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| Titel |
An automated system for selective and continuous measurements of vertical thoron profiles for the determination of transport times near the ground |
| VerfasserIn |
D. Plake, I. Trebs |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 6, no. 4 ; Nr. 6, no. 4 (2013-04-16), S.1017-1030 |
| Datensatznummer |
250017875
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| Publikation (Nr.) |
 copernicus.org/amt-6-1017-2013.pdf |
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| Zusammenfassung |
| The quantification of in-canopy transport times is of major importance for
the investigation of sources, sinks and net fluxes of reactive trace gases
within plant canopies. The Damköhler number, which compares timescales of
chemical reactions with transport times, is a widely applied measure to
evaluate flux divergences. In this study we present and evaluate a novel
automated measurement system for selective vertical thoron (Tn) profiles near
the Earth's surface and demonstrate its suitability for the direct and
reliable determination of transport times within a natural grassland canopy.
For the first time, we perform a rigorous determination of systematic and
random uncertainties of Tn (and Rn) concentrations under field conditions for
this type of measurement system. The obtained median precisions for three
concentration classes (> 100 Bq m−3,
100–15 Bq m−3, < 15 Bq m−3) were 8.8%,
23.2% and 132.1% for Tn (and 16.6%, 25.0%, 99.2% for
Rn). We calculate in-canopy transport times (τ) and propagate their
uncertainty from the individual errors of the Tn concentration measurements.
A quality assessment of τ for the field experiment during a period of
51 days revealed good data quality with 44% of the relative
uncertainties below 50%. The occurrence of transport time uncertainties
higher than 100% was caused by absolute Tn gradients lower than
70 Bq m−3 m−1, which was found for 22% of all determined
transport times. In addition, the method was found to be highly sensitive to
the Tn concentrations at the upper of the two inlet heights (zu).
Low values of CTnzu result in high absolute
uncertainties of the transport time. A comparison with empirical
parameterizations revealed a much lower scatter for the τ values
determined from our measurements. We found an excellent agreement with τ
values obtained by the in-canopy resistance approach used, e.g., in the
SURFATM model during daytime, while the SURFATM model significantly
overestimated transport times during nighttime. |
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