<p><strong>Abstract.</strong> Methyl iodide (CH₃I) is a volatile organic halogen compound that contributes significantly to the transport of iodine from the ocean to the atmosphere, where it plays an important role in tropospheric chemistry. CH₃I is naturally produced and occurs in the global ocean. The processes involved in the formation of CH₃I, however, are not fully understood. In fact, there is an ongoing debate whether production by phytoplankton or

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... emical degradation of organic matter is the main source term. Here, both the biological and photochemical production mechanisms are considered in a biogeochemical module that is coupled to a one-dimensional water column model for the eastern tropical Atlantic. The model is able to reproduce observed subsurface maxima of CH₃I concentrations. But, the dominating source process cannot be clearly identified as subsurface maxima can occur due to both direct biological and photochemical production. However, good agreement between the observed and simulated difference between surface and subsurface methyl iodide concentrations is achieved only when direct biological production is taken into account. Production rates for the biological CH₃I source that were derived from published laboratory studies are shown to be inappropriate for explaining CH₃I concentrations in the eastern tropical Atlantic.</p>