Measurement of the rate coefficient for the reaction of O(1D) with H2O and re-evaluation of the atmospheric OH production rate

Abstract

The rate coefficient for the reaction of O(¹D) with H₂O (Reaction 1), whose value is critical for calculating production rates of the OH radical in the atmosphere, was measured over a range of atmospherically relevant temperatures. The temporal profile of O(³P) following photolytic production of O(¹D) in the presence of water vapor was used to determine a temperature dependent value for k₁ of (1.45 ± 0.34) × 10⁻¹⁰ exp((89 ± 65)/T) cm³ molecule⁻¹ s⁻¹, where the quoted errors are at the 95% confidence level, include estimated systematic errors and σ_A = Aσ_lnA. In addition, ratios of rate coefficients for quenching of O(¹D) by N₂ and O₂ (k₂ and k₃) to that for Reaction 1 were determined to be k₂/k₁ = (0.13 ± 0.04) and k₃/k₁ = (0.19 ± 0.09) by measuring the relative OH concentration produced from Reaction 1 in the presence of various concentrations of N₂ or O₂. Combining our results of this work with previous measurements of k₁, we recommend a value for k₁ = (1.62 ± 0.27) × 10⁻¹⁰ exp((65 ± 50)/T) cm³ molecule⁻¹ s⁻¹, where the quoted errors are 95% confidence level, include estimated systematic errors and σ_A = Aσ_lnA. Based on these results, the uncertainty in the calculated atmospheric OH production rate due to the uncertainty in these rate coefficients is reduced from ±50% to ±20%.