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| Titel |
Propagating Spectroscopic Effects through WPL Terms when Using a Fast Laser-Based Open-Path CH4 Analyzer |
| VerfasserIn |
George Burba, Dayle McDermitt, Tyler Anderson, Anatoly Komissarov |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250071733
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| Zusammenfassung |
| Eddy flux is computed using a covariance between fast changes in gas density and vertical
wind speed. The measured changes in gas density happen due to gas flux itself, thermal
expansion and contraction of the sampled gas, water vapor dilution, and pressure-related
expansions and contractions. These are standard processes described by the Ideal Gas Law
and by the Law of Partial Pressures, and are often called density effects. The gas flux
is usually corrected for such density effects using Webb-Pearman-Leuning terms
(WPL).
When gas density is measured by laser spectroscopy, there are also spectroscopic effects
affecting measured gas density depending on fluctuations in temperature, water vapor and
pressure, 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 temperature, pressure and the presence
of water vapor. These effects are specific for each specific absorption line, and the
measurement technique.
The majority of density effects and spectroscopic effects are reduced or eliminated in the
closed-path analyzers, when: (a) intake tube is very long, (b) gas sample is dried, and (c)
pressure fluctuations are very small. However, the use of long intake tubes and
drying of the air sample also lead to a significant increase in power demand, and to
increased uncertainties due to excess attenuation of the fluctuations of the gas in the
drier. Not drying the air sample leads to a need for applying a 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 high-speed inside the closed-path device, which increases measurements
costs.
In addition, current fast closed-path analyzers based on laser spectroscopy have to
operate under significantly reduced pressures, and require powerful pumps and grid
power (400-1500 Watts). Power demands may be why these instruments are often
deployed at locations with infrastructure and grid power, and not where the gas is
produced.
Open-path gas analyzers can require very low-power (e.g., 5-10 Watts), permitting
solar-powered deployments, cost-effectively permitting an addition of a single new gas
measurement to the present array of CO2 and H2O measurements, and avoiding attenuation
of gas fluctuations in the intake tube. These features enable long-term deployments of
permanent, portable or mobile open-path flux stations at remote locations with high
production of the gas of interest. However, in open-path analyzers, density and spectroscopic
effects cannot be neglected.
Here we propose a new way to account for spectroscopic effects due to fast fluctuations in
air temperature, water vapor and pressure in the same manner as Webb et al. (1980) proposed
a way of accounting for respective density effects. Since both density effects and
spectroscopic effects are known from Gas Laws and HITRAN, respectively, they can be
incorporated into the WPL correction. We use an example of a fast open-path CH4 gas
analyzer, the LI-7700, yet the proposed approach would also apply to any closed-path design
where fluctuations in temperature, water vapor and pressure are not fully eliminated. |
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