ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-12-1423-2012 Bromine and iodine chemistry in a global chemistry-climate model: description and evaluation of very short-lived oceanic sources Ordóñez C. 1 Lamarque J.-F. 2 Tilmes S. 2 Kinnison D. E. 2 Atlas E. L. 3 Blake D. R. 4 Sousa Santos G. 5 Brasseur G. 6 Saiz-Lopez A. 1 Laboratory for Atmospheric and Climate Science (CIAC), CSIC, Toledo, Spain Atmospheric Chemistry Division, NCAR, Boulder, CO, USA University of Miami, Miami, FL, USA University of California, Irvine, CA, USA Institute of Atmospheric and Climate Science, ETH, Zurich, Switzerland Climate Service Center, Hamburg, Germany 07 02 2012 12 3 1423 1447 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/12/1423/2012/acp-12-1423-2012.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/1423/2012/acp-12-1423-2012.pdf

The global chemistry-climate model CAM-Chem has been extended to incorporate an expanded bromine and iodine chemistry scheme that includes natural oceanic sources of very short-lived (VSL) halocarbons, gas-phase photochemistry and heterogeneous reactions on aerosols. Ocean emissions of five VSL bromocarbons (CHBr<sub>3</sub>, CH<sub>2</sub>Br<sub>2</sub>, CH<sub>2</sub>BrCl, CHBrCl<sub>2</sub>, CHBr<sub>2</sub>Cl) and three VSL iodocarbons (CH<sub>2</sub>ICl, CH<sub>2</sub>IBr, CH<sub>2</sub>I<sub>2</sub>) have been parameterised by a biogenic chlorophyll-<i>a</i> (chl-<i>a</i>) dependent source in the tropical oceans (20° N–20° S). Constant oceanic fluxes with 2.5 coast-to-ocean emission ratios are separately imposed on four different latitudinal bands in the extratropics (20°–50° and above 50° in both hemispheres). Top-down emission estimates of bromocarbons have been derived using available measurements in the troposphere and lower stratosphere, while iodocarbons have been constrained with observations in the marine boundary layer (MBL). Emissions of CH<sub>3</sub>I are based on a previous inventory and the longer lived CH<sub>3</sub>Br is set to a surface mixing ratio boundary condition. The global oceanic emissions estimated for the most abundant VSL bromocarbons – 533 Gg yr<sup>−1</sup> for CHBr<sub>3</sub> and 67.3 Gg yr<sup>−1</sup> for CH<sub>2</sub>Br<sub>2</sub> – are within the range of previous estimates. Overall the latitudinal and vertical distributions of modelled bromocarbons are in good agreement with observations. Nevertheless, we identify some issues such as the reduced number of aircraft observations to validate models in the Southern Hemisphere, the overestimation of CH<sub>2</sub>Br<sub>2</sub> in the upper troposphere – lower stratosphere and the underestimation of CH<sub>3</sub>I in the same region. Despite the difficulties involved in the global modelling of the shortest lived iodocarbons (CH<sub>2</sub>ICl, CH<sub>2</sub>IBr, CH<sub>2</sub>I<sub>2</sub>), modelled results are in good agreement with published observations in the MBL. Finally, sensitivity simulations show that knowledge of the diurnal emission cycle for these species, in particular for CH<sub>2</sub>I<sub>2</sub>, is key to assess their global source strength.

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