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| Titel |
Significance of High-Speed Air Temperature Measurements in the Sampling Cell of a Closed-Path Gas Analyzer with a Short Tube |
| VerfasserIn |
James Kathilankal, Gerardo Fratini, George Burba |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250102012
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| Publikation (Nr.) |
 EGU/EGU2015-1289.pdf |
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| Zusammenfassung |
| Eddy covariance gas analyzers measure gas content in a known volume, thus
essentially measuring gas density. The fundamental flux equation, however, is
based on the dry mole fraction. The relationship between dry mole fraction
and density is regulated by the ideal gas law describing the processes of
temperature- and pressure-related expansions and contractions, and by the law
of partial pressures, describing the process of dilution. As a result, this
relationship depends on water vapor content, temperature and pressure of the
air sample.
If the instrument is able to output precise high-speed dry mole fraction, the
flux processing is significantly simplified and WPL density terms accounting
for the air density fluctuations are no longer required. This should also lead to
the reduction in uncertainties associated with the density terms resulting from
the eddy covariance measurements of sensible and latent heat fluxes used in
these terms. In this framework, three main measurement approaches may be
considered:
Open-path approach
Outputting correct high-speed dry mole fraction from the open-path
instrument is difficult because of complexities with maintaining reliable fast
temperature measurements integrated over the entire measuring path, and also
because of extraordinary challenges with accurate measurements of fast
pressure in the open air flow.
Classical long-tube closed-path approach
For instruments utilizing traditional long-tube closed-path design, with
tube length 1000 or more times the tube diameter, the fast dry mole fraction
can be used successfully when instantaneous fluctuations in the air
temperature of the sampled air are effectively dampened to negligible levels,
instantaneous pressure fluctuations are regulated or negligible, and
water vapor is measured simultaneously with gas or the air sample is
dried.
Short-tube closed-path approach, the enclosed design
For instruments with a short-tube enclosed design, most - but not all - of
the temperature fluctuations are attenuated, so calculating unbiased fluxes
using fast dry mole fraction requires high-speed, precise temperature
measurements of the air stream inside the cell. Fast pressure and water vapor
content of the sampled air should also be measured in the sampling cell, and
carefully aligned in time with gas and temperature measurements.
This presentation examines data from the three different sites equipped
with enclosed short-tube analyzers in order to evaluate the significance of
high-speed, precise air temperature measurements in the sampling cell of the
analyzer. Short-term and long-term effects are examined using half hourly
fluxes of carbon dioxide and water vapor, as well as long-term carbon and
water budgets. |
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