A high volume sampling system for isotope determination of volatile halocarbons and hydrocarbons

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