*Corresponding author. Department of Physics, University of Toronto, 60 St. George Street, Toronto, Ont., M5S 1A5, Canada. e-mail: matt@atmosp.physics.utoronto.ca. Atmospheric Environment 33 (1999) 2373—2383 The concurrent observation of methyl iodide and dimethyl sulphide in marine air; implications for sources of atmospheric methyl iodide Matthew R. Bassford*, Graham Nickless, Peter G. Simmonds, Alastair C. Lewis, Michael J. Pilling, Mathew J. Evans School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 ITS, UK The Environment Centre (ACL) and School of Chemistry (MJP), University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK Centre for Atmospheric Science, University of Cambridge Chemical Laboratory, Lensfield Road, Cambridge, CB2 1EW, UK Received 8 April 1998; received in revised form 19 October 1998; accepted 11 November 1998 Abstract Continuous atmospheric measurements of methyl iodide and dimethyl sulphide were carried out at Mace Head, western Ireland, over a 4-week period in July 1996. The concurrent observations of methyl iodide and dimethyl sulphide reported here display a clear association, indeed statistical analysis indicated a very significant degree of covariance. A simple yet informative use of modelled 5-day back trajectories was employed in tandem with examination of local meteorology to illuminate the geographical source regions of methyl iodide and dimethyl sulphide. The interpretation of the atmospheric observations in terms of air-mass flow has elucidated part of the global methyl iodide cycle and provides evidence for two distinct source regions of methyl iodide: 1. Under certain synoptic meteorological conditions, long-range transport of methyl iodide and dimethyl sulphide was observed from discrete areas of the sub-tropical Atlantic Ocean located in a region between 30 —50°N and 20 —50°W. 2. Measurements taken under different conditions led us to believe that there 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.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Iodine; Air-mass trajectory; Long-range transport; Phytoplankton; Ocean; Macroalgae 1. Introduction It is generally accepted that atmospheric methyl iodide (CH  I) is predominantly derived from marine organisms (Lovelock et al., 1973; Singh et al., 1983). Although other atmospheric organoiodine compounds such as CH  ClI, CH  I  and C  H  I have been detected (Klick and Ab- rahamsson, 1992), CH  I is thought to play the dominant role in biogeochemical cycling of iodine. Compared with the other monohalomethanes present in the atmosphere (CH  Cl and CH  Br), CH  I is relatively active in tropo- spheric chemistry because the C—I bond is far more photochemically labile than the analogous C—Cl or C—Br bonds. In fact, the major sink for atmospheric CH  I is photolysis (i.e. CH  I#hPCH  #I) and the calculated 1352-2310/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 4 0 3 - 8