Metrological calibration approaches for atmospheric measurement of δ13C-CH4 employing CRDS

Accurate measurement of atmospheric greenhouse gases is essential for tracking emissions and assessing the effectiveness of mitigation strategies. Methane (CH₄), a potent greenhouse gas with diverse sources, requires isotope analysis to determine its origin (e.g. biogenic, anthropogenic, thermogenic, etc.) in the atmosphere. Precise and consistent measurements of atmospheric δ¹³C-CH₄ are critical for the source attribution and understanding global methane budget.

Cavity ring-down spectroscopy (CRDS) is a versatile technique for monitoring atmospheric δ¹³C in CH_4, providing rapid, continuous measurements with high precision (better than 0.8 ‰). CRDS isotope ratio analyzers like the Picarro G2201 offer flexibility for both laboratory and field applications. However, δ¹³Cmeasurements can be affected by instrument biases, drift, concentration dependence and matrix gas effects. These challenges highlight the need for a careful and traceable calibration strategy to ensure accurate, reproducible, and comparable results.

There are two approaches to calibration laser spectroscopic isotope analyzers: (a) Isotope-ratio (δ-based) approach: Reference gases with assigned δ¹³C values, traceable to e.g. the Vienna Pee Dee Belemnite (VPDB) scale, are used to directly calibrate δ¹³C-CH₄ (or CO₂). Samples are measured between two reference gases in a bracketing sequence, which allows correction for short-term instrumental drift and assignment of δ values. This approach is most effective when sample and reference CH₄ amount fractions (concentrations) are close. (b) Isotopologue approach: The amount fractions of individual isotopologue (¹²CH₄ and ¹³CH₄) are calibrated separately. δ¹³C-CH₄ is then calculated from their ratio using the conventional delta notation relative to e.g. VPDB. This approach is well suited for measurements over a wider range of CH₄ amount fractions.

In this work, we carefully evaluate the capabilities of both calibration approaches for δ¹³C measurement in CH₄ (also for CO₂), targeting field site application. We compare results from both approaches, assess their relative accuracy, precision and suitability for long term in situ atmospheric δ¹³C monitoring. Metrological data qualities, such as traceability of the results and Guide to the expression of uncertainty in measurement (GUM) complaint uncertainties evaluation, are addressed. The use of accurate and reliable reference gases traceable to the VPDB scale ensure that the isotope ratio measurements for CH₄ (and CO₂) are metrologically reliable to ensure comparability across instruments and laboratories.

References

• 19ENV05 STELLAR; D5: Good practice guide for specification and application of OIRS for atmospheric measurements, including sample handling protocol, optimised analytical procedures, traceability to the international standards and target uncertainties (0.05 ‰ for δ¹³C-CO₂ and δ¹⁸O-CO₂).
• Srivastava, A., ... & Nwaboh, J. (2025). Developing calibration and measurement capabilities for atmospheric CH₄ stable isotope ratios at NMIs/DIs: metrology for global comparability. Metrologia, 62(3), 032001.

Acknowledgement  

This work was carried out within project 24GRD03-MetHIR, which has received funding from the European Partnership on Metrology, co-financed by the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States.