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

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 latitudeand altitudedependent changes in the HNO3 /NOx ratio of between −6 and +6%. Effective HNO3 absorption cross sections (184.95 and 213.86 nm, units of cmmolecule) were obtained as part of this work: σ213.86= 4.52 +0.23 −0.12× 10 −19 and σ184.95= 1.61 +0.08 −0.04× 10 .