Temperature-(208–318 K) and pressure-(18–696 Torr) dependent rate coefficients for the reaction between OH and HNO₃

Abstract. Rate coefficients (k₅) for the title reaction were obtained using pulsed laser photolytic generation of OH coupled to its detection by laser-induced fluorescence (PLP–LIF). More than 80 determinations of k₅ were carried out in nitrogen or air bath gas at various temperatures and pressures. The accuracy of the rate coefficients obtained was enhanced by in situ measurement of the concentrations of both HNO₃ reactant and NO₂ impurity. The rate coefficients show both temperature and pressure dependence with a rapid increase in k₅ at low temperatures. The pressure dependence was weak at room temperature but increased significantly at low temperatures. The entire data set was combined with selected literature values of k₅ and parameterised using a combination of pressure-dependent and -independent terms to give an expression that covers the relevant pressure and temperature range for the atmosphere. A global model, using the new parameterisation for k₅ rather than those presently accepted, indicated small but significant latitude- and altitude-dependent changes in the HNO₃ ∕ NO_x ratio of between −6 and +6 %. Effective HNO₃ absorption cross sections (184.95 and 213.86 nm, units of cm² molecule⁻¹) were obtained as part of this work: σ_213.86  =  4.52_−0.12^+0.23  ×  10⁻¹⁹ and σ_184.95  =  1.61_−0.04^+0.08  ×  10⁻¹⁷.