Effect of water on the HNO3 pressure dependence of the reaction between gas-phase HNO3 and NaCl surfaces

We show here that the dissociative adsorption of HNO3(g) on NaCl to form Na NO3(s) and HCl(g) follows single-site Langmuir adsorption behavior. X-ray p hotoelectron spectroscopy is also used to show that the amount of "strongly adsorbed water" on the surfaces of NaCl particles strongly depends on the particle size. Particles of 1-10 mu m diameter show large quantities of ads orbed water that remain on the sample up to temperatures of 200 degrees C. Particles in the size range of 500 mu m diameter have less, but still easil y measurable, amounts of strongly adsorbed water. Water desorbs completely from low defect density NaCl(100) surfaces under vacuum at temperatures wel l below room temperature. We present a model for the recently reported HNO3 pressure dependence of the reactive sticking coefficient of HNO3 on NaCl u nder steady-state reaction conditions. The origin of the pressure dependenc e in the model is the competition between site blocking on the surface by t he build-up of the NaNO3 reaction product and the water-induced 3-D recryst allization of the NaNO3 that frees up reactive sites for further reaction.