Pressure, Temperature, and Water Vapor Dependencies of the Bimolecular Rate Coefficients for the Reaction OH + NO + M → HONO + M

OH+NO is an important termolecular association reaction in the troposphere and stratosphere that influences the atmospheric ozone budget. In this study, rate coefficients for the reaction of OH + NO + M → HONO + M were measured under conditions relevant to the troposphere/lower stratosphere over a temperature range of 228–298 K and pressure range of 50–750 Torr using N₂ as a bath gas. Time-resolved kinetics were studied by pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) detecting OH by laser-induced fluorescence. Data for the temperature range 258–298 K were fit to two falloff expressions, with the JPL expressions (k_1,0^N₂ = 7.37 × 10^–31(T/300 K)^−2.90 cm⁶ molecule^–2 s^–1 and k_1,∞ = 3.44 × 10^–11(T/300 K)^−0.1cm³ molecule^–1 s^–1) and IUPAC expression (k_1,0^N₂ = 6.80 × 10^–31(T/300 K)^−2.81 cm⁶ molecule^–2 s^–1, F_C = 0.81, k_1,∞ = 1.96 × 10^–11(T/300 K)^−0.3 cm³ molecule^–1 s^–1). At temperatures T < 258 K, the measured rate coefficients were significantly higher than the IUPAC and JPL fits. To accommodate the rate coefficient deviation from the two expressions, data across the entire temperature range (228–298 K) was fit with two approaches. First, rate coefficients were fit with an empirical modification by adding a second falloff term to the JPL expression with a second low-pressure rate coefficient of k_1,0^N₂ = 5.20 × 10^–35(T/300 K)^−30.4 cm⁶ molecule^–2 s^–1. Second, k_1,0^N₂, k_1,∞, and n were fit globally to the entire temperature data set, but F_C was varied for each individual temperature, which increased with decreasing temperature. In the second portion of the study, the influence of H₂O on the reaction rate was investigated using a N₂–H₂O mixture as the bath gas at 50 Torr and 273 and 298 K. The JPL and IUPAC falloff expressions were modified to include H₂O as a third-body collisional partner consistent with a nonlinear mixture model. Fits to the data yielded the low pressure termolecular rate coefficients in H₂O, k_1,0^H₂O = 3.81 × 10^–30(T/300 K)^−6.04 and k_1,0^H₂O = 3.31 × 10^–30(T/300 K)^−5.81 cm⁶ molecule^–2 s^–1, respectively. Experimental data were fit using MESMER give energy relaxation parameters of <ΔE_{down,295 K, N2}> = 170 ± 10 cm^–1 and <ΔE_{down,295 K,H2O}> = 634 ± 20 cm^–1, indicating that H₂O is a 4× more efficient collisional quencher than N₂ alone. The modified JPL expressions with the newly derived low pressure rate coefficients were implemented into a STOCHEM-CRI atmospheric model. Predictions of HONO concentrations with the new rates were up to 15% higher in remote tropical regions.