Abstract. Active bromine (e.g., Br2, BrCl, BrO, HOBr) promotes atmospheric ozone destruction and mercury removal. Here we report a previously unidentified participant in active-Br chemistry, cyanogen bromide (BrCN), measured during the NASA Atmospheric Tomography (ATom) mission. BrCN was confined to polar boundary layers, often appearing at concentrations higher than other Br compounds. The chemistry of BrCN determines whether it promotes or inhibits ozone and mercury removal. This dataset provides evidence that much of the BrCN was from atmospheric Br chemistry involving surface reactions with reduced nitrogen compounds. Since gas phase loss processes are known to be relatively slow, surface reactions must also be the major loss processes, with vertical profiles implying a BrCN atmospheric lifetime in the range 1–10 days. Liquid phase reactions of BrCN tend to convert Br to bromide (Br¯) or C-Br bonded organics, constituting a loss of active Br. Thus, accounting for BrCN chemistry is crucial to understanding polar Br cycling.;
