Rate Coefficients for OH + NO (+N(2)) in the Fall-off Regime and the Impact of Water Vapor

[Image: see text] The termolecular, association reaction between OH and NO is a source of nitrous acid (HONO), an important atmospheric trace gas. Rate coefficients for the title reaction as recommended by evaluation panels differ substantially at the temperatures and pressures that prevail in the Earth’s boundary layer where the reaction is in the fall-off regime between low- and high-pressure limiting rate coefficients. Using pulsed laser methods for generation and detection of OH, we have reinvestigated the kinetics of the title reaction at pressures of 22–743 Torr (1 Torr = 1.333 hPa) and temperatures (273, 298, and 333 K) in pure N(2) and in N(2)–H(2)O bath gases. In situ optical absorption measurements were used to rule out any bias due to NO(2) or HONO impurities. Our rate coefficients (k(1)) in N(2) bath gas are parametrized in terms of low-pressure (k(0)) and high-pressure (k(∞)) rate coefficients and a fall-off parameter (F(C)) with k(1,0)(N(2)) = 7.24 × 10(–31) (T/300 K)(−2.17) cm(6) molecule(–2) s(–1), k(1,∞) = 3.3 × 10(–12) (T/300 K)(−0.3) cm(3) molecule(–1) s(–1), and F(C) = 0.53. Used with the “Troe” expression for termolecular reactions, these parameters accurately reproduce the current data in the fall-off regime and also capture literature rate coefficients at extrapolated temperatures. The presence of water vapor was found to enhance the rate coefficients of the title reaction significantly. The low-pressure limiting rate coefficient in H(2)O bath gas is a factor 5–6 larger than in N(2), at room temperature (k(1,0)(H(2)O) = 4.55 × 10(–30) (T/300 K)(−4.85) cm(6) molecule(–2) s(–1)) indicating that H(2)O is much more efficient in quenching the association complex HONO* through collisional energy transfer. Based on measurements in N(2)–H(2)O mixtures, a parametrization of k(1) including both N(2) and H(2)O as third-body quenchers was derived. Neglecting the effect of H(2)O results, e.g., in an underestimation of k(1) by >10% in the tropical boundary layer.