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 NaNO3(s) and HCl(g) follows single-site Langmuir adsorption behavior. X-ray photoelectron 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 adsorbed 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 easily measurable, amounts of strongly adsorbed water. Water desorbs completely from low defect density NaCl(100) surfaces under vacuum at temperatures well below room temperature. We present a model for the recently reported HNO3 pressure dependence of the reactive sticking coefficient of HNO3 on NaCl under steady-state reaction conditions. The origin of the pressure dependence in the model is the competition between site blocking on the surface by the build-up of the NaNO3 reaction product and the water-induced 3-D recrystallization of the NaNO3 that frees up reactive sites for further reaction.