October 04, 2019

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Field Evaluation of Gas Analyzers for Measuring Ecosystem Fluxes

How gas analyzer type and correction method impact measured fluxes.

The Science

A side-by-side comparison of five gas analyzers commonly used to measure ecosystem fluxes of water and carbon dioxide, in observation networks such as AmeriFlux, was conducted. Findings demonstrate that correction methods applied play a significant role in the measured fluxes.

The Impact

The work describes a new spectral correction method for use in eddy covariance flux calculations which was found to improve upon existing methods across a range of gas analyzers. Due to the variability of fluxes arising solely from choice of correction method, researchers emphasize the importance of reporting the correction method as metadata when publishing and sharing flux data.

Summary

The eddy covariance technique (EC) is used at hundreds of field sites worldwide to measure trace gas exchange between the surface and the atmosphere. Data quality and correction methods for EC have been studied empirically and theoretically for many years. The recent development of new gas analyzers has led to an increase in technological options for users. Open-path (no inlet tube) and closed-path (long inlet tube) sensors have been used for a long time, whereas enclosed-path (short inlet tube) sensors are relatively new.  Researchers from Lawrence Berkeley National Lab and the AmeriFlux Network used five gas analyzers and three sonic anemometers deployed in an agricultural research field in Davis, CA. Two different experimental setups were evaluated for 3 month periods. Two established spectral correction methods as well as a new approach (described in the manuscript) were applied and evaluated for all analyzers. All gas analyzers were found to measure fluxes comparably, if appropriate corrections are applied and quality control measures are taken. Compared to CO2 fluxes, water vapor fluxes were most variable and sensitive to gas analyzer type and correction method. Gas analyzers with inlet tubes exhibited larger signal attenuation for water vapor and should be corrected with empirical correction methods.  These study provides valuable information for the eddy covariance community to help determine the best sensor, approach and correction method at sites that meet their specific research questions, as well as potential issues with comparing multiple field sites.

Principal Investigator

Sébastien Biraud
Lawrence Berkeley National Laboratory
scbiraud@lbl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

The work was supported by the Terrestrial Ecosystem Science program of the Office of Biological and Environmental Research within the U.S. Department of Energy (DOE) Office of Science under the Contract DEAC0205CH11231 to Lawrence Berkeley National Laboratory.

References

Polonik, P., W. S. Chan, D. P. Billesbach, and G. Burba, et al. "Comparison of gas analyzers for eddy covariance: Effects of analyzer type and spectral corrections on fluxes". Agriculture and Forest Meteorology 272–273 128–142  (2019). https://doi.org/10.1016/j.agrformet.2019.02.010.