Abstract
Abstract. 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.
Highlights
Emission of iodine- and bromine-containing trace organic compounds into the atmosphere affects the chemical balance of both the troposphere and the stratosphere and has implications for climate
H2O2 is produced by normal metabolic activity within the plants cells, when macroalgae are subjected to oxidative stress, additional H2O2 is released as part of a defence mechanism, and can reach levels that exceed the plants cellular scavenging capacity (Theiler et al, 1978; Pedersen et al, 1996; Palmer et al, 2005)
It is apparent that biological and physical environmental factors such as the inter-tidal range, wind speed and direction, site topography, seaweed speciation and seaweed distribution greatly influence emissions of reactive halocarbons, and as such caution should be taken in any attempt to scale up coastal measurements from any one site in order to infer global coastal halocarbon emission terms
Summary
Emission of iodine- and bromine-containing trace organic compounds into the atmosphere affects the chemical balance of both the troposphere and the stratosphere and has implications for climate. Short-lived volatile halogenated organic compounds (VHOCs) are produced by marine algae, and the exact purpose of their production is not fully understood, possible explanations include release of halocarbons by means of a chemical defence mechanism against grazing herbivores (Wever et al, 1991) and as by-products of reactions to scavenge oxidants such as H2O2, OH and O3 which may cause damage to the plant (Pedersen et al, 1996).
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