Release of Gas-Phase Halogens by Photolytic Generation of OH in Frozen Halide-Nitrate Solutions: An Active Halogen Formation Mechanism?

To better define the mechanisms by which condensed-phase halides may be oxidized to form gas-phase halogens under polar conditions, experiments have been conducted whereby frozen solutions containing chloride (1 M), bromide (1.6 x 10(-3) to 5 x 10(-2) M), iodide (<1 x 10(-5) M), and nitrate (0.01 to 1 M) have been illuminated by ultraviolet light in a continually flushed cell. Gas-phase products are quantified using chemical ionization mass spectrometry, and experiments were conducted at both 248 and 263 K. Br-2 was the dominant product, along with smaller yields of IBr and trace BrCl and I-2. The Br-2 yields were largely independent of the Br-/Cl- ratio of the frozen solution, down to seawater composition. However, the yields of halogens were strongly dependent on the levels of NO3- and acidity in solution, consistent with a mechanism whereby NO3- photolysis yields OH that oxidizes the condensed-phase halides. In support, we observed the formation of gas-phase NO2, formed simultaneously with OH. Gas-phase HONO was also observed, suggesting that halide oxidation by HONO in the condensed phase may also occur to some degree. By measuring the production rate of condensed-phase OH, using benzoic acid as a radical trap, we determine that the molar yield of Br-2 formation relative to OH generation is 0.6, consistent with each OH being involved in halide oxidation. These studies suggest that gas-phase halogen formation should occur simultaneously with NO, release from frozen sea ice and snow surfaces that contain sufficient halides and deposited nitrate.