Coastal zone production of IO precursors

Introduction Conclusions References Tables Figures Back Close Print Version Interactive Discussion Abstract Introduction Conclusions References Tables Figures Back Close Print Version Interactive Discussion 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 CH 2 I 2 and IO at low water indicate that the intertidal region is a strong source of organo-iodines. Atmospheric measurements of CH 2 I 2 made in marine air are used in conjunction with 5 a 2-dimensional model incorporating horizontal and vertical dispersion to provide estimates of the intertidal and offshore fluxes of CH 2 I 2 upwind of Mace Head. The strong signature of photolysis in the CH 2 I 2 observations indicates that the emissions are not entirely local/coastal, but must include an additional offshore source. Good agreement between model and measured CH 2 I 2 concentrations is achieved with an offshore flux 10 of 2 × 10 4 molecules cm −2 s −1 and an intertidal flux ranging from 1.3 × 10 9 molecules cm −2 s −1 (low water) to 6.5 × 10 8 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, 15 their contribution to the measured CH 2 I 2 concentrations at 10 m height is only ∼ 50%.