The ocean is the major global reservoir of the halogens chlorine, bromine and iodine. These elements exist as halide ions in seawater, with the major form of iodine being present as the oxidised state, iodate (IO₃^-). It has been shown that in addition to the flux of particulate halides in seasalt, volatile organohalogen compounds are also important in the transfer of these elements from the ocean to the atmosphere [1-3]. Once in the atmosphere, the organohalogens are photodissociated to form reactive radical species that catalytically destroy ozone. The longer-lived bromine and chlorine species are involved in stratospheric ozone destruction whereas the shorter-lived iodine-containing species primarily affect tropospheric ozone levels [4], although Solomon et al., [5] showed that iodine could also play a role in ozone destruction in the lower stratosphere. The gaseous flux of iodine is also important for supplying terrestrial organisms with this essential element [6] and may contribute to new particle formation in the marine boundary layer [7].

Organohalogens with a significant oceanic source include the iodocarbons, methyl iodide (CH₃I), chloroiodomethane (CH₂CII), diiodomethane (CH₂I₂), iodoethane (C₂H₅I), 1- and 2-iodopropanes, 1- and 2-iodobutanes and the novel polyhalogenated compounds CH₂IBr, CHI₂Cl and CHIBr₂. For the bromocarbons, bromoform (CHBr₃) is considered to be the most abundant form of biogenic reactive organic bromine, which also comprises dibromomethane (CH₂Br₂), dibromochloromethane (CHBr₂Cl), dichlorobromomethane (CHBrCl₂), and bromochloromethane (CH₂BrCl).

Measurements of CH₃I, CH₂CII, CHBr₃, CH₂Br₂, CHBr₂Cl and CHBrCl₂ have been made during three open ocean cruises in the Atlantic and Southern Oceans between 50°N and 50°S (Atlantic Meridional Transect 9; ANT XVIII/1&2, Meteor M55). Surface seawater and air concentrations were measured as well as depth profiles in the top 250 m of the ocean. The saturation anomalies and air-sea fluxes of the gases have been calculated which show methyl iodide and chloroiodomethane to be highly supersaturated (>1000%) throughout the temperate and tropical regions[8]. The oceanic source of the brominated compounds is more variable and further field studies with a greater temporal and spatial sampling resolution are required to assess the oceanic flux of these gases.

Current research within the Trace Gas Biogeochemistry Group is focused on the biological and photochemical mechanisms for production and destruction of the organohalogens in seawater.

1. Lovelock, J.E., Natural halocarbons in air and in the sea. Nature, 1975. 256: p. 193-194.
2. Rasmussen, R.A., et al., Concentration distribution of methyl chloride in the atmosphere. Journal of Geophysical Research, 1980. 85: p. 7350-7356.
3. Singh, H.B., L.J. Salas, and R.E. Stiles, Methyl halides in and over the Eastern Pacific (40ºN-32ºS). Journal of Geophysical Research, 1983a. 88(C6): p. 3684-3690.
4. Chameides, W.L. and D.D. Davis, Iodine: Its possible role in tropospheric photochemistry. Journal of Geophysical Research, 1980. 85: p. 7383-7398.
5. Solomon, S., R. Garcia, R., and A.R. Ravishankara, On the role of iodine in ozone depletion. Journal of Geophysical Research, 1994. 99(D10): p. 20 491-20499.
6. Fuge, R. and C.C. Johnson, The Geochemistry of Iodine - a Review. Environmental Geochemistry and Health, 1986. 8(2): p. 31-54.
7. O'Dowd, C., et al., Marine aerosol formation from biogenic iodine emissions. Nature, 2002. 417: p. 632-636.
8. Chuck, A.L., Biogenic halocarbons and light alkyl nitrates in the marine environment, in School of Environmental Sciences. 2002, University of East Anglia: Norwich.