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

摘要

Tunable diode laser absorption spectrometry (TDLAS) is gaining in popularityfor measuring the mole fraction [CO] and stable isotopic composition(δC) of carbon dioxide (CO) in air in studies ofbiosphere-atmosphere gas exchange. Here we present a detailed examination ofthe performance of a commercially-available TDLAS located in a high-altitudesubalpine coniferous forest (the Niwot Ridge AmeriFlux site), providing thefirst multi-year analysis of TDLAS instrument performance for measuringCO isotopes in the field. Air was sampled from five to nine verticallocations in and above the forest canopy every ten minutes for 2.4 years. Avariety of methods were used to assess instrument performance. Measurementof two compressed air cylinders that were in place over the entire studyestablish the long-term field precision of 0.2 μmol mol for[CO] and 0.35‰ for δC, but after fixing severalproblems the isotope precision improved to 0.2permil (over the last severalmonths). The TDLAS provided detail on variability of δC ofatmospheric CO that was not represented in weekly flask samples, aswell as information regarding the influence of large-scale (regional)seasonal cycle and local forest processes on [CO] and δC of CO. There were also clear growing season and winterdifferences in the relative contributions of photosynthesis and respirationon the [CO] and δC of forest air.