An improved method for mobile characterisation of δ 13 CH 4 source signatures and its application in Germany

Abstract

Abstract. The carbon isotopic signature ( δ13CH4 ) of several methane\nsources in Germany (around Heidelberg and in North Rhine-Westphalia) were\ncharacterised. Mobile measurements of the plume of CH4 sources are carried\nout using an analyser based on cavity ring-down spectroscopy (CRDS). To achieve precise results\na CRDS analyser, which measures methane ( CH4 ), carbon dioxide ( CO2 ) and\ntheir 13C -to- 12C ratios, was characterised especially with regard\nto cross sensitivities of composition differences of the gas matrix in air\nsamples or calibration tanks. The two most important gases which affect\n δ13CH4 are water vapour ( H2O ) and ethane ( C2H6 ). To\navoid the cross sensitivity with H2O , the air is dried with a Nafion dryer\nduring mobile measurements. C2H6 is typically abundant in natural gases\nand thus in methane plumes or samples originating from natural gas. A\n C2H6 correction and calibration are essential to obtain accurate\n δ13CH4 results, which can deviate by up to 3 \u2009‰ \ndepending on whether a C2H6 correction is applied. The isotopic signature is determined with the Miller–Tans approach and the\nYork fitting method. During 21 field campaigns the mean\n δ13CH4 signatures of three dairy farms\n( - 63.9 ± 0.9 ‰ ), a biogas plant\n( - 62.4 ± 1.2 ‰ ), a landfill\n( - 58.7 ± 3.3 ‰ ), a wastewater treatment plant\n( - 52.5 ± 1.4 ‰ ), an active deep coal mine\n( - 56.0 ± 2.3 ‰ ) and two natural gas storage and gas compressor\nstations ( - 46.1 ± 0.8 ‰ ) were recorded. In addition, between December\xa02016 and November\xa02018 gas samples from the\nHeidelberg natural gas distribution network were measured with a mean\n δ13CH4 value of - 43.3 ± 0.8 ‰ . Contrary to\nprevious measurements between 1991 and 1996 by Levin et al. ( 1999 ) , no\nstrong seasonal cycle is shown.