Quantifying the contribution of marine organic gases to atmospheric iodine

Oceanic emissions of gaseous organic iodine-atom precursors have the potential to significantly affect atmospheric chemistry and climate, however there is currently considerable uncertainty associated with quantifying their sources. We present sea-air fluxes calculated from simultaneous air and seawater measurements of a comprehensive range of volatile organic iodine compounds (VOICs), including CH3I and the less commonly reported dihalomethanes CH2ICl, CH2IBr and CH2I2, made during two cruises in the Atlantic Ocean between 15-58N. The combined dihalomethane flux provides a global iodine source (∼0.33 0.19 Tg I y -1) comparable to that of CH3I, and a surface iodine atom source 3-4 times higher. However, a 1D atmospheric model reveals that, in the tropical east Atlantic Ocean in the vicinity of Cape Verde, even these combined VOIC fluxes are capable of supporting only ∼10-25% of the observed IO levels, and suggests that a substantial (340-640 nmol I m-2 d -1) additional photochemical source of iodine is required. © 2010 by the American Geophysical Union.