Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 8 17 2008 10.5194/acp-8-5263-2008 http://www.atmos-chem-phys.net/8/5263/2008/ http://www.atmos-chem-phys.net/8/5263/2008/acp-8-5263-2008.html http://www.atmos-chem-phys.net/8/5263/2008/acp-8-5263-2008.pdf 5263 5277 2008-09-05 Long-term field performance of a tunable diode laser absorption spectrometer for analysis of carbon isotopes of CO₂ in forest air S. M. Schaeffer J. B. Miller B. H. Vaughn J. W. C. White D. R. Bowling bowling@biology.utah.edu Department of Biology, University of Utah, Salt Lake City, UT 84112, USA National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Boulder, CO 80305, USA Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO 80309, USA Institute for Arctic and Alpine Research, University of Colorado, Boulder, CO 80302, USA Stable Isotope Ratio Facility of Environmental Research, University of Utah, Salt Lake City, UT 84112, USA now at: Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA 93106, USA Tunable diode laser absorption spectrometry (TDLAS) is gaining in popularity for measuring the mole fraction [CO₂] and stable isotopic composition (δ¹³C) of carbon dioxide (CO₂) in air in studies of biosphere-atmosphere gas exchange. Here we present a detailed examination of the performance of a commercially-available TDLAS located in a high-altitude subalpine coniferous forest (the Niwot Ridge AmeriFlux site), providing the first multi-year analysis of TDLAS instrument performance for measuring CO₂ isotopes in the field. Air was sampled from five to nine vertical locations in and above the forest canopy every ten minutes for 2.4 years. A variety of methods were used to assess instrument performance. Measurement of two compressed air cylinders that were in place over the entire study establish the long-term field precision of 0.2 μmol mol^−1 for [CO₂] and 0.35‰ for δ¹³C, but after fixing several problems the isotope precision improved to 0.2\permil (over the last several months). 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