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Abstract:
The isotopic composition of volatile organic compounds (VOCs) can
provide valuable information on their sources and fate not deducible
from mixing ratios alone. In particular the reported carbon stable
isotope ratios of chloromethane and bromomethane from different sources
cover a delta(13)C-range of almost 100 parts per thousand making isotope
ratios a very promising tool for studying the biogeochemistry of these
compounds. So far, the determination of the isotopic composition of C(1)
and C(2) halocarbons others than chloromethane is hampered by their low
mixing ratios.
In order to determine the carbon isotopic composition of C(1) and C(2)
halocarbons with mixing ratios as low as 1 pptv (i) a field suitable
cryogenic high volume sampling system and (ii) a chromatographic set up
for processing these samples have been developed and validated. The
sampling system was tested at two different sampling sites, an urban and
a coastal location in Northern Germany. The average delta(13)C-values
for bromomethane at the urban site were -42.9 +/- 1.1 parts per thousand
and agreed well with previously published results. But at the coastal
site bromomethane was substantially enriched in (13)C by almost 10 parts
per thousand. Less pronounced differences were observed for
chlorodifluoromethane, 1,1,1-trichloroethane and chloromethane. We
suggest that these differences are related to the turnover of these
compounds in ocean surface waters. Furthermore we report first carbon
isotope ratios for iodomethane (-40.4 parts per thousand to -79.8 parts
per thousand), bromoform (-13.8 parts per thousand to 22.9 parts per
thousand), and other halocarbons.
