Nitrate Ion Photolysis in Thin Water Films in the Presence of Bromide Ions

Nitrate ions commonly coexist with halide ions in aged sea salt particles, as well as in the Arctic snowpack, where NO3- photochemistry is believed to be an important source of NO gamma (NO + NO2 + HONO + ... ). The effects of bromide ions on nitrate ion photochemistry were investigated at 298 +/- 2 K in air using 311 nm photolysis lamps. Reactions were carried out using NaBr/NaNO3 and KBr/KNO3 deposited on the walls of a Teflon chamber. Gas phase halogen products and NO2 were measured as a function of photolysis time using long path FTIR, NO gamma, chemiluminescence and atmospheric pressure ionization mass spectrometry (API-MS). Irradiated NaBr/NaNO3 mixtures show an enhancement in the rates of production of NO2 and Br-2 as the bromide mole fraction (chi(NaBr)) increased. However, this was not the case for KBr/KNO3 mixtures where the rates of production of NO2 and Br-2 remained constant over all values of chi(KBr). Molecular dynamics (MD) simulations show that the presence of bromide in the NaBr solutions pulls sodium toward the solution surface, which in turn attracts nitrate to the interfacial region, allowing for more efficient escape of NO2 than in the absence of halides. However, in the case of KBr/KNO3, bromide ions do not appreciably affect the distribution of nitrate ions at the interface. Clustering of Br- with NO3- and H2O predicted by MD simulations for sodium salts may facilitate a direct intermolecular reaction, which could also contribute to higher rates of NO2 production. Enhanced photochemistry in the presence of halide ions may be important for oxides of nitrogen production in field studies such as in polar snowpacks where the use of quantum yields from laboratory studies in the absence of halide ions would lead to a significant underestimate of the photolysis rates of nitrate ions.