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

Abstract. Rate coefficients (k5) for the title reaction were ob- tained using pulsed laser photolytic generation of OH cou- pled 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 pres- sures. The accuracy of the rate coefficients obtained was en- hanced 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 in- crease 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 se- lected literature values of k5 and parameterised using a com- bination of pressure-dependent and -independent terms to give an expression that covers the relevant pressure and tem- perature range for the atmosphere. A global model, using the new parameterisation for k 5 rather than those presently ac- cepted, indicated small but significant latitude- and altitude- dependent changes in the HNO 3 / NO x ratio of between − 6 and + 6 %. Effective HNO 3 absorption cross sections (184.95 and 213.86 nm, units of cm 2 molecule − 1 ) were ob- tained as part of this work: σ 213 . 86 = 4.52 + 0 . 23 − 0 . 12 × 10 − 19 and σ 184 . 95 = 1.61 + 0 . 08 − 0 . 04 × 10 − 17 .