Ground-based FTIR Measurements of Carbon Dioxide Isotopic Concentrations and Fluxes and Ecosystem-Atmosphere Exchange

Analysis of the isotopic content of atmospheric carbon dioxide provides a wealth of information about the complex interaction between the biosphere and the atmosphere. When combined with micrometeorological flux measurements, a much improved picture emerges of ecosystem-scale carbon cycling dynamics. Since photosynthesis and respiration impart a unique isotopic signature to the atmosphere, knowledge of the concentrations and fluxes of the individual CO2 isotopologues can be used to constrain and partition net ecosystem exchange into its gross photosynthetic and respiration components. In this work, concentrations and fluxes of individual isotopologues of carbon dioxide in a forest ecosystem were determined using Fourier Transform Infrared (FTIR) spectroscopy and the Disjunct Eddy Covariance (DEC) flux measurement technique. We separately measure the concentrations of the 12CO2 and 13CO2 isotopomers of carbon dioxide at approximately one minute intervals with very high signal-to-noise ratio using molecular absorption in a 1-meter cell in the 2100 to 2600 cm-1 region of the isotopic vibration-rotation bands. The FTIR - DEC system was deployed during four field campaigns in the summers of 2005 and 2006 in north central Oregon. Fixed height or vertical profile concentration measurements of 12CO2 and 13CO2 were carried out in the canopy using a 25-meter tower fitted with air inlets at 0.3, 4.1, 7.5, 10.8, 14.0, and 20.6 meters. Isotopic flux measurements above the canopy were taken at constant height of 20.6 meters. Temporal and vertical profiles of CO2 concentrations and carbon isotope ratios will be shown together with the total net ecosystem exchange and isotopic CO2 fluxes.