A Knudsen cell study of the heterogeneous reactivity of nitric acid on oxide and mineral dust particles

Abstract

The heterogeneous reactivity of gaseous nitric acid on α-Al2O3, α-Fe2O3, SiO2, MgO, and CaO, as well as authentic samples of Gobi dust and Saharan sand, was investigated at 295 K using a Knudsen cell reactor. Through total uptake calculations and mass dependent studies, nitric-acid diffusion into the underlying layers is shown to be an important process for all of the systems studied. As such, models that account for the increased surface area and the concomitant increase in "internal collisions" were used to calculate uptake coefficients. The initial uptake coefficients for α-Al2O3, α-Fe2O3, SiO2, MgO, and CaO all lie in the 10-5 to 10-3 region at gas concentrations between 1011 and 1012 molecules/cm3. As expected, on the basis of their high SiO2 composition, the measured initial uptake coefficients for the authentic dust samples are close to the value found for SiO2 and are between 2 and 6 × 10-5. Uptake of nitric acid on most of the oxide particles and the authentic dust samples results in irreversible adsorption. The effect of surface adsorbed water on both CaO and Gobi dust was investigated and is found to significantly enhance the initial uptake coefficient. Atmospheric implications of these results in terms of the importance of heterogeneous reactions of nitric acid on mineral aerosol are discussed.