Evaluation of continuous δ13CH4 measurements in Heidelberg and at Schauinsland, Germany

Instrumental development in measurement technique now allows continuous in-situ isotope analysis of ¹³CH₄ by Cavity Ring-Down Spectroscopy (CRDS). Analyses of the isotopic composition of methane in ambient air can potentially be used to partition between different CH₄ source categories.

Since 2014 a CRDS G2201-i analyser has been used to continuously measure CH₄ and its ¹³C/¹²C ratio in ambient air at the Institute of Environmental Physics (IUP) in Heidelberg (116m a.s.l.), South-West Germany. Furthermore, the CRDS G2201-i analyser was installed twice for a month at the measurement station of the German Environment Agency at Schauinsland (1205m a.s.l.). In September 2018 and in February 2019 the analyser was moved to Schauinsland to examine the validity of evaluations of continuous δ¹³CH₄ measurements at a semi-rural station.

As an urban station, the seasonal and daily variations of the measured CH₄ mole fraction and isotopic composition in Heidelberg vary much stronger than at the mountain station Schauinsland. The precision of the isotopic source signature calculation using a Keeling plot strongly depends on the CH₄ peak height and instrumental precision. Therefore, at Schauinsland station the lower variability in the CH₄ mole fraction makes the evaluation challenging. Different methods such as monthly/weekly interval evaluations and moving Keeling/Miller Tans methods has been used to calculate the isotopic source signature in ambient air.

The isotopic methane source signatures of the air in Heidelberg was found to be between -75 ‰ and -35 ‰, with an average of (-54 ± 2) ‰. An annual cycle can be noticed with more depleted values (-56 ‰) in summer and more enriched values (-51 ‰) in winter, due to larger biogenic emissions in summer and more thermogenic (e.g. natural gas) emissions in winter. The mean isotopic source signature calculated at Schauinsland shows variations, too, with more enriched values (−56 ‰) in winter and more depleted (−60 ‰) ones in autumn. The more depleted values in summer/autumn at Schauinsland corresponds to more biogenic methane and can be explained by dairy cows grazing near the station especially during this time.

The generally more enriched values at Schauinsland are caused by the more rural surrounding. Emission estimates of county provided by the LUBW Landesanstalt für Umwelt Baden-Württemberg shows that around Schauinsland 60 % of the CH₄ emissions are emitted by livestock farming and around Heidelberg only 28 %. The mean isotopic source signature calculated using these emissions is (-58 ± 2) ‰ for Schauinsland and (-53 ± 2) ‰ for Heidelberg. These results agreed well with the mean source signatures determined out of continuous isotopic measurements.