Six years of continuous CH4 mole fraction and δ13C-CH4 measurements in Heidelberg (Germany) [data]

δ13C-CH4 and the mole fraction of CH4 have been measured continuously since April 2014 using a cavity ring-down spectroscopy (CRDS) analyser in Heidelberg, Germany. This 6-year time series shows an increasing trend of (6.8 ± 0.3)ppb a−1 for the CH4 mole fraction between 2014 and 2020. δ13C-CH4 decreases by (−0.028 ± 0.002)‰ a−1 over this time period. In this study, seasonal variations and trends of CH4 emissions in the catchment area of Heidelberg are analysed using three approaches by applying the Keeling plot method to atmospheric measurements on different time scales. The mean δ13C isotopic source signature for the Heidelberg catchment area is (−52.3 ± 0.3)‰ (moving Keeling plot approach). In all three approaches, there is no significant trend in the monthly mean source signature over the last six years. However, more depleted source signature values occur in summer. This annual cycle in 13C-CH4 sources, with a peak-to-peak amplitude of −5.8‰, can only be partially explained by seasonal variations in CH4 emissions from heating. Additional seasonal variations needs to occur in biogenic CH4 emissions from waste water, landfills or dairy cows. Furthermore, the source contributions derived from atmospheric measurements are used to evaluate the CH4 emissions reported by two emission inventories: the Emissions Database for Global Atmospheric Research (EDGARv6.0) and the inven- tory of the State Institute for the Environment Baden-Württemberg (LUBW - Landesanstalt für Umwelt Baden-Württemberg). The mean δ13C-CH4 source signature determined from the LUBW inventory agrees well with the result from atmospheric measurements. However, the signature determined from EDGARv6.0 data is less depleted by about 7‰. Thus, EDGARv6.0 seems to overestimate CH4 emissions from more enriched sources.