Marked seasonality in the concentrations and sea-to-air flux of volatile iodocarbon compounds in the western English Channel

In the first seasonal study of volatile iodinated organic compounds (VICs) in the open sea, concentrations of five VICs were measured approximately weekly at four depths, over 20 months from July 2002 to April 2004, in the western English Channel. Seawater concentrations varied seasonally by an order of magnitude for all five compounds, with winter minima and, generally, late summer/autumn maxima. The average contribution to the dissolved VIC pool was chloroiodomethane (39%), diiodomethane (33%), iodomethane (22%), iodoethane (6%), and bromoiodomethane (4%). Total sea-to-air flux of iodine atoms carried by the VICs (15.5 mu mol I m(-2) yr(-1)) was approximately fourfold higher than that for iodomethane alone. This contrasts with previous studies that indicated that iodomethane was the main vector of iodine away from macroalgal beds. The estimated sea-to-air flux takes into account the significant airside control of the gas exchange of certain VICs, particularly diiodomethane; for which a 30% reduction in average daily flux was observed when an additional airside transfer velocity was included in the calculations. Because of their high reactivity, chloroiodomethane and diiodomethane are likely to drive the atmospheric organic iodine chemistry over these shelf seas, rather than the monohalogenated VICs.