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| Titel |
Field examination of low temperature control setting for mediating surface heating effect in open-path flux measurements under cold conditions |
| VerfasserIn |
George Burba, Jason Hupp, Dayle McDermitt, Robert Eckles |
| 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 |
250045487
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| Zusammenfassung |
| Open-path gas analyzers are extensively used for measurements of CO2 and H2O fluxes and
concentrations. They have advantages of excellent frequency response, long-term stability,
low sensitivity to window contamination, low-power pump-free operation, and infrequent
calibration requirements. They also have limitations such as susceptibility to precipitation and
icing, and a potential need for instrument surface heating correction in extremely
cold environments. In spite of these limitations, open-path measurements often
provide data coverage that would not have been possible using traditional closed-path
approach.
Losses from precipitation and icing may not always be prevented for the open-path
instruments, while heating effect does not pose a problem for CO2 flux in warm
environments. Even in cold environments, the impact of heating on CO2 flux is much
smaller than other well-known effects, such as WPL terms, or frequency response
corrections for closed-path analyzers. Nonetheless, instrument surface heating effect in
cold environments could be addressed scientifically, via developing the theoretical
corrections, and instrumentally, via measuring fast integrated air temperature in the
optical path, or via enclosing the open-path instrument into a low-power short-intake
design.
Here we provide an alternative way to minimize or eliminate open-path heating effect,
achieved by minimizing or eliminating the temperature gradient between the instrument
surface and ambient air. Open-path low temperature controlled design is discussed in
comparison with two other approaches (e.g., traditional open-path design and closed-path
design) in terms of their field performance for Eddy Covariance flux measurements in the
cold.
This study presents field data from a new open-path CO2/H2O gas analyzer, LI-7500A,
based on the LI-7500 model modified to produce substantially less heat during extremely
cold conditions. Two prototypes of LI-7500A were tested in the field and the lab in
2009-2010. Two regiments of the temperature control for internal electronics were
examined across a wide range of temperatures: (i) the traditional control temperature
of about +30oC, and (ii) new regiment controlling parts of internal electronics at
+5oC.
When new +5oC regiment was activated, the following changes were observed: heat
dissipation from the surface reduced several folds, surface-to-air temperature gradients
reduced 2-50 times; and the number of false uptake hours were reduced by 3.5 times, to the
same level as a closed-path standard. Significant advantage of the new regiment was also
observed in the magnitude of CO2 fluxes, especially in cold weather below -10oC. At such
cold temperatures, CO2 fluxes from a +30oC controlled LI-7500 were 19% below those of the
closed-path standard, while fluxes from a +5oC controlled LI-7500A were, on average, within
1% of the standard.
Strong experimental evidence are presented demonstrating that open-path instrument
heating could be substantially reduced or eliminated via such a simple hardware-based
solution. This allows continued and expanded use of the open-path gas analyzer
design, the ultimately lowest-power remote solution for fast gas measurements. |
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