Coastal zone production of IO precursors: A 2-dimensional study
1Department of Chemistry, University of York, York, YO10 5DD, U.K.
2Institut für Umweltphysik, University of Heidelberg, INF 229, D-69120 Heidelberg, Germany
3School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K.
Abstract. 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%.