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| Titel |
Accounting for spectroscopic effects in eddy covariance calculations of methane flux |
| VerfasserIn |
George Burba, Tyler Anderson, Dayle McDermitt, Liukang Xu, Anatoly Komissarov, Bradley Riensche, Douglas Allyn, Kevin Ediger |
| 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 |
250046247
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| Zusammenfassung |
| Closed-path and open-path designs of high-speed gas analyzers are widely used in Eddy
Covariance research to quantify CO2 and H2O fluxes. Typically such devices use NDIR
technology due to combination of good performance, low cost, field robustness, low
maintenance and power demands. Recently, new analyzers were developed to measure fluxes
of gases beyond CO2 and H2O using laser spectroscopy. The new way of measuring gas
concentrations may also bring a need for a new additional correction to the flux
values.
Traditionally, when Eddy Covariance flux is computed, the fast changes in gas density are
correlated with fast changes in vertical wind speed. Measured changes in gas density happen
due to gas flux, thermal expansion and contraction of the sampled gas, water vapor dilution,
and pressure-related expansion and contraction. These are standard processes described by
Ideal Gas Law and by Law of Partial Pressures, and often are called density effects. The gas
flux is usually corrected for these density effects using Webb-Pearman-Leuning correction
[1].
When gas density is measured by the means of laser spectroscopy, there are also
spectroscopic effects affecting measured gas density depending on fluctuations in temperature
(T), water vapor (q) and pressure (P), in addition to the density effects. The spectroscopic
effects are related to changes in the shape of the absorption line due to changes in gas T, q,
and P. These effects are individual for each specific absorption line [2], and the method for
compensating these effects are also highly dependent on the measurement technique.
Depending on the absorption line probed, spectroscopic effects may add or subtract from
density effects, or may have a positive slope with T and negative with P, or vice
versa.
For closed-path gas analyzers, the majority of density effects and spectroscopic effects
could be reduced or eliminated, when: (i) intake tube is very long, (ii) air sample is dry, and
(iii) pressure fluctuations are very small. This way fast fluctuations in T are attenuated, fast
fluctuations in q are removed entirely, and fast fluctuations in P are neglected. While
minimizing uncertainty related to density and spectroscopic effects, use of long intake tubes
and drying air sample also lead to significant increase in power demand, and to increased
uncertainties due to excess attenuation of the fluctuations of the gas of interest in the
drier.
Not drying air sample leads to the need for applying density correction for dilution, and
spectroscopic corrections for gas absorption due to fast fluctuations in water vapor pressure.
For both of these corrections water vapor should be measured accurately at hi-speed inside
the closed-path device, which increases measurement costs.
In addition, such closed-path analyzers have to work under significantly reduced
pressures, and require powerful pumps and grid power (600-1500 Watts). Power
and labor demands may be reasons why these instruments are often deployed at
locations with good infrastructure and grid power, and not where the gas of interest is
produced.
Alternatively, open-path design can offer very low-power (e.g. 5-10 Watts) solution
permitting solar-powered deployments. It is cost-effective permitting an addition of a single
new gas measurement to the present array of CO2 and H2O measurements. Measurements are
truly in-situ avoiding attenuation of gas fluctuations in the intake tube. These features enable
long-term deployment of permanent, portable or mobile open-path flux stations at remote |
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