Long-term field performance of a tunable diode laser absorption spectrometer for analysis of carbon isotopes of CO2 in forest air

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‰ (over the last several months). The TDLAS provided detail on variability of δ13C of atmospheric CO2 that was not represented in weekly flask samples, as well as information regarding the influence of large-scale (regional) seasonal cycle and local forest processes on [CO2] and δ13C of CO2. There were also clear growing season and winter differences in the relative contributions of photosynthesis and respiration on the [CO2] and δ13C of forest air.