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Long-term field performance of a tunable diode laser absorption spectrometer for analysis of carbon isotopes of CO 2 in forest air

2008; Centre National de la Recherche Scientifique; Linguagem: Inglês

Sean M. Schaeffer, J. B. Miller, Bruce H. Vaughn, James W. C. White, D. R. Bowling,

Atmospheric chemistry and aerosols

Tunable diode laser absorption spectrometry (TD- LAS) is gaining in popularity for measuring the mole frac- tion (CO2) and stable isotopic composition ( 13 C) of carbon dioxide (CO2) in air in studies of biosphere-atmosphere gas exchange. Here we present a detailed examination of the per- formance 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 lo- cations in and above the forest canopy every ten minutes for 2.4 years. A variety of methods were used to assess in- strument 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 13 C, but after fixing several problems the isotope precision improved to 0.2‰ (over the last several months). The TDLAS provided detail on variability of 13 C of atmo- spheric CO2 that was not represented in weekly flask sam- ples, as well as information regarding the influence of large- scale (regional) seasonal cycle and local forest processes on (CO2) and 13 C of CO2. There were also clear growing sea- son and winter differences in the relative contributions of photosynthesis and respiration on the (CO2) and 13 C of for- est air.