Gas-phase Br₂ production in heterogeneous reactions of Cl₂, HOCl, and BrCl with halide-ice surfaces

In order to evaluate the relative importance of different heterogeneous mechanisms that may release gas-phase Br₂ into the Arctic boundary layer, interactions of Cl₂, HOCl, and BrCl with halide-containing ice surfaces were studied in the laboratory. Results are presented from experiments using a coated-wall, low-pressure flow tube coupled to a mass spectrometer. Gas-surface reaction probabilities were calculated from the first-order loss of Cl₂, HOCl, or BrCl on bromide-ice surfaces at 233 K and on chloride/bromide-ice surfaces at 248 K. In all cases, the reactions were relatively fast, and the primary product was gas-phase Br₂. To determine the main factors that influence the kinetics of these reactions, experiments were conducted over a range of temperatures, hydrogen ion concentrations in ice, and halide concentrations in ice. Cl₂ reaction probability increases with increasing bromide concentration in the ice film, up to approximately 3% bromide. At this point, the ice surface is completely saturated with bromide ions, and the maximum reaction probability, is 0.1. In contrast, HOCl reaction probability is highest when the ice film is made from a solution with low pH. BrCl reaction probability on bromide-ice films is less than that of Cl₂ and less than or equal to that of HOCl, depending on hydrogen ion concentration. No reaction between Cl₂ or HOCl and chloride-ice films was observed. The impact of Cl₂, HOCl, and BrCl heterogeneous chemistry on catalytic ozone destruction in the Arctic boundary layer is discussed.