Continuous field measurements of δ¹³C–CO₂ and trace gases by FTIR spectroscopy

Abstract

Continuous analysis of the ¹³C/¹²C ratio of atmospheric CO₂ (δ¹³C–CO₂) is a powerful tool to quantify CO₂ flux strengths of the two major ecosystem processes assimilation and respiration. Traditional laboratory techniques such as isotope ratio mass spectrometry (IRMS) in combination with flask sampling are subject to technical limitations that do not allow to fully characterising variations of atmospheric δ¹³C–CO₂ at all relevant timescales. In our study, we demonstrate the strength of Fourier transform infrared (FTIR) spectroscopy in combination with a PLS-based calibration strategy for online analysis of δ¹³C–CO₂ in ambient air. The ability of the instrument to measure δ¹³C–CO₂ was tested on a grassland field-site and compared with standard laboratory-based IRMS measurements made on field-collected flask samples. Both methods were in excellent agreement, with an average difference of 0.4‰ (n=81). Simultaneously, other important trace gases such as CO, N₂O and CH₄ were analysed by FTIR spectroscopy.

Keywords:

• carbon-13
• carbon dioxide
• FTIR
• infrared spectroscopy
• isotope ratio mass spectrometry

Acknowledgements

Kerstin Zeyer and Christoph Zellweger are acknowledged for CO₂, N₂O, CO and CH₄ analysis of the multi-component standard gases. We thank Bela Tuzson for helpful discussion. The authors are grateful to Willi Brand and Michael Rothe from the Max Planck Institute for Biochemistry for δ¹³C–CO₂ and analysis of the IRMS laboratory standards. We acknowledge E. Pieper from the Section Mechanical Engineering/Workshop and W. Knecht and R. Gross from the Section Electronics/Metrology for technical assistance. M.J. Zeeman, R.A. Werner and W. Eugster were supported by the Swiss National Science Foundation, grant 200021-105949.