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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/amt-2019-316
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2019-316
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 23 Oct 2019

Submitted as: research article | 23 Oct 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

Impact of isotope composition on the humidity dependency correction of water vapour isotope measurements with infra-red cavity ring-down spectrometers

Yongbiao Weng1,2, Alexandra Touzeau1,2, and Harald Sodemann1,2 Yongbiao Weng et al.
  • 1Geophysical Institute, University of Bergen, Bergen, Norway
  • 2Bjerknes Centre for Climate Research, Bergen, Norway

Abstract. Advances in laser spectroscopic techniques now enable high-frequency in situ measurements of the water vapour isotope composition on platforms such as ship and aircraft. However, the isotope compositions and the d-excess measured by this technique can be substantially affected by water concentrations, especially at low humidity. This measurement artefact is known as the "humidity dependency", and is commonly corrected for uniformly across all isotope compositions. Here we systematically investigate the behaviour of the humidity dependency on three commercial cavity ring-down spectrometers for different water standards within a humidity range of 500 to 23'000 ppmv. We find that the isotope compositions of measured water do have a substantial impact on the shape of the humidity dependency at low humidity (below 4'000 ppmv). This isotope composition dependency can, for example, create an offset of ±0.5 and ±6 ‰ for δ18O and δD at ~2'000 ppmv in the case of a Picarro L2130-i, resulting in an offset of 2–3 ‰ for the d-excess. We show that the combined isotope composition–humidity dependency can be expressed by a surface function, which allows to build a correction framework. The impact of this correction is illustrated for a case of aircraft measurements and ship-based measurements in cold environments. A ystematic assessment of the robustness of our findings for three laser spectrometers shows overall constant behaviour with time (for periods ranging from one month to two years), independent of the method of characterisation (discrete liquid injection and continuous vapour streaming), dry gas supply (N2 and synthetic air), or the sequence of humidity levels. Based on our analysis, we recommend a procedure for the characterisation and subsequent correction of the combined isotope composition–humidity dependency for this type of laser spectrometers when performing water vapour measurements in cold and dry conditions.

Yongbiao Weng et al.
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Short summary
We find that the known humidity dependence of laser spectrometers for water vapour isotope measurements varies with the isotope composition. We have developed a scheme to correct for this isotope-composition dependent bias. The correction is most substantial at low humidity and for d-excess. Stability tests indicate that the dependency is to first order a constant instrument characteristic. Water vapour isotope measurements at low humidities can now be corrected following our proposed procedure.
We find that the known humidity dependence of laser spectrometers for water vapour isotope...
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