Production of Gas Phase NO2 and Halogens from the Photochemical Oxidation of Aqueous Mixtures of Sea Salt and Nitrate Ions at Room Temperature

Nitrate and halide ions coexist in a number of environmental systems, including sea salt particles, the Arctic snowpack, and alkaline dry lakes. However, little is known about potential synergisms between halide and nitrate ions. The effect of sea salt on NO3- photochemistry at 311 nm was investigated at 298 K using thin films of deliquesced NaNO3-synthetic sea salt mixtures. Gas phase NO2, NO, and halogen products were measured as a function of photolysis time using NOy chemiluminescence and atmospheric pressure ionization mass spectrometry (API-MS). The production of NO2 increases with the halide-to-nitrate ratio, and is similar to that for mixtures of NaCl with NaNO3. Gas phase halogen production also increased with the halide-to-nitrate ratio, consistent with NO3- photolysis yielding OH which oxidizes halide ions in the film. Yields of gas phase halogens and NO were strongly dependent on the acidity of the solution, while that of NO2 was not. An additional halogen formation mechanism in the dark involving molecular HNO3 is proposed that may be important in other systems such as reactions on surfaces. These studies show that the yield of Br-2 relative to NO2 during photolysis of halide-nitrate mixtures could be as high as 35% under some atmospheric conditions.