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| Titel |
True eddy accumulation and eddy covariance methods and instruments intercomparison for fluxes of CO2, CH4 and H2O above the Hainich Forest |
| VerfasserIn |
Lukas Siebicke |
| 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 |
250153134
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| Publikation (Nr.) |
 EGU/EGU2017-18076.pdf |
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| Zusammenfassung |
| The eddy covariance (EC) method is state-of-the-art in directly measuring vegetation-atmosphere
exchange of CO2 and H2O at ecosystem scale. However, the EC method is currently limited
to a small number of atmospheric tracers by the lack of suitable fast-response analyzers or
poor signal-to-noise ratios. High resource and power demands may further restrict the
number of spatial sampling points.
True eddy accumulation (TEA) is an alternative method for direct and continuous flux
observations. Key advantages are the applicability to a wider range of air constituents such as
greenhouse gases, isotopes, volatile organic compounds and aerosols using slow-response
analyzers. In contrast to relaxed eddy accumulation (REA), true eddy accumulation
(Desjardins, 1977) has the advantage of being a direct method which does not require
proxies. True Eddy Accumulation has the potential to overcome above mentioned limitations
of eddy covariance but has hardly ever been successfully demonstrated in practice in the
past.
This study presents flux measurements using an innovative approach to true eddy
accumulation by directly, continuously and automatically measuring trace gas fluxes using a
flow-through system. We merge high-frequency flux contributions from TEA with
low-frequency covariances from the same sensors. We show flux measurements of CO2, CH4
and H2O by TEA and EC above an old-growth forest at the ICOS flux tower site “Hainich”
(DE-Hai).
We compare and evaluate the performance of the two direct turbulent flux measurement
methods eddy covariance and true eddy accumulation using side-by-side trace gas flux
observations.
We further compare performance of seven instrument complexes, i.e. combinations of
sonic anemometers and trace gas analyzers. We compare gas analyzers types of open-path,
enclosed-path and closed-path design. We further differentiate data from two gas analysis
technologies: infrared gas analysis (IRGA) and laser spectrometry (open path and CRDS
closed-path laser spectrometers).
We present results of CO2 and H2O fluxes from the following six instruments, i.e. combinations
of sonic anemometers/gas analyzers (and methods): METEK-uSonic3/Picarro-G2301 (TEA),
METEK-uSonic3/LI-7500 (EC), Gill-R3/LI-6262 (EC), Gill-R3/LI-7200 (EC),
Gill-HS/LI-7200 (EC), Gill-R3/LGR-FGGA (EC).
Further, we present results of much more difficult to measure CH4 fluxes from the
following three instruments, i.e. combinations of sonic anemometers/gas analyzers
(and methods): METEK-uSonic3/Picarro-G2301 (TEA), Gill-R3/LI-7700 (EC),
Gill-R3/LGR-FGGA (EC).
We observed that CO2, CH4 and H2O fluxes from the side-by-side measurements by true
eddy accumulation and eddy covariance methods correlated well. Secondly, the difference
between the TEA and EC methods using the same sonic anemometer but different gas
analyzer was often smaller than the mismatch of the various side-by-side eddy covariance
measurements using different sonic anemometers and gas analyzers.
Signal-to-noise ratios of CH4 fluxes from the true eddy accumulation system system were
superior to both eddy covariance sensors (open-path LI-7700 and closed-path CRDS
LGR-FGGA sensors).
We conclude that our novel implementation of the true eddy accumulation method
demonstrated high signal-to-noise ratios, applicability to slow-response gas analyzers, small
power consumption and direct proxy-free ecosystem-scale trace gas flux measurements of
CO2, CH4 and H2O. The current results suggest that true eddy accumulation would be
suitable and should be applied as the method-of-choice for direct flux measurements
of a large number of atmospheric constituents beyond CO2 and H2O, including
isotopes, aerosols, volatile organic compounds and other trace gases for which eddy
covariance might not be a viable alternative. We will further develop true eddy
accumulation as a novel approach using multiplexed systems for spatially distributed flux
measurements. |
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