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| Titel |
Stratospheric age tracers: re-evaluating old friends and making new ones |
| VerfasserIn |
Emma Leedham Elvidge, Harald Bönisch, Andreas Engel, Paul J. Fraser, Eileen Gallacher, Lauren Gooch, Jens Muhle, David E. Oram, Eric A. Ray, Thomas Röckmann, William T. Sturges, Ray F. Weiss, Johannes C. Laube |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250144101
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| Publikation (Nr.) |
 EGU/EGU2017-7886.pdf |
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| Zusammenfassung |
| Stratospheric transport, specifically the mean meridional or Brewer-Dobson circulation,
cannot be measured directly, but can be inferred from trace gas distributions. For example, the
transit time of air from the troposphere to a given location in the stratosphere is
described by the ‘age of the air’, determined by observations of inert chemical tracers.
Ideal tracers should have no stratospheric sources or sinks and should have shown a
linear tropospheric trend for at least a decade. Sulphur hexafluoride, SF6, is a very
long-lived compound that is often used as an atmospheric transport tracer. Use
of SF6 assumes an accurate understanding of its atmospheric lifetime, which is
currently estimated to be around 3200 years. However, as SF6 loss mainly occurs in
the rarely-sampled mesosphere, loss rates must be determined indirectly. Recent
evidence suggests that SF6 loss mechanisms may be underestimated, reducing its
atmospheric lifetime. This would complicate the use of SF6 as a tracer of stratospheric
transport. In this work we collate data from five stratospheric aircraft and balloon
campaigns stretching over 17 years with long-term tropospheric trends from Cape Grim,
Tasmania to re-investigate the suitability of this age tracer. At the same time, we assess
alternative transport tracers, such as CF4(PFC-14), C2F6 (PFC-116), C3F8 (PFC-218)
and CHF3 (HFC-23), HFC-125 and HFC-227ea. Mean ages derived from each
tracer are compared, along with an in-depth analysis of the uncertainties involved
in these calculations. Key uncertainties associated with calculating the age of air
from chemical tracers include: (1) uncertainties in the atmospheric measurements,
(2) uncertainties during the processing (namely applying a polynomial fit) of the
tropospheric trend for input into the age calculation and (3) uncertainties in the
assumptions involved in the age of air calculation. Our results indicate good suitability
for some of these gases, in terms of their inertness, tropospheric growth rates and
measurement precisions, to serve as tracers to investigate stratospheric transport. |
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