Abstract
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.
Highlights
In most commercially available laser absorption spectrometers, the accuracy and precision of trace gas measurements are sensitive to the composition of the background gas
We explore this issue in the context of a class of laser absorption spectrometers that is of increasing importance for environmental research, the cavity ring-down spectroscopy (CRDS) analyzers
Measurement errors emerge from the interaction between two factors: first, collisional shifting and broadening of the trace gas absorption transitions; and second, the spectral acquisition and analysis strategies employed by the CRDS analyzers (Hendry et al, 2011; Gralher et al, 2016; Sprenger et al, 2017)
Summary
In most commercially available laser absorption spectrometers, the accuracy and precision of trace gas measurements are sensitive to the composition of the background gas. While the CRDS analyzers can accurately and precisely measure the concentration and isotopic composition of trace gases in situations where the background gas has a constant composition, they make substantial measurement errors in situations where the background gas has a variable composition (Chen et al, 2010; Friedrichs et al, 2010; Aemisegger et al, 2012; Becker et al, 2012; Nara et al, 2012; Long et al, 2013; Volkmann and Weiler, 2014). Measurement errors emerge from the interaction between two factors: first, collisional shifting and broadening of the trace gas absorption transitions; and second, the spectral acquisition and analysis strategies employed by the CRDS analyzers (Hendry et al, 2011; Gralher et al, 2016; Sprenger et al, 2017). The fitting routine compares this modeled spectrogram to the measured spectrogram in three stages
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