Abstract
Abstract. 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 photochemical 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.
Highlights
IntroductionIn logical and photochemical production mechanisms are con- the open ocean, CH3I sources are unclear, and uncertainsidered in a biogeochemical module that is coupled to a ties remain with regard to origin of the source as well as one-dimensional water column model for the eastern tropi- production rates
The CH3I production rates that have for explaining CH3I concentrations in the eastern tropical At- been independently derived for the same species by different lantic
When photochemical production is parameterized using light that is efficiently absorbed in the surface layers (i.e. UV light identical to the parameterization of photolysis) CH3I production is restricted to the upper model levels and never takes place below the mixed layer
Summary
In logical and photochemical production mechanisms are con- the open ocean, CH3I sources are unclear, and uncertainsidered in a biogeochemical module that is coupled to a ties remain with regard to origin of the source as well as one-dimensional water column model for the eastern tropi- production rates. The model is able to reproduce observed sub- or a photochemical prOoduccetioannpaSthcwiaey.nLcaeboratory experisurface maxima of CH3I concentrations. The dominat- ments in which filtered sea water was irradiated show phoing source process cannot be clearly identified as subsurface tochemical production of CH3I in absence of living phytomaxima can occur due to both direct biological and photo- plankton cells that could account for at least 50 % of obchemical production. Good agreement between the served CH3I emissions from the tropical Atlantic
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