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

We report rate coefficients for the reaction of OH with HNO<INF>3</INF>, k<INF>1</INF>, between 10 and 500 Torr of He, SF<INF>6</INF>, N<INF>2</INF>, and O<INF>2</INF> and at 10 different temperatures between 200 and 375 K. We generated OH via pulsed photolysis of HNO<INF>3</INF> and monitored the [OH] temporal profile via pulsed laser induced fluorescence. Below 300 K the value of k<INF>1</INF> increases rapidly with decreasing temperature and depends on pressure. The pressure dependence of k<INF>1</INF> 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 k<INF>1</INF> that is applicable to atmospheric conditions. The new rate constant alters the calculated NO<INF>2</INF> to HNO<INF>3</INF> ratio in the lower stratosphere.