Effects of variation in background mixing ratios of N₂, O₂, and Ar on the measurement of δ¹⁸O–H₂O and δ²H–H₂O values by cavity ring-down spectroscopy

Abstract. Cavity ring-down spectrometers have generally been designed to operate under conditions in which the background gas has a constant composition. However, there are a number of observational and experimental situations of interest in which the background gas has a variable composition. In this study, we examine the effect of background gas composition on a cavity ring-down spectrometer that measures δ¹⁸O–H₂O and δ²H–H₂O values based on the amplitude of water isotopologue absorption features around 7184 cm⁻¹ (L2120-i, Picarro, Inc.). For background mixtures balanced with N₂, the apparent δ¹⁸O values deviate from true values by −0.50 ± 0.001 ‰ O₂ %⁻¹ and −0.57 ± 0.001 ‰ Ar %⁻¹, and apparent δ²H values deviate from true values by 0.26 ± 0.004 ‰ O₂ %⁻¹ and 0.42 ± 0.004 ‰ Ar  %⁻¹. The artifacts are the result of broadening, narrowing, and shifting of both the target absorption lines and strong neighboring lines. While the background-induced isotopic artifacts can largely be corrected with simple empirical or semi-mechanistic models, neither type of model is capable of completely correcting the isotopic artifacts to within the inherent instrument precision. The development of strategies for dynamically detecting and accommodating background variation in N₂, O₂, and/or Ar would facilitate the application of cavity ring-down spectrometers to a new class of observations and experiments.