The concurrent observation of methyl iodide and dimethyl sulphide in marine air; implications for sources of atmospheric methyl iodide

Continuous atmospheric measurements of methyl iodide and dimethyl sulphide were carried out at Mace Head, western Ireland, over a 4-week period in Jul y 1996. The concurrent observations of methyl iodide and dimethyl sulphide reported here display a clear association, indeed statistical analysis indi cated a very significant degree of covariance. A simple yet informative use of modelled 5-day back trajectories was employed in tandem with examinatio n of local meteorology to illuminate the geographical source regions of met hyl iodide and dimethyl sulphide. The interpretation of the atmospheric obs ervations in terms of air-mass flow has elucidated part of the global methy l iodide cycle and provides evidence for two distinct source regions of met hyl iodide: 1. Under certain synoptic meteorological conditions, long-range transport o f methyl iodide and dimethyl sulphide was observed from discrete areas of t he sub-tropical Atlantic Ocean located in a region between 30-50 degrees N and 20-50 degrees W. 2. Measurements taken under different conditions led us to believe that the re was an additional source of methyl iodide that influenced the Mace Head atmosphere, most likely produced by coastal macroalgae which inhabit waters off the western coast of Ireland. (C) 1999 Elsevier Science Ltd. All right s reserved.