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| Titel |
Comparison of CO2 and H2O eddy fluxes derived from density and from mixing ratio measured by an enclosed gas analyzer |
| VerfasserIn |
George Burba, Andres Schmidt, Russell Scott, Gerardo Fratini, James Kathilankal, Beverly Law, Dayle McDermitt, Chad Hanson |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250045492
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| Zusammenfassung |
| Enclosed gas analyzer with short intake tube is a blend of a traditional long-tube closed-path
design and a traditional open-path design. Analogous to closed-path, the enclosed design
leads to minimal data loss during precipitation events and icing, and it does not have surface
heating issues. Analogous to the open-path design, the enclosed design has good
frequency response due to the short intake tube, does not require frequent calibration,
needs minimal maintenance, and could be low power when used with short intake
tube.
In addition to these advantages, enclosed design could provide measurement of fast
mixing ratio, or dry mole fraction, because native density measurements can be converted to
mixing ratio units using fast measurements of temperature, water vapor and pressure inside
the sampling cell. Fast mixing ratio implies that the thermal expansion and water
dilution of the sampled air have been accounted for in such a conversion. Thus, no
density corrections are required to compute fluxes when the fast mixing ratio is
used.
Such a way of calculating fluxes has been used frequently with traditional closed-path
analyzers (e.g., LI-6262 and LI-7000), because fast fluctuations in the air temperature of the
sample were attenuated in the long intake tube, and because water vapor was simultaneously
measured and dry mole fraction was output from the instrument. In an enclosed design, such
as the LI-7200 used with short tube, most but not all of the fast temperature fluctuations are
attenuated, so calculating fluxes using the mixing ratio output of such an instrument requires
validation.
The CO2 and H2O Eddy Covariance fluxes of from eight experiments with new LI-7200
enclosed analyzer were examined here: five deployments of the Ameriflux Roving
Intercomparison Station in California, Arizona, New Mexico and Oregon; one deployment at
a USDA flux site in Arizona; and two deployments at the LI-COR flux test facility in
Nebraska. Fluxes were computed in two ways: (i) via the traditional way using the density
corrections, and (ii) via a mixing ratio output from the instrument without applying the
density corrections.
The results of these comparisons have important implications for future gas flux
measurements, because avoiding half-hourly or hourly density corrections could help to
minimize at least two kinds of uncertainties: (i) the uncertainties associated with correcting
the product of fast covariances of gas density using sensible and latent heat flux calculated
over half-hour or an hour; and (ii) the uncertainties in the magnitudes of the sensible and
latent heat fluxes used in correcting gas flux.
When used alongside fast measurements of sample air temperature, water vapor and
pressure, fluxes could be computed from raw covariance of mixing ratio and vertical wind
speed, multiplied by a frequency response correction. There are some unknowns associated
with the latter, especially when applied to mixing ratio, and primarily related to tube
attenuation. It is because the related functions, parameters and coefficients for the
correction were originally developed and tested using gas density and not mixing
ratio, so caution should be used in this step of the flux data processing. However,
such unknowns did not seem to significantly affect resulted fluxes at the studied
sites.
The absence of a need for density corrections in the flux calculations using mixing ratio
brings the advantages of increased flux measurement quality and temporal resolution, and
may reduce the magnitude of minimum detectable flux. |
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