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| Titel |
CO2-gradient measurements using a parallel multi-analyzer setup |
| VerfasserIn |
L. Siebicke, G. Steinfeld, T. Foken |
| 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 ; 4, no. 3 ; Nr. 4, no. 3 (2011-03-01), S.409-423 |
| Datensatznummer |
250001792
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| Publikation (Nr.) |
 copernicus.org/amt-4-409-2011.pdf |
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| Zusammenfassung |
| Accurate CO2 concentration gradient measurements are needed for the
computation of advective flux terms, which are part of the full Net Ecosystem
Exchange (NEE) budget equation. A typical draw back of current
gradient measurement designs in advection research is the inadequate sampling
of complex flow phenomena using too few observation points in space and time.
To overcome this draw back, a new measurement design is presented which
allows the parallel measurement of several sampling points at a high
frequency. Due to the multi-analyzer nature of the design, inter-instrument
bias becomes more of a concern compared to conventional setups. Therefore a
statistical approach is presented which allows for accurate observations of
concentration gradients, which are typically small in relation to analyzer
accuracy, to be obtained. This bias correction approach applies a
conditional, time dependent signal correction. The correction depends on a
mixing index based on cross correlation analysis, which characterizes the
degree of mixing of the atmosphere between individual sample points. The
approach assumes statistical properties of probability density functions
(pdf) of concentration differences between a sample point and the
field average which are common to the pdf's from several sample
points. The applicability of the assumptions made was tested by Large Eddy
Simulation (LES) using the model PALM and could be verified for a test case
of well mixed conditions. The study presents concentration time series before
and after correction, measured at a 2 m height in the sub-canopy at the
FLUXNET spruce forest site Waldstein-Weiden\-brunnen (DE-Bay), analyzes the
dependence of statistical parameters of pdf's from atmospheric
parameters such as stratification, quantifies the errors and evaluates the
performance of the bias correction approach. The improvements that are
achieved by applying the bias correction approach are one order of magnitude
larger than possible errors associated with it, which is a strong incentive
to use the correction approach. In conclusion, the presented bias correction
approach is well suited for – but not limited to – horizontal gradient
measurements in a multi-analyzer setup, which would not have been reliable
without this approach. Finally, possible future improvements of the bias
correction approach are outlined and further fields of application indicated. |
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