Pratt et al. paper published in Nature Geoscience!
04-14-2013
Photochemical production of molecular bromine in Arctic surface snowpacks
Kerri A. Pratt1*, Kyle D. Custard1, Paul B. Shepson1,2, Thomas A. Douglas3, Denis Pöhler4, Stephan General4, Johannes Zielcke4,William R. Simpson5, Ulrich Platt4, David J. Tanner6, L. Gregory Huey6, Mark Carlsen1 and Brian H. Stirm7
Following the springtime polar sunrise, ozone concentrations in the lower troposphere episodically decline to near-zero levels1. These ozone depletion events are initiated by an increase in reactive bromine levels in the atmosphere2-5. Under these conditions, the oxidative capacity of the Arctic troposphere is altered, leading to the removal of numerous transported trace gas pollutants, including mercury6. However, the sources and mechanisms leading to increased atmospheric reactive bromine levels have remained uncertain, limiting simulations of Arctic atmospheric chemistry with the rapidly transforming sea-ice landscape7,8. Here, we examine the potential for molecular bromine production in various samples of saline snow and sea ice, in the presence and absence of sunlight and ozone, in an outdoor snow chamber in Alaska. Molecular bromine was detected only on exposure of surface snow (collected above tundra and first-year sea ice) to sunlight. This suggests that the oxidation of bromide is facilitated by a photochemical mechanism, which was most efficient for more acidic samples characterized by enhanced bromide to chloride ratios. Molecular bromine concentrations increased significantly when the snow was exposed to ozone, consistent with an interstitial air amplification mechanism. Aircraft-based observations confirm that bromine oxide levels were enhanced near the snow surface. We suggest that the photochemical production of molecular bromine in surface snow serves as a major source of reactive bromine, which leads to the episodic depletion of tropospheric ozone in the Arctic springtime.
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