A Comparison of Infrared Gas Analyzers above a Subalpine Forest in Complex Terrain

Infrared gas analyzers (IRGAs) are a key component to the eddy covariance measurement of water vapor and carbon dioxide exchange between the surface and atmosphere (Aubinet et al., 2012). Historically, closed-path IRGAs designed for laboratory use (such as the LI-COR, model LI-6262) were used to measure H2O and CO2 fluxes in the atmosphere (e.g., McDermitt, 1997). These closed-path IRGAs worked best in climate-controlled conditions. In order to use them in the field these IRGAs were typically housed in temperature-controlled enclosures or buildings that were tens of meters away from the actual measurement location near the sonic anemometer. This necessitated the use of long tubing and high-power pumps to bring the air sample to the IRGA cell. Attenuation of H2O and CO2 fluctuations within the tubing was a persistent problem with such a setup, especially for H2O (Fratini et al., 2012). As an alternative, open-path IRGAs are frequently utilized, but the key trade-offs with the open-path design are: (i) precipitation and dew affecting the measurements and creating data gaps, and (ii) the need to account for effects of air density changes on measured H2O and CO2 along the air sampling path (e.g., WPL terms). Over the past five years a new type of closed-path IRGA has emerged. This newly-designed IRGA is weather-proof, compact, and low-maintenance. Furthermore, because of its small size, short intake tubing can be used which places the sampling cell as close as possible to the sonic anemometer and reduces high frequency signal loss (e.g., Clement et al., 2009; Burba et al., 2010; Nakai et al., 2011; Burba et al., 2012; Novick et al., 2013). Two such IRGAs are the LI-COR LI-7200 and the Campbell Scientific EC155, which is part of the CPEC200 closed-path eddy covariance system.

At the University of Colorado (CU) AmeriFlux tower near Niwot Ridge, Colorado a closed-path IRGA (LI-COR, model LI-6262) has been deployed since 1998 to measure ecosystem fluxes with a 10m long 1/4 inch dekaron tube transporting the air sample to the LI-6262 (Monson et al., 2002). The LI-6262 has been out of production for over 10 years and requires factory maintenance about every two years. To take advantage of the new design features mentioned above and reduce instrument maintenance costs, we wanted to upgrade the LI-6262 to a newer model IRGA. However, one difficulty with changing the analyzer in the middle of such a long-term measurement program is that the upgraded sensor can potentially bias conclusions about the phenomena being measured. Therefore, prior to replacing the LI-6262 IRGA on the AmeriFlux tower, we deemed it crucial to better understand any instrument-dependent measurement differences over the full range of environmental conditions experienced at this specific site. Consequently, starting in summer 2013, a LI-7200 and EC155/CPEC200 (along with an open-path LI-7500) were deployed side-by-side with the LI-6262 inlet at 21.5m on the Niwot Ridge AmeriFlux tower. The preliminary results presented in our study will compare the H2O and CO2 spectra and cospectra measured by each IRGA alongside the final calculated fluxes.

References: -----------

Aubinet, M., T. Vesala, and D. Papale, 2012: Eddy Covariance: A Practical Guide to Measurement and Data Analysis. Springer Atmospheric Sciences, Dordrecht, The Netherlands, 438 pp.

Burba, G. G., D. K. Mcdermitt, D. J. Anderson, M. D. Furtaw, and R. D. Eckles (2010), Novel design of an enclosed CO2/H2O gas analyser for eddy covariance flux measurements, Tellus Series B-Chemical And Physical Meteorology, 62, 743–748.

Burba, G., et al. (2012), Calculating CO2 and H2O eddy covariance fluxes from an enclosed gas analyser using an instantaneous mixing ratio, Global Change Biology, 18, 385–399.

Clement, R. J., G. G. Burba, A. Grelle, D. J. Anderson, and J. B. Moncrieff (2009), Improved trace gas flux estimation through IRGA sampling optimization, Agricultural and Forest Meteorology, 149, 623–638.

Fratini, G., A. Ibrom, N. Arriga, G. Burba, and D. Papale (2012), Relative humidity effects on water vapour fluxes measured with closed-path eddy-covariance systems with short sampling lines, Agricultural and Forest Meteorology, 165, 53–63.

McDermitt, D. K., 1997: Some recommendations for using LI-COR Gas Analyzers in Eddy Correlation Measurements. Topics discussed at Ameriflux Workshop, 1996. Application Note #118, LI-COR Inc., Lincoln, Nebraska, 7pp., (Available at www.licor.com).

Monson, R. K., A. A. Turnipseed, J. P. Sparks, P. C. Harley, L. E. Scott-Denton, K. Sparks, and T. E. Huxman (2002), Carbon sequestration in a high-elevation, subalpine forest, Global Change Biol., 8, 459–478.

Nakai, T., H. Iwata, and Y. Harazono (2011), Importance of mixing ratio for a long-term CO2 flux measurement with a closed-path system, Tellus Series B-Chemical and Physical Meteorology, 63, 302–308.

Novick, K. A., J. Walker, W. S. Chan, A. Schmidt, C. Sobek, and J. M. Vose (2013), Eddy covariance measurements with a new fast-response, enclosed-path analyzer: Spectral characteristics and cross-system comparisons, Agricultural And Forest Meteorology, 181, 17–32.