Observations of Cyanogen Bromide (BrCN) in the Global Atmosphere during the NASA Atmospheric Tomography mission (ATom) and Implications for Active Bromine Chemistry.

<p>Bromine activation (the production of Br in an elevated oxidation state) represents a mechanism for ozone destruction and mercury removal in the global troposphere, and has been a common feature of both polar boundary layers, often accompanied by nearly complete ozone destruction. The chemistry and budget of active bromine compounds (e.g. Br<sub>2</sub>, BrCl, HOBr) reflects the cycling of Br and ultimately its impact on the environment. Cyanogen bromide (BrCN) has recently been measured by iodide ion high resolution time-of-flight mass spectrometry (I<sup>-</sup> CIMS) during the NASA Atmospheric Tomography mission, and could be a previously unquantified participant in active Br chemistry. BrCN mixing ratios ranged from below detection limit (1.5pptv) up to as high as 48 pptv (10sec avg) and enhancements were almost exclusively confined to the polar boundary layers (PBL). Likely BrCN formation pathways involve the reactions of active Br (Br<sub>2</sub>, HOBr) with reduced nitrogen compounds. Gas phase loss processes due to reaction with radical species are likely quite slow and photolysis is known to be relatively slow. These features, and the lack of BrCN enhancements above the PBL, imply that surface reactions must be the major loss processes. Known liquid phase reactions of BrCN result in the conversion of the Br to bromide (Br<sup>-</sup>) or formation of C-Br bonded organic species, hence a loss of atmospheric active Br from that chemical cycle. Thus, accounting for the chemistry of BrCN will be an important aspect of understanding polar Br cycling.</p>