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| Titel |
Tropospheric chlorine isotope measurements in CFC-11, CFC-12 and CFC-113 |
| VerfasserIn |
Sam Allin, Jan Kaiser, Johannes Laube, Bill Sturges |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250100812
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| Publikation (Nr.) |
 EGU/EGU2014-16811.pdf |
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| Zusammenfassung |
| In 2010, we reported the first measurements of chlorine isotope fractionation in stratospheric
dichlorodifluoromethane (CF2Cl2, CFC-12) (Laube et al., Science 329:1167, 2010). We
found an increase in the isotope delta, δ(37Cl), with altitude and a tight correlation between
ln[1 + δ(37Cl)] and ln(mixing ratio). The derived apparent isotope fractionation was
εapp = (-12 ± 2) o.
The stratospheric isotopic fractionation should lead to a continuous increase of the
tropospheric chlorine isotope delta while CFC-12 is still emitted into the atmosphere.
Provided the source signature has not changed, we predict a 2.5 to 3 o increase since when
CFC emissions started in the 1930s until 2010, with the strongest increase in recent years
(about 1 o per decade since the mid-1990s).
We have now measured the chlorine isotope delta of CFC-12 as well as CFC-11
(trichlorofluoromethane, CFCl3) and CFC-113 (1,1,2-trichloro-1,2,2-trifluoroethane,
C2F3Cl3) in the Cape Grim Air Archive (1978 to 2010) and Arctic (NEEM, Greenland) and
Antarctic firn samples (Fletcher, West Antarctica). The deepest firn samples include a
significant proportion of older air from before 1978. The repeatability for individual samples
was ±2.6o for CFC-12 and ±2.7o for CFC-11 and ±3.7o for CFC-113. The results
show no significant trends in δ(37Cl) over the whole time period; however, there is a small
positive trend for the latter period of the samples of (0.3 ± 0.1) o per decade for CFC-12
since 1997, which explains at third of the predicted trend. The discrepancy between observed
and predicted trends may be due to offsetting changes in the source isotope signature: If the
there was decrease in the isotope delta of the emissions over time, this would reduce the
predicted increase caused by downward transport of 37Cl-enriched stratospheric air. |
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