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Abstract:
Methyl bromide (CH3Br) emitted from plants constitutes a natural source of bromine to the atmosphere, and is a component in the currently unbalanced global CH3Br budget. In the stratosphere, CH3Br contributes to ozone loss processes. Studies of stable isotope composition may reduce uncertainties in the atmospheric CH3Br budget, but require well-constrained isotope fingerprints of the source end members. Here we report the first measurements of stable bromine isotopes (delta Br-81) in CH3Br from abiotic plant emissions. Incubations of both KBr-fortified pectin, a ubiquitous cell-stabilizing macromolecule, and of a natural halophyte (Salicornia fruticosa), yielded an enrichment factor (epsilon) of -2.00 +/- 0.23 parts per thousand (1 sigma, n = 8) for pectin and -1.82 +/- 0.02 parts per thousand (1 sigma, n = 4) for Salicornia (the relative amount of the heavier Br-81 was decreased in CH3Br compared to the substrate salt). For short incubations, and up to 10% consumption of the salt substrate, this isotope effect was similar for temperatures from 30 up to 300 degrees C. For longer incubations of up to 90 h at 180 degrees C the delta Br-81 values increased from -2 parts per thousand to 0 parts per thousand for pectin and to -1 parts per thousand for Salicornia. These delta Br-81 source signatures of CH3Br formation from plant matter combine with similar data for carbon isotopes to facilitate multidimensional isotope diagnostics of the CH3Br budget. (C) 2013 Elsevier Ltd. All rights reserved.
