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 NO(3)(-) and acidity in solution, consistent with a mechanism whereby NO(3)(-) photolysis yields OH that oxidizes the condensed-phase halides. In support, we observed the formation of gas-phase NO(2), 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(x) release from frozen sea ice and snow surfaces that contain sufficient halides and deposited nitrate.