Air-sea exchange provides the mechanism by which iodine is transferred from the oceans to the atmosphere and land. This exchange occurs primarily by the emission of gases, such as methyl iodide, from the oceans. In the atmosphere methyl iodide is broken down to produce inorganic iodine. This break down may be an important control on ozone cycling in the remote atmosphere. The iodine formed is ultimately deposited either back to the oceans or to land. On land this input is an important source of iodine in the human diet. Insufficient iodine in the diet can lead to the medical condition goitre and other iodine deficiency disorders. (Click for a description of the iodine cycle).
Our work aims to improve our understanding of the cycling of iodine. We do not understand how methyl iodide is produced or the complex chemistry of iodine after methyl iodide is broken down in the atmosphere. Until we do understand these processes, it is not possible to describe quantitatively the role that iodine plays in atmospheric chemistry and how this role might change as climatic conditions change.
We have tried to consider the entire cycle of iodine, including work on methyl iodide and the inorganic forms of iodine in the oceans and atmosphere, and the return fluxes of iodine from the atmosphere back to land and oceans.
Our studies confirm that methyl iodide is produced in the oceans, though the mechanisms involved are uncertain. We have shown that these mechanisms are different to those producing other important gases in the oceans such as dimethyl sulphide (DMS). We have also shown that seaweed provide a much stronger source of methyl iodide than phytoplankton, though on a global scale phytoplankton will dominate the total production. The release from seaweed appears to be a response to stress. Under such stress the seaweed also release large amounts of inorganic iodine. The reasons for such iodine release in response to stress are unclear, but may be related to the antibiotic properties of many iodine compounds.
Once the methyl iodide is released to the atmosphere it has a lifetime of a few days before it is converted to other forms of iodine associated with the smallest aerosol particles in the atmosphere. We have also demonstrated that some of this aerosol iodine appears to be in an organic form.
The iodine is eventually lost from the atmosphere through deposition in aerosol and rain. Using the results of a 15-month field campaign sampling rain and aerosol at the Weybourne Atmospheric Observatory we have determined the relative contributions of wet and dry deposition to the total flux of iodine to land. We have also interpreted the different chemical forms of iodine in our samples to infer information on the chemical processing of iodine in the atmosphere.