Field Evaluation of Gas Analyzers for Measuring Ecosystem Fluxes
How gas analyzer type and correction method impact measured fluxes.
A side-by-side comparison was conducted of five gas analyzers commonly used to measure ecosystem fluxes of water and carbon dioxide in observation networks such as AmeriFlux. Findings demonstrate that the correction methods applied play a significant role in the measured fluxes.
The work describes a new spectral correction method for use in eddy covariance flux calculations that improves upon existing methods across a range of gas analyzers. Due to the variability of fluxes arising solely from the correction method used, researchers emphasize the importance of reporting the correction method as metadata when publishing and sharing flux data.
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 long been used, whereas enclosed-path (short inlet tube) sensors are relatively new. Researchers from Lawrence Berkeley National Laboratory and the AmeriFlux Network used five gas analyzers and three sonic anemometers deployed in an agricultural research field in Davis, California. 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 carbon dioxide fluxes, water vapor fluxes were the most variable and sensitive to the 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. This 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.
Lawrence Berkeley National Laboratory
The work was supported by the Office of Biological and Environmental Research within the U.S. Department of Energy’s Office of Science as part of the Terrestrial Ecosystem Science program under contract DEAC0205CH11231 to Lawrence Berkeley National Laboratory.
Polonik, P., et al.. “Comparison of gas analyzers for eddy covariance: Effects of analyzer type and spectral corrections on fluxes.” Agriculture and Forest Meteorology 272–273, 128–42 (2019). [DOI: 10.1016/j.agrformet.2019.02.010]