Is the Reaction Rate Coefficient for OH + HO₂ → H₂O + O₂ Dependent on Water Vapor?

A critical reaction affecting the oxidation chemistry in the middle-to-upper atmosphere occurs between hydroxyl (OH) and hydroperoxyl (HO₂). The reaction rate coefficient for OH + HO₂ → H₂O + O₂, 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 cm³ molecule^-1 s^-1, but instead, a water-dependent (∼1 × 10^-11 + 2.17 × 10^-28[H₂O]) cm³ 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 HO₂. The observed OH decays were due only to the OH reaction with HO₂ 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 cm³ molecule^-1 s^-1, 68% confidence, independent of water vapor and lower than but consistent with the recommended value.