Heterogeneous reactivity of gaseous nitric acid on Al2O3, CaCO3, and atmospheric dust samples:: A Knudsen cell study

The heterogeneous reaction between HNO3 and various authentic and synthetic mineral dust/mineral oxide surfaces has been investigated using a low-pressure Knudsen reactor operating at 298 K. The surfaces used were Saharan dust from Cape Verde, Arizona dust, CaCO3, and Al2O3 In ail cases, a large irreversible uptake was observed. An uptake coefficient of gamma = (II +/- 3) x 10(-2) was determined for Saharan dust, and gamma = (6 +/- 1.5) x 10(-2) was obtained for Arizona dust. The uptake coefficients for HNO3 on heated CaCO3 and on unheated CaCO3 are given by gamma = (10 +/- 2.5) x 10(-2) and (18 +/- 4.5) x 10(-2), respectively, and are in good agreement with previous results. CO2 and H2O were formed as gas-phase products. Measurements of the uptake coefficient of HNO3 on grain-size selected samples of Al2O3, gamma = (13 +/- 3.3) x 10(-2), and systematic variation of sample mass enabled us to show that the geometrical surface area of the dust sample is appropriate for calculation of uptake coefficients in these experiments. The high reactivity of HNO3 toward dust samples highlights the potentially important role of mineral dust in redistributing nitrate from the gaseous to the particulate phase and modifying tropospheric photochemical oxidation cycles.