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

The heterogeneous reactivity of gaseous nitric acid on alpha -Al2O3, alpha -Fe2O3, SiO2, MgO, and CaO, as well as authentic samples of Gobi dust and S aharan sand, was investigated at 295 K using a Knudsen cell reactor. Throug h total uptake calculations and mass dependent studies, nitric-acid diffusi on into the underlying layers is shown to be an important process for all o f the systems studied. As such, models that account for the increased surfa ce area and the concomitant increase in "internal collisions" were used to calculate uptake coefficients. The initial uptake coefficients for alpha -A l2O3, alpha -Fe2O3, SiO2, MgO, and CaO all lie in the 10(-5) to 10(-3) regi on at gas concentrations between 10(11) and 10(12) molecules/cm(3). As expe cted, on the basis of their high SiO2 composition, the measured initial upt ake coefficients for the authentic dust samples are close to the value foun d for SiO2 and are between 2 and 6 x 10(-5). Uptake of nitric acid on most of the oxide particles and the authentic dust samples results in irreversib le adsorption. The effect of surface adsorbed water on both CaO and Gobi du st was investigated and is found to significantly enhance the initial uptak e coefficient. Atmospheric implications of these results in terms of the im portance of heterogeneous reactions of nitric acid on mineral aerosol are d iscussed.