The reactions of NO3 with OH and H

Abstract

Rate constants for the reaction NO₃+ OH → products (1) have been measured in a flow system. NO₃ radicals were generated from the reaction of F with HNO₃, and OH radicals were generated by the reaction of H with NO₂. Concentrations of NO₃ were determined by multipass optical absorption in the visible region, while [OH] was measured by resonance fluorescence. Numerical modelling allowed a rate coefficient to be derived for the reaction at room temperature of k₁=(2.0 ± 0.6)× 10^–11 cm³ molecule^–1 s^–1. The modelling shows that secondary reactions are of only minor importance. We describe a hitherto-unknown reaction between H and NO₃. NO₃+ H → OH + NO₂. (2) Production of OH was demonstrated explicitly from the resonance fluorescence measurements. The reaction is fast, with a room-temperature rate constant given by k₂=(1.1 ± 0.6)× 10^–10 cm³ molecule^–1 s^–1. Reaction (1) follows the formation of OH in reaction (2), so that the time-dependence of [OH] can be used to give further information about k₁. The rate constants so obtained were entirely consistent with those obtained in the direct study. The possible influence of steric effects and orbital degeneracy on the relative rates of the H and OH interactions with NO₃ are pointed out. Atmospheric involvement of the reactions, in either troposphere or stratosphere, is shown to be improbable, although the new data illustrate that NO₃ can be highly reactive towards suitable partners.