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.