Long-term field performance of a tunable diode laser absorption spectrometer for analysis of carbon isotopes of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; in forest air

S M Schaeffer,B H Vaughn,D R Bowling,J W C White,J B Miller

doi:10.5194/acp-8-5263-2008

Abstract

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.

Highlights

Measurement of the stable isotope ratios of gases that participate in biosphere/atmosphere exchange processes is necessary to understand those processes and the natural environment (Keeling et al, 1995; Trolier et al, 1996)
The objective of the present study is to evaluate the longterm performance of a tunable diode laser absorption spectroscopy (TDLAS) used to monitor [CO2] and δ13C of atmospheric CO2within a subalpine forest at Niwot Ridge, Colorado, USA
We describe the analytical details of the sampling system as well as the sampling and data analysis procedures that were used to assess the performance of this fielddeployed TDLAS system

Summary

Introduction

Measurement of the stable isotope ratios of gases that participate in biosphere/atmosphere exchange processes is necessary to understand those processes and the natural environment (Keeling et al, 1995; Trolier et al, 1996). Laser absorption spectrometry has regularly been used to measure the mole fractions of atmospheric trace gases such as CH4, CO, N2O, and others (Gulluk et al, 1997; Webster et al, 2001; Provencal et al, 2005) These instruments measure absorption of mid-infrared (mid-IR) radiation using a variety of techniques including Fourier-transform IR spectroscopy (Esler et al, 2000; Griffith et al, 2000), quantum cascade laser spectroscopy (Weidmann et al, 2004; McManus et al, 2005; Saleska et al, 2006), cavity ring-down spectroscopy (Crosson et al, 2002; Provencal et al, 2005; Wahl et al, 2006), and laser optogalvanic effect spectroscopy (Murnick and Okil, 2005). These studies examined the δ13C and δ18O of ecosystem-respired CO2 (Griffis et al, 2004; Bowling et al, 2005; Griffis et al, 2005a; Griffis et al, 2005b; Zhang et al, 2006; Griffis et al, 2007), δ13C and δ18O of leaf-respired CO2 (Barbour et al, 2007a, 2007b), and δ18O of water vapor in forest air (Lee et al, 2005)

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