Coastal measurements of short-lived reactive iodocarbons and bromocarbons at Roscoff, Brittany during the RHaMBLe campaign

Atmospheric concentrations of the volatile reactive iodocarbons C2H5I, 1-C3H7I, 2-C3H7I, CH2ICl, CH2IBr, CH2I2 and bromocarbons CH2Br2 and CHBr3 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 CH2IBr and CH2I2 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. CH2Br2 and CHBr3 were strongly correlated both in air and in seawater, with CH2Br2/CHBr3 ratios of 0.19 in air and 0.06 in water. The combined midday I atom flux from the photolabile diahlomethanes CH2I2, CH2IBr and CH2ICl of ~5×103 molecules cm−3 s−1 is several orders of magnitude lower than the estimated I atom flux from I2 based on coinciding measurements at the same site, which indicates that at Roscoff the major I atom precursor was I2 rather than reactive iodocarbons.