Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 1 1 2001 10.5194/acpd-1-193-2001 http://www.atmos-chem-phys-discuss.net/1/193/2001/ http://www.atmos-chem-phys-discuss.net/1/193/2001/acpd-1-193-2001.html http://www.atmos-chem-phys-discuss.net/1/193/2001/acpd-1-193-2001.pdf 193 220 2001-09-03 Coastal zone production of IO precursors: A 2-dimensional study L. J. Carpenter K. Hebestreit U Platt P. S. Liss Department of Chemistry, University of York, York, YO10 5DD, U.K. Institut für Umweltphysik, University of Heidelberg, INF 229, D-69120 Heidelberg, Germany School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K. 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 CH₂I₂ and IO at low water indicate that the intertidal region is a strong source of organoiodines. Atmospheric measurements of CH₂I₂ made in marine air are used in conjunction with a 2-dimensional model incorporating horizontal and vertical dispersion to provide estimates of the intertidal and offshore fluxes of CH₂I₂ upwind of Mace Head. The strong signature of photolysis in the CH₂I₂ observations indicates that the emissions are not entirely local/coastal, but must include an additional offshore source. Good agreement between model and measured CH₂I₂ concentrations is achieved with an offshore flux of 2 × 10⁴ molecules cm^-2 s^-1 and an intertidal flux ranging from 1.3 × 10⁹ molecules cm^-2 s^-1 (low water) to 6.5 × 10⁸ molecules cm^-2 s^-1 (high water), the latter over a 100 m wide coastal belt. The coastal emissions at low water are in good agreement with independent estimates made from seaweed emission data. We estimate that, although intertidal emissions are ~4 orders of magnitude higher than those offshore, their contribution to the measured CH₂I₂ concentrations at 10 m height is only ~50%.