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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-2020-365
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-365
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  12 Nov 2020

12 Nov 2020

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This preprint is currently under review for the journal AMT.

Simultaneous measurement of δ13C, δ18O and δ17O of atmospheric CO2 – Performance assessment of a dual-laser absorption spectrometer

Pharahilda M. Steur1, Hubertus A. Scheeren1, Dave D. Nelson2, J. Barry McManus2, and Harro A. J. Meijer1 Pharahilda M. Steur et al.
  • 1Centre for Isotope Research, University of Groningen, Nijenborgh 6, 9747 AG Groningen, The Netherlands
  • 2Aerodyne Research Inc., 45 Manning Road, Billerica, MA 01821-3976, USA

Abstract. Using laser absorption spectrometry for the measurement of stable isotopes of atmospheric CO2 instead of the traditional Isotope Ratio Mass Spectrometry (IRMS) method decreases sample preparation time significantly, and uncertainties in the measurement accuracy due to CO2 extraction and isobaric interferences are avoided. In this study we present the measurement performance of a new dual-laser instrument developed for the simultaneous measurement of the δ13C, δ18O and δ17O of atmospheric CO2 in discrete air samples, referred to as the Stable Isotopes of CO2 Absorption Spectrometer (SICAS). We compare two different calibration methods: the ratio method (RM) based on measured isotope ratio and a CO2 mole fraction dependency correction (CMFD), and the isotopologue method (IM) based on measured isotopologue abundances. Calibration with the RM and IM is based on three different assigned whole air references calibrated on the VPBD scale. An additional quality control tank (QC) is included in both methods to follow long-term instrument performance. Measurements of the QC tank show that best performance is achieved with the RM for both the δ13C and δ18O measurements with mean residuals of 0.007 ‰ and 0.016 ‰ and mean standard errors of 0.009 ‰ and 0.008 ‰ respectively, during periods of optimal measurement conditions. The δ17O standard error in the same measurement period is 0.013 ‰. In addition, intercomparing a total of 14 different flask samples covering a CO2 mole fraction range of 344–439 ppm with the Max Planck Institute for Biogeochemistry shows a mean residual of 0.002 ‰ and a standard deviation of 0.063 ‰ for δ13C, using the RM. The δ18O could not be compared due to depletion of the δ18O signal in our sample flasks because of too long storage times. Finally, we evaluated the potential of our Δ17O measurements as a tracer for Gross Primary Production (GPP) by vegetation through photosynthesis. Here, a measurement precision of < 0.01 ‰ would be a prerequisite for capturing seasonal variations in the Δ17O signal. So far, a mean standard error of 0.016 ‰ was achieved for Δ17O measurements of our QC tank using the RM. Improvements in our measurement procedure, spectral fit and δ17O calibration are due to reach the required precision.

Pharahilda M. Steur et al.

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Short summary
For understanding the sources and sinks of atmospheric CO2, measurement of stable isotopes has proven to be highly valuable. We present a new method using laser absorption spectroscopy to simultaneously conduct measurements of three CO2 isotopes, directly on dry air samples. This new method reduces sample preparation time significantly, compared to the conventional method where measurements are conducted on pure CO2, and avoids measurement biases introduced by CO2 extraction.
For understanding the sources and sinks of atmospheric CO2, measurement of stable isotopes has...
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