Reconsideration of the Rate Constant for the Reaction of Hydroxyl Radicals with Nitric Acid

We report rate coefficients for the reaction of OH with HNO3, k1, between 10 and 500 Torr of He, SF6, N2, and O2 and at 10 different temperatures between 200 and 375 K. We generated OH via pulsed photolysis of HNO3 and monitored the [OH] temporal profile via pulsed laser induced fluorescence. Below 300 K the value of k1 increases rapidly with decreasing temperature and depends on pressure. The pressure dependence of k1 at low temperature is significantly larger than that obtained by extrapolation of the currently available data. The pressure and temperature dependence is most likely due to a competition between direct abstraction and reactive complex formation. A rate constant expression derived from such a mechanism gives a global fit for k1 that is applicable to atmospheric conditions. The new rate constant alters the calculated NO2 to HNO3 ratio in the lower stratosphere.