Submitted to: Journal of Geophysical Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/4/2008
Publication Date: 6/15/2008
Publication URL: www.agu.org/pubs/crossref/2008/2007JD009297.shtml
Citation: Griffis, T.J., Sargent, S.D., Baker, J.M., Lee, X., Tanner, B.D., Greene, J., Swaitek, E., Billmark, K. 2008. Direct Measurement of Biosphere-Atmosphere Isotopic Exchange Using the Eddy Covariance Technique. Journal of Geophysical Research. Available: http://www.agu.org/pubs/crossref/2008/2007JD009297.shtml. Interpretive Summary: Measurement of CO2 exchange and reconciliation of measurements with models of changes in atmospheric CO2 composition are central problems of climate change research. Stable isotopes of CO2 are particularly important in this research, because various ecosystems differ in their discrimination against the heavier isotopes. Eddy covariance is the most accurate means to measure surface/atmosphere gas exchange, but to this point there have been no methods capable of measuring the individual isotopes with sufficient accuracy and frequency response. We deployed and tested a new tunable diode laser spectroscopy system (TDL) to see if it was capable of making such measurements above a soybean canopy. The sum of the individually measured isotope fluxes was compared against a conventional eddy covariance system that measures the combined flux of all CO2 isotopes, and the two systems were found to agree within 9%. We did find some high frequency flux loss in the TDL system, particularly under daytime, windy conditions, but there was little evidence of fractionation in the sampling tube, and there was spectral similarity amongst the isotopes. Thus, despite the flux loss, there was no bias in the estimation of the isotope ratio of net ecosystem exchange (delta N), and good agreement with flux-gradient estimates of delta N. We conclude that this approach will be suitable for obtaining the long-term, continuous measurements of isotopic exchange necessary to evaluate and constrain models of surface/atmosphere CO2 exchange.
Technical Abstract: Quantifying isotopic CO2 exchange between the biosphere and atmosphere presents a significant measurement challenge, but has the potential to provide important constraints on local, regional, and global carbon cycling. Past approaches have indirectly estimated isotopic CO2 exchange using relaxed eddy accumulation, the flask-based isoflux method, and flux-gradient techniques. Eddy covariance (EC) is an attractive method because it has the fewest theoretical assumptions and the potential to give a direct measure of isotopic CO2 flux, but it requires a highly sensitive and relatively fast-response instrument. To date, no such field measurements have been reported. Here, we describe the use of a closed-path tunable diode laser absorption spectroscopy and eddy covariance (EC-TDL) system for isotopic (C16O2, 13CO2, C18O16O) flux measurements. Results are presented from an intensive field experiment conducted over a soybean canopy from July 18 to September 20, 2006. This experiment represents a rigorous field-test of the EC-TDL technique because the transport was dominated by relatively high frequency eddies. Net ecosystem CO2 exchange (FN) measured with the EC-TDL system showed strong correlation (r2 = 0.99) in the half-hourly fluxes with an EC open-path infrared gas analyzer (EC-IRGA) over the 60-day period. Net CO2 flux measured with the EC-IRGA and EC-TDL systems agreed to within 9%. Flux loss associated with diminished frequency response beyond 1 Hz for the EC-TDL system was approximately 8% during daytime windy (> 4 m s-1) conditions. There was little evidence of a kinetic-type fractionation effect related to a phase shift among isotopologues due to tube attenuation. Considering a worst case scenario of laminar flow through the sampling system we predicted a maximum fractionation effect of 0.19‰ and 0.41‰ for 13CO2 and C18O16O, respectively. Investigation of isotopic spectral similarity in the flux ratio for both 13CO2 and C18O16O transport showed that was relatively independent of eddy scale for this ecosystem type. Flux loss, therefore, did not significantly bias. Uncertainty analyses of the half-hourly flux ratios indicated that the uncertainty was dominated by meteorological and biological variations and not instrument noise. There was excellent agreement between isotopic fluxes and measured using the flux-gradient and eddy covariance methods. Application of the EC-TDL technique over rougher surfaces or below canopy, where the flux-gradient approach is difficult to apply, appears promising for obtaining continuous long-term measurements of isotopic CO2 exchange.