Abstract. The carbon isotopic signature (δ13CH4) of several methane sources in Germany (around Heidelberg and in North Rhine-Westphalia) were characterised. Mobile measurements of the plume of CH4 sources are carried out using an analyser based on cavity ring-down spectroscopy (CRDS). To achieve precise results a CRDS analyser, which measures methane (CH4), carbon dioxide (CO2) and their 13C-to-12C ratios, was characterised especially with regard to cross sensitivities of composition differences of the gas matrix in air samples or calibration tanks. The two most important gases which affect δ13CH4 are water vapour (H2O) and ethane (C2H6). To avoid the cross sensitivity with H2O, the air is dried with a Nafion dryer during mobile measurements. C2H6 is typically abundant in natural gases and thus in methane plumes or samples originating from natural gas. A C2H6 correction and calibration are essential to obtain accurate δ13CH4 results, which can deviate by up to 3 ‰ depending on whether a C2H6 correction is applied. The isotopic signature is determined with the Miller–Tans approach and the York fitting method. During 21 field campaigns the mean δ13CH4 signatures of three dairy farms (-63.9±0.9‰), a biogas plant (-62.4±1.2‰), a landfill (-58.7±3.3‰), a wastewater treatment plant (-52.5±1.4‰), an active deep coal mine (-56.0±2.3‰) and two natural gas storage and gas compressor stations (-46.1±0.8‰) were recorded. In addition, between December 2016 and November 2018 gas samples from the Heidelberg natural gas distribution network were measured with a mean δ13CH4 value of -43.3±0.8‰. Contrary to previous measurements between 1991 and 1996 by Levin et al. (1999), no strong seasonal cycle is shown.

Abstract. The carbon isotopic signature (δ13CH4) of several methane sources in Germany (around Heidelberg and in North Rhine-Westphalia) were characterised. Therefore, mobile measurements of the plume of CH4 sources are carried out using a cavity ring-down spectrometer (CRDS). To achieve precise results a CRDS analyser, which measures methane (CH4), carbon dioxide (CO2) and their 13C to 12C ratios, was characterised especially with regard to cross sensitivities of the gas matrix. The two most important gases which affect the measurements are water vapour (H2O) and ethane (C2H6). To avoid the cross sensitivity with H2O, the air is dried with a nafion dryer during mobile measurements. C2H6 is abundant in natural gas and thus in methane plumes or samples originating from natural gas. A C2H6 correction and calibration are essential to obtain accurate δ13CH4 results, which can deviate up to 3 ‰ depending on whether an ethane correction is applied. The isotopic signature is determined with the Miller-Tans approach and the York fitting method. During 21 field campaigns the mean δ13CH4 values of three dairy farms (−63.9 ± 0.9 ‰), a biogas plant (−62.4 ± 1.2 ‰), a landfill (−58.7 ± 3.3 ‰), a wastewater treatment plant (−52.5 ± 1.4 ‰), an active deep coal mine (−56.0 ± 2.3 ‰) and two natural gas storage and gas compressor stations (−46.1 ± 0.8 ‰) were recorded. In addition, between December 2016 and June 2018 gas samples from the Heidelberg natural gas distribution network were measured. Contrary to former measurements between 1991 and 1996 (Levin et al., 1999) no strong seasonal cycle is shown. The mean δ13CH4 value of this study is −43.1 ± 0.8 ‰ which is 2.8 ‰ more depleted than in former years.

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Abstract. The CO, CO2, and CH4 mole fractions have been measured since 2002 at the Environmental Research Station Schneefernerhaus (ZSF), which is located approximately 300 m below the summit of Mount Zugspitze. Although the station is located remotely at an altitude of 2666 m a.s.l., local pollution events by snow blowers and snow groomers can be detected in the high temporal resolution time series of seconds or minutes. Therefore, a time-consuming flagging process, carried out manually by the station manager, is necessary. To examine local influences and the effectiveness of data flagging, a 290 m long intake line to the higher Zugspitz ridge was used to measure CO, CO2 and CH4 mole fractions at a potentially less polluted location between October 2018 and October 2020. The comparison of these two time series shows that the mountain ridge measurement is almost unaffected by local pollution. It also demonstrates that the influence of local pollution events on the Schneefernerhaus measurements is successfully removed by the station manager. Only a small deviation up to 0.24 ppm can be observed during the day between the CO2 time series of Schneefernerhaus and the mountain ridge in winter, probably due to anthropogenic sources.

<p>Instrumental development in measurement technique now allows continuous in-situ isotope analysis of <sup>13</sup>CH<sub>4</sub> by Cavity Ring-Down Spectroscopy (CRDS). Analyses of the isotopic composition of methane in ambient air can potentially be used to partition between different CH<sub>4</sub> source categories.</p><p>Since 2014 a CRDS G2201-i analyser has been used to continuously measure CH<sub>4</sub> and its <sup>13</sup>C/<sup>12</sup>C 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 δ<sup>13</sup>CH<sub>4 </sub>measurements at a semi-rural station.</p><p>As an urban station, the seasonal and daily variations of the measured CH<sub>4</sub> 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 CH<sub>4</sub> peak height and instrumental precision. Therefore, at Schauinsland station the lower variability in the CH<sub>4</sub> 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.</p><p>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.</p><p>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 CH<sub>4</sub> 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.</p>