Nitrate Photochemistry on Laboratory Proxies of Mineral Dust Aerosol: Wavelength Dependence and Action Spectra

Nitrate ion adsorbed on the surface of mineral dust particles from heterogeneous reaction of nitric acid, nitrogen pentoxide, and nitrogen dioxide is thought to be a sink for nitrogen oxides. However, it has the potential to release gas-phase nitrogen oxides back into the atmosphere when irradiated with UV 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 represent non-photoactive oxides, photoactive semiconductor oxides, and porous aluminosilicate materials, respectively, present in mineral dust aerosol. Nitrate photochemistry on mineral dust particles is governed by the wavelength of light, physicochemical properties of the dust particles, and the adsorption mode of the nitrate ion. Most interestingly, in some cases, nitrate ion adsorbed on oxide particles can undergo photochemistry over a broader wavelength region of the solar spectrum compared to nitrate ion in solution. As shown here, gas-phase NO2 is the major photolysis product formed from nitrate adsorbed on the surface of oxide particles under dry conditions. The NO2 yield and the initial rate of production is highest on TiO2, indicating that nitrate photochemistry is more efficient on photoactive oxides present in mineral dust. Nitrite ion complexed to Na+ sites in aluminosilicate zeolite pores is the major photolysis product found for zeolites. Mechanisms for the formation of gas-phase and surface-adsorbed products and a discussion of the wavelength dependence of nitrate ion photochemistry are presented, as is a discussion of the atmospheric implications.