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-pres sure Knudsen reactor operating at 298 K. The surfaces used were Saharan dus t from Cape Verde, Arizona dust, CaCO3, and Al2O3 In ail cases, a large irr eversible 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 C aCO3 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 th e 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 u s to show that the geometrical surface area of the dust sample is appropria te for calculation of uptake coefficients in these experiments. The high re activity of HNO3 toward dust samples highlights the potentially important r ole of mineral dust in redistributing nitrate from the gaseous to the parti culate phase and modifying tropospheric photochemical oxidation cycles.