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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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Atmos. Meas. Tech., 10, 3073-3091, 2017
https://doi.org/10.5194/amt-10-3073-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
24 Aug 2017
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
Jennifer E. Johnson1,a and Chris W. Rella2 1Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, USA
2Picarro, Inc., Santa Clara, CA, USA
acurrent address: Department of Global Ecology, Carnegie Institution, Stanford, CA, USA
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.

Citation: Johnson, J. E. and Rella, C. W.: 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, Atmos. Meas. Tech., 10, 3073-3091, https://doi.org/10.5194/amt-10-3073-2017, 2017.
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Short summary
Cavity ring-down spectrometers (CRDS) can make accurate measurements of trace gases when the inert gas background is constant but are prone to errors when the background is variable. Here, we report that in one CRDS analyzer (L2120-i, Picarro), these errors result from broadening, narrowing, and shifting of the target absorption lines as well as neighboring lines. New spectral acquisition and analysis strategies are needed to make CRDS measurements robust to variable backgrounds.
Cavity ring-down spectrometers (CRDS) can make accurate measurements of trace gases when the...
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