Inter-comparison of two high-accuracy fast-response spectroscopic sensors of carbon dioxide: a case study

Abstract. Tunable diode laser absorption (TDL) and cavity ring-down spectroscopic (CRDS) sensors for atmospheric carbon dioxide were co-deployed during summer and fall of 2010 in field and laboratory conditions at Los Alamos National Laboratory. Both sensors were characterized for accuracy and precision for ambient carbon dioxide measurements at ground level and compared using both laboratory and ambient field data. After post-processing that included water vapor correction and calibration to WMO reference standards, overall mean [¹²C¹⁶O₂] = 392.05 ± 8.92 ppm and [¹²C¹⁶O₂] = 392.22 ± 9.05 ppm were observed between 29 July and 16 August 2010. The mean difference between the CRDS and TDL data for ¹²CO₂ was 0.04 ± 1.80 ppm (±1σ in 60 s) for ambient field data, demonstrating the sensors meet the WMO/IAEA compatibility standard. The observations show over the 19-day period the [CO₂]_CRDS'/[CO₂]_TDL ratio exhibits a Gaussian distribution centered at x₀ = 1.003 ± 3.38 × 10⁻⁵ (±1σ), indicating the ratio is dominated by random noise as opposed to a bias in the output of either sensor. The CRDS sensor is capable of measuring [¹²C¹⁶O₂] to a precision of 23 ppb in 1 min and decreases to 6.5 ppb in 58 min. At one and 58-min, the TDL exhibits precisions of 29 ppb and 53 ppb. The CRDS is compact, fast, and stable; the TDL is larger and requires frequent calibrations to maintain its precision. The sensors also exhibit consistent hourly averaged diurnal values underscoring the interplay of biological, anthropogenic, and transport processes regulating CO₂ at the site.