Optimizing a calibration strategy for precise δ13CH4 measurements of ambient air using a CRDS analyzer

Methane (CH₄) is an important greenhouse gas with multiple natural and anthropogenic sources. Precise δ¹³CH₄ measurements are essential for distinguishing these sources, understanding biogeochemical cycles, and improving climate models. While in-situ CRDS (Cavity Ring-Down Spectroscopy) measurements may have limited absolute precision, well-calibrated continuous measurements of δ¹³CH₄ provide high-temporal-resolution data that are essential for reliably attributing atmospheric CH₄ sources.

Here, we present an instrumental characterization, determination of cross-sensitivities and an improved calibration strategy for high-precision δ¹³CH₄ measurements in ambient air using a Picarro G2201-i CRDS analyzer. This approach combines the determination of internal correction parameters from regular measurements (every 5-6 hours) of a single calibration gas at atmospheric concentration with annual multi-point calibrations using reference gases at 10 ppm CH₄ spanning an isotopic range of -60 to -37 ‰ in δ¹³CH₄. This strategy corrects the non-linearity in δ¹³CH₄ with changing CH₄ mole fraction, which can reach up to 3 ‰ in δ¹³CH₄ over the 2-10 ppm CH4 range.

Applying the Keeling-plot method to nightly CH₄ enhancements in Heidelberg, Germany, the new calibration leads to δ¹³CH₄ source signatures for individual events differing up to 4.6 ‰ to the previous one-point δ-calibration. Using this new calibration scheme, the mean δ¹³CH₄ source signature for 2021-2025 was (-52.3 ± 0.3) ‰, slightly more enriched compared to 2014-2020 ((-53.9 ± 0.3) ‰, presented by Hoheisel and Schmidt, 2024). Only a careful instrument characterization combined with an adapted calibration strategy can ensure the high precision required for δ¹³CH₄ data suitable for quantitative atmospheric studies.