Reconsideration of the rate constant for the reaction of hydroxyl radicalswith nitric acid

We report rate coefficients for the reaction of OH with HNO3, k(1), between 10 and 500 Torr of He, SF6, N-2, and O-2 and at 10 different temperatures between 200 and 375 K. We generated OH via pulsed photolysis of HNO3 and mo nitored the [OH] temporal profile via pulsed laser induced fluorescence. Be low 300 K the value of k(1) increases rapidly with decreasing temperature a nd depends on pressure. The pressure dependence of k(1) at low temperature is significantly larger than that obtained by extrapolation of the currentl y available data. The pressure and temperature dependence is most likely du e to a competition between direct abstraction and reactive complex formatio n. A rate constant expression derived from such a mechanism gives a global fit for k(1) that is applicable to atmospheric conditions. The new rate con stant alters the calculated NO2 to HNO3 ratio in the lower stratosphere.