Mechanism and Kinetics of the Reactions of NO2 or HNO3 with Alumina as a Mineral Dust Model Compound

Abstract

The reaction of alumina as a model substance for mineral aerosols with NO2 or HNO3 was studied using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The formation of nitrate on the Al2O3 surface was observed in both cases. In addition, during the initial phase of the NO2 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···NO2} adduct, and the formation of acidic OH groups. The reaction order in NO2 of 1.86 ± 0.1 was determined from a quantitative kinetic evaluation of a series of experiments with NO2 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 Al2O3 Brunauer−Emmett-Teller (BET) surface area. γ depended linearly on the NO2 concentration and varied from γ = 7.3 × 10-10 to 1.3 × 10-8 for [NO2] = 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 NO2 reaction would be negligible.