Mechanism and kinetics of the reactions of NO2 or HNO3 with alumina as a mineral dust model compound

The reaction of alumina as a model substance for mineral aerosols with NO2 or HNO3 was studied using diffuse reflectance infrared Fourier transform sp ectroscopy (DRIFTS). The formation of nitrate on thr Al2O3 surface was obse rved in both cases. In addition, during the initial phase of the NO2 reacti on, intermediate nitrite formation was observed. The DRIFTS data provide in sight into the reaction mechanism, which involves reaction of surface OH gr oups, die formation of a (AlOOH ... NO2) adduct, and the formation of acidi c 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 conce ntrations in the range of 10(13) to 10(15) molecules cm(-3). The reactive u ptake coefficient, gamma, was determined from the infrared absorbance, whic h was calibrated by ion chromatography, and from the Al2O3 Brunauer-Emmett- Teller (BET) surface erect. gamma depended linearly on the NO2 concentratio n and varied from gamma = 7.3 x 10-(10) to 1.3 x 10(-8) for [NO2] = 2.5 x 1 0(13) to 8.5 x 10(14) molecules cm(-3). Estimations of the atmospheric impa ct showed that at these above conditions (gamma = 10(-9)) nitrate formation on mineral aerosol from the NO2 reaction would be negligible.