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

Abstract. 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.