Mechanism and Kinetics of the Reactions of NO<INF>2</INF> or HNO<INF>3</INF> with Alumina as a Mineral Dust Model Compound

The reaction of alumina as a model substance for mineral aerosols with NO<INF>2</INF> or HNO<INF>3</INF> was studied using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The formation of nitrate on the Al<INF>2</INF>O<INF>3</INF> surface was observed in both cases. In addition, during the initial phase of the NO<INF>2</INF> reaction, intermediate nitrite formation was observed. The DRIFTS data provide insight into the reaction mechanism, which involves reaction of surface OH groups, the formation of a {AlOOH···NO<INF>2</INF>} adduct, and the formation of acidic OH groups. The reaction order in NO<INF>2</INF> of 1.86 ± 0.1 was determined from a quantitative kinetic evaluation of a series of experiments with NO<INF>2</INF> concentrations in the range of 10¹³ to 10¹⁵ molecules cm^-3. The reactive uptake coefficient, γ, was determined from the infrared absorbance, which was calibrated by ion chromatography, and from the Al<INF>2</INF>O<INF>3</INF> Brunauer−Emmett-Teller (BET) surface area. γ depended linearly on the NO<INF>2</INF> concentration and varied from γ = 7.3 × 10^-10 to 1.3 × 10^-8 for [NO<INF>2</INF>] = 2.5 × 10¹³ to 8.5 × 10¹⁴ molecules cm^-3. Estimations of the atmospheric impact showed that at these above conditions (γ = 10^-9) nitrate formation on mineral aerosol from the NO<INF>2</INF> reaction would be negligible.