In the atmosphere,nitric acid, nitrogen pentoxide and nitrogen dioxide react with mineral dust particle surfaces to form adsorbed nitrate. This process is thought to be a sink for nitrogen oxides but in fact there is the potential to release gas-phase nitrogen oxides back into the atmosphere due to surface photochemistry of adsorbed nitrate initiated by solar light. In this study, the wavelength dependence of nitrate ion photochemistry when adsorbed onto model laboratory proxies of mineral dust aerosol including Al2O3, TiO2 and NaY Zeolite was investigated using FTIR spectroscopy. These proxies are used to represent non-photoactive oxides, photoactive semiconductor oxides and aluminosilicate materials, respectively, present in mineral dust. Nitrate photochemistry on these materials is shown to depend on the wavelength of light, physicochemical properties of the dust particles and the adsorption mode of the nitrate ion. As will be shown, nitrate ions adsorbed on oxide particles undergo photochemistry over a broader wavelength region of solar spectrum compared to nitrate ion in solution. Gas-phase NO2 is the major photolysis product under dry conditions. Nitrate photochemistry is more efficient on TiO2 compared to Al2O3 as expected because of its semiconductor properties. Nitrite ion complexed to Na+ sites is the major photolysis product found in nitrate photochemistry in NaY zeolite. The wavelength dependence of nitrate photochemistry and proposed mechanisms for the formation of photoproducts are presented.

Surface photochemistry of adsorbed nitrate on oxide and zeolite surfaces

248th National Meeting (2014)

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