Surface Adsorbed Water on NaCl and Its Effect on Nitric Acid Reactivity with NaCl Powders

Abstract

Adsorbed water has been shown to enhance the ionic mobility on NaCl surfaces at water vapor pressures well below the deliquescence point. This has important implications for the steady-state reactivity of NaCl, in atmospheric sea salt particles, with nitrogen oxides in the atmosphere. In the absence of surface ionic mobility the surface passivates due to the buildup of a product layer. It has also been suggested that the actual reaction probability of the reaction of HNO3 with NaCl surfaces may depend on the water content of the surface. Using X-ray photoelectron spectroscopy and electron microscopy, we show that the specifics (particle size and water exposure) of the NaCl sample preparation can have significant effect on the amount of strongly adsorbed water (SAW) on the surface. Specifically, large single crystals of NaCl(100) do not adsorb water strongly. Crystallites in the size range of 500 μm adsorb small amounts of water strongly. In comparison, small crystallites in the size range of 1−10 μm can adsorb large amounts of water strongly. The strongly adsorbed water remains on the surface when exposed to vacuum at temperatures above 100 °C. The NaCl small crystallites, with the largest amount of strongly adsorbed water, do show a small increase in the initial reaction probability for the reaction with HNO3. This increase in initial reaction probability is at most a factor of 4. By combining the results presented here with our previously reported quantitative measurement of the reaction probability for HNO3 on NaCl(100) single-crystal surfaces we recommend a value of γ = (5.2 ± 3) × 10-3 for the reaction probability of gaseous HNO3 with micrometer-size NaCl particles that contain strongly adsorbed surface water.