
A critical reaction affecting the oxidation chemistry in the middle-to-upper atmosphere occurs between hydroxyl (OH) and hydroperoxyl (HO(2)). The reaction rate coefficient for OH + HO(2) → H(2)O + O(2), here called k (OH+HO2), has challenged laboratory kineticists for 50 years. However, several measurements from the past 30 years had approached a rough consensus until the publication of a new study that examined, for the first time, the water vapor dependence of this reaction. According to the study, k (OH+HO2) is not the recommended value of 11.0 × 10(-11) cm(3) molecule(-1) s(-1), but instead, a water-dependent (∼1 × 10(-11) + 2.17 × 10(-28)[H(2)O]) cm(3) molecule(-1) s(-1). Our study examines the water dependence of k(OH+HO2) using water vapor photolysis of moist air at atmospheric pressure in a flow tube, with direct detection of both OH and HO(2). The observed OH decays were due only to the OH reaction with HO(2) and, to a lesser extent, the OH loss to the flow tube wall and trace impurities. The resulting k(OH+HO2) is (8.54 ± 2.90) × 10(-11) cm(3) molecule(-1) s(-1), 68% confidence, independent of water vapor and lower than but consistent with the recommended value.