ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-1-9-2001 Coastal zone production of IO precursors: a 2-dimensional study Carpenter L. J. 1 Hebestreit K. 2 Platt U. 2 Liss P. S. 3 Department of Chemistry, University of York, York, YO10 5DD, UK Institut f¨ur Umweltphysik, University of Heidelberg, INF 229, D-69120 Heidelberg, Germany School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK 23 11 2001 1 1 9 18 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/1/9/2001/acp-1-9-2001.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/1/9/2001/acp-1-9-2001.pdf

At Mace Head, Eire, in the coastal East Atlantic, diiodomethane has been identified as an important precursor of iodine oxide radicals. Peak concentrations of both CH<sub>2</sub>I<sub>2</sub> and IO at low water indicate that the intertidal region is a strong source of organo-iodines. Atmospheric measurements of CH<sub>2</sub>I<sub>2</sub> made in marine air are compared with the concentrations predicted by a 2-dimensional model incorporating horizontal and vertical dispersion of surface emissions. The model shows that micrometeorological variability, proximity of the site to emissions, and photolysis all play important roles in determining the CH<sub>2</sub>I<sub>2</sub> concentrations at Mace Head. In addition to a tidal-height dependent intertidal flux, which was estimated from seaweed production data, a contribution from offshore (non-local) sources was required in order to reproduce the strong signature of photolysis in the CH<sub>2</sub>I<sub>2</sub> observations. A combination of an offshore flux and an intertidal flux (of up to 1.4 × 10<sup>9</sup> molecules cm<sup>-2</sup>s<sup>-1</sup> at low water) results in good agreement between the measured and modelled CH<sub>2</sub>I<sub>2</sub> concentrations. Although this study does not necessarily infer emission of CH<sub>2</sub>I<sub>2</sub> from the open ocean, it suggests that air-sea exchange of CH<sub>2</sub>I<sub>2</sub> in coastal waters does occur.