Global measurements of methyl iodide (CH3I) show that typical concentrations in background air are between 1 and 3 pptv (1 ppt = 10−12) in the boundary layer and drop to half of these values above the boundary layer. Concentrations of CH3I over cities in Oregon were found to be similar to background levels. These low concentrations suggest that CH3I may not affect atmospheric concentrations of free radicals, O3, or the NO2/NO ratio, as recent model calculations had shown. We also found that near‐oceanic regions characterized by high biomass productivity, there is 10 pptv −20 pptv of CH3I. By using these results in conjunction with measurements of CH3I in seawater we calculated a global flux of about 1.3 Tg/year from the oceans, most of it (1 Tg/year) from the small fraction of the ocean with high biomass productivity. All these results are based on more than 450 measurements of CH3I at locations ranging from the arctic circle to the south pole. Uncertainties relating to the global distribution, sources, and sinks are identified and discussed.

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