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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/amt-2020-305
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-305
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  02 Sep 2020

02 Sep 2020

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This preprint is currently under review for the journal AMT.

Characterising water vapour concentration dependence of commercial cavity ring-down spectrometers for continuous onsite atmospheric water vapour isotope measurements in the tropics

Shujiro Komiya1, Fumiyoshi Kondo2, Heiko Moossen1, Thomas Seifert1, Uwe Schultz1, Heike Geilmann1, David Walter1,3, and Jost V. Lavric1 Shujiro Komiya et al.
  • 1Max Planck Institute for Biogeochemistry, Jena, 07745, Germany
  • 2Japan Coast Guard Academy, Kure, 737-8512, Japan
  • 3Max Planck Institute for Chemistry, Mainz, 55128, Germany

Abstract. The recent development and improvement of commercial laser-based spectrometers have expanded in situ continuous observations of water vapour (H2O) stable isotope ratios (e.g., δ18O, δ2H, etc.) in a variety of sites worldwide. However, we still lack continuous observations in the Amazon, a region that significantly influences atmospheric and hydrological cycles on local to global scales. In order to achieve accurate on-site observations, commercial water isotope analysers require regular in situ calibration, including H2O concentration dependence ([H2O]-dependence) of isotopic accuracy. Past studies have assessed [H2O]-dependence for air with H2O concentrations up to 35,000 ppm, a value that is frequently surpassed in tropical rainforest settings like the central Amazon where we plan continuous observations. Here we investigated the performance of two commercial analysers (L1102i and L2130i models, Picarro, Inc., USA) for measuring δ18O and δ2H in atmospheric moisture at four different H2O levels from 21,500 to 41,000 ppm. These H2O levels were created by a custom-built calibration unit designed for regular in situ calibration. Measurements on the newer analyser model (L2130i) had better precision for δ18O and δ2H and demonstrated less influence of H2O concentration on the measurement accuracy at each moisture level compared to the older L1102i. Based on our findings, we identified the most appropriate calibration strategy for [H2O]-dependence, adapted to our calibration system. The best strategy required using two pairs of a two-point calibration with four different H2O concentration levels. The smallest uncertainties in calibrating [H2O]-dependence of isotopic accuracy of the two analysers were achieved using a linear-surface fitting method and a 28 h calibration interval, except for the δ18O accuracy of the L1102i analyser for which the cubic fitting method gave best results. The uncertainties in [H2O]-dependence calibration did not show any significant difference using calibration intervals from 28 h up to 196 h; this suggested that one [H2O]-dependence calibration per week for the L2130i and L1102i analysers is sufficient.

Shujiro Komiya et al.

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Shujiro Komiya et al.

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Data for amt-2020-305 (Discussion) Shujiro Komiya and Jost Lavric https://doi.org/10.17617/3.4n

Shujiro Komiya et al.

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Short summary
The Amazon basin influences the atmospheric and hydrological cycles on local to global scales. To better understand how, we plan to perform continuous on-site measurements of the stable isotope composition of atmospheric water vapour. For making accurate on-site observations possible, we have investigated the performance of two commercial analysers and determined the best calibration strategy. Well-calibrated, both analysers will allow us to record natural signals in the Amazon rainforest.
The Amazon basin influences the atmospheric and hydrological cycles on local to global scales....
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