AUGUST 2001: Oceanic distribution of methyl iodide

Methyl iodide (CH₃I) is emitted by the oceans and photolyzes in the atmosphere on a time scale of 4 days in the tropics. In a recently submitted paper [Bell et al., 2001], we have developed a global simulation of methyl iodide to test marine convective transport in atmospheric models, complementing the common use of ²²²Rn to test convective transport over continents. Methyl iodide is also of interest as a major source of iodine radicals to the atmosphere. The present figure shows the distribution of seawater methyl iodide concentrations computed with an oceanic mixed layer model coupled to the GEOS-CHEM atmospheric transport model. In that oceanic model, methyl iodide is produced photochemically from dissolved organic carbon (DOC), and is lost from the oceans by SN2 reaction with Cl^- and by (reversible) escape to the atmosphere. Observations from ship cruises are superimposed on the figure as circles; the model captures 40% of the variance in these observations. The model simulates maximum oceanic concentrations at subtropical latitudes in the summer season where solar radiation is high but seawater temperature is relatively low (suppressing the SN2 reaction). Similar subtropical maxima are seen in the observations. Summer midlatitude concentrations in the model are too high, suggesting the possibility of a biological sink missing from the model. We compute a global emission of oceanic methyl iodide to the atmosphere of 214 Gg yr^-1, at the low end of previous estimates which were based on global extrapo lation of regional data. Comparison of the atmospheric vertical profiles of methyl iodide simulated with GEOS-CHEM to t he ensemble of aircraft observations in different regions of the world indicates no significant global bias and implies a good overall simulation of marine convective transport in the GEOS meteorological data. This work was conducted by Nadine Bell.