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 fall-off expressions, with the JPL expressions (k_1,0^(N_2 )=7.37×10-31(T/300 K)-2.90 cm6 molecule-2 s-1 and k1,∞ = 3.44×10-11(T/300 K)-0.1cm3 molecule-1 s-1 and IUPAC expression (k_1,0^(N_2 )= 6.80 ×10-31(T/300 K)-2.81 cm6 molecule-2 s-1, FC = 0.81 , k1,∞= 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 constants were fit with an empirical modification by adding a second fall-off term to the JPL expression with a second low-pressure rate coefficient of k_1,0^(N_2 )=5.20 ×10-35(T/300K)-30.4 cm6 molecule-2 s-1. Second, k_1,0^(N_2 ), k1,∞, and n were fit globally to the entire temperature dataset, but FC 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 fall-off 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^(H_2 O )= 3.81×10-30(T/300 K)-6.04 and k_1,0^(H_2 O )= 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 <Edown,295 K, N2> = 170 ± 10 cm-1 and <Edown,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.