HONO and NO₂ evolution from irradiated nitrate-doped ice and frozen nitrate solutions

Abstract. Nitrate photolysis in the wavelength range 250–1200 nm was studied on ice in a controlled laboratory experiment. Monolayer coverage of nitrate was achieved by dosing well-known amounts of HNO₃ from the gas phase onto a frozen water surface. Fluxes of HONO and NO₂ into the gas phase with time were quantified at temperatures between 193 K and 258 K and as a function of illumination wavelength in the range: 250–345 nm. Whereas HONO release showed a strong temperature dependence at colder temperatures, attributed to reversible adsorption processes, NO₂ fluxes were independent of temperature. The observed fluxes of HONO and NO₂ at high temperature were not affected by diffusion or adsorption processes, and could be used to estimate a quantum yield for HONO formation of (3.8±0.6)×10⁻⁴. A different wavelength dependence for HONO and NO₂ fluxes indicates that additional reactions besides nitrate photolysis and subsequent release of the products contribute to the emission of nitrogen oxides.