Effect of air composition (N₂, O₂, Ar, and H₂O) on CO₂ and CH₄ measurement by wavelength-scanned cavity ring-down spectroscopy: calibration and measurement strategy

Abstract. We examined potential interferences from water vapor and atmospheric background gases (N₂, O₂, and Ar), and biases by isotopologues of target species, on accurate measurement of atmospheric CO₂ and CH₄ by means of wavelength-scanned cavity ring-down spectroscopy (WS-CRDS). Changes of the background gas mole fractions in the sample air substantially impacted the CO₂ and CH₄ measurements: variation of CO₂ and CH₄ due to relative increase of each background gas increased as Ar < O₂ < N₂, suggesting similar relation for the pressure-broadening effects (PBEs) among the background gas. The pressure-broadening coefficients due to variations in O₂ and Ar for CO₂ and CH₄ are empirically determined from these experimental results. Calculated PBEs using the pressure-broadening coefficients are linearly correlated with the differences between the mole fractions of O₂ and Ar and their ambient abundances. Although the PBEs calculation showed that impact of natural variation of O₂ is negligible on the CO₂ and CH₄ measurements, significant bias was inferred for the measurement of synthetic standard gases. For gas standards balanced with purified air, the PBEs were estimated to be marginal (up to 0.05 ppm for CO₂ and 0.01 ppb for CH₄) although the PBEs were substantial (up to 0.87 ppm for CO₂ and 1.4 ppb for CH₄) for standards balanced with synthetic air. For isotopic biases on CO₂ measurements, we compared experimental results and theoretical calculations, which showed excellent agreement within their uncertainty. We derived instrument-specific water correction functions empirically for three WS-CRDS instruments (Picarro EnviroSense 3000i, G-1301, and G-2301), and evaluated the transferability of the water correction function from G-1301 among these instruments. Although the transferability was not proven, no significant difference was found in the water vapor correction function for the investigated WS-CRDS instruments as well as the instruments reported in the past studies within the typical analytical precision at sufficiently low water concentrations (<0.7% for CO₂ and <0.6% for CH₄). For accurate measurements of CO₂ and CH₄ in ambient air, we concluded that WS-CRDS measurements should be performed under complete dehumidification of air samples, or moderate dehumidification followed by application of a water vapor correction function, along with calibration by natural air-based standard gases or purified air-balanced synthetic standard gases with the isotopic correction.