Atmospheric concentrations of the volatile reactive iodocarbons C[subscript 2]H[subscript 5]I, 1-C[subscript 3]H[subscript 7]I, 2-C[subscript 3]H[subscript 7]I, CH[subscript 2]ICl, CH[subscript 2]IBr, CH[subscript 2]I[subscript 2] and bromocarbons CH[subscript 2]Br[subscript 2] and CHBr[subscript 3] were determined by GC/MS analysis of marine boundary layer air at Roscoff, Brittany on the northwest coast of France during September 2006. Comparison with other coastal studies suggests that emissions of these trace gases are strongly influenced by site topography, seaweed populations and distribution, as well as wind speed and direction and tide height. Concentrations of the very short-lived dihalomethanes CH[subscript 2]IBr and CH[subscript 2]I[subscript 2] in particular showed evidence of tidal dependence, with higher concentrations observed at low tide during maximum exposure of seaweed beds. We also present a limited number of halocarbon measurements in surface seawater and estimate sea-air fluxes based on these and simultaneous air measurements. CH[subscript 2]Br[subscript 2] and CHBr[subscript 3] were strongly correlated both in air and in seawater, with CH[subscript 2]Br[subscript 2]/CHBr[subscript 3] ratios of 0.19 in air and 0.06 in water. The combined midday I atom flux from the photolabile diahlomethanes CH[subscript 2]I[subscript 2], CH[subscript 2]IBr and CH[subscript 2]ICl of ~5×10[superscript 3] molecules cm[superscript −3] s[superscript −1] is several orders of magnitude lower than the estimated I atom flux from I[subscript 2] based on coinciding measurements at the same site, which indicates that at Roscoff the major I atom precursor was I[subscript 2] rather than reactive iodocarbons
