Understanding uncertainties in net ecosystem CO2 exchange measurements by open-path gas analysers

Manuel Helbig, Université de Montréal, Département de géographie and Centre d’études nordiques, Montréal, QC, Canada, Karoline Wischnewski, Université de Montréal, Département de géographie, Montréal, QC, Canada, Elyn Humphreys, Carleton University, Geography & Environmental Studies, Ottawa, ON, Canada, Ivan Bogoev, Campbell Scientific, Inc., Logan, UT, United States, George G Burba, LI-COR, Lincoln, United States and Oliver Sonnentag, Université de Montréal, Département de géographie, Montreal, Canada

Contact First Author: Manuel Helbig; helbig@gfz-potsdam.de

Previously Published Material: Parts of this study are intended to be presented at the EGU Annual Assembly in Vienna in April 2015.

Abstract ID#: 36438

 

English Abstract:
Open-path (OP) infrared gas analysers (IRGAs) are often used to estimate net ecosystem CO2 exchange (NEE) using the eddy covariance technique. A main focus of this research is to better understand the variability of NEE dynamics across contrasting ecosystems. A sophisticated understanding and documentation of potential uncertainties inherent to the use of OP IRGA is necessary to ensure inter-site comparability.

In contrast to closed-path (CP) IRGAs, OP IRGAs are exposed to high-frequency temperature and humidity variations affecting CO2 density fluctuations not related to biological sources or sinks of CO2. The Webb-Pearman-Leuning (WPL) term accounts for these gas density fluctuations. However, the application of the WPL term introduces different sources of uncertainty compared to NEE estimates based on CP IRGAs. Potential sources of such errors in NEE are uncertainties in the measurements of sensible and latent heat fluxes, biases in absolute gas density measurements, sensor surface heating, and non-sufficient corrections for high-frequency band-broadening effects.

With this contribution, we present results from a sensor inter-comparison study conducted at the Mer Bleue peatland near Ottawa, Ontario. Eddy covariance systems with a CP IRGA (LI7000) and with an OP IRGA (EC150) were compared between September 2012 and January 2013, whereas the same CP IRGA system was compared to a second OP IRGA (LI7500) between January and April 2006. The results indicate significant biases in NEE estimates obtained with the OP IRGAs compared to CP IRGA NEE estimates. For both OP IRGAs, the differences in NEE scaled with the magnitude of the sensible heat flux. Since the absolute magnitude of the bias seems to be directly related to the magnitude of the sensible heat flux rather than to the magnitude of NEE, the relative error is likely larger at sites with small NEE. Preliminary results indicate multiple error sources for the OP IRGAs and highlight the importance of site-specific uncertainty analyses.

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