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

Abstract

The carbon isotopic signature (δ 13 CH 4 ) of several methane sources in Germany (around Heidelberg and in North Rhine-Westphalia) were characterised. Mobile measurements of the plume of CH 4 sources are carried out using an analyser based on cavity ring-down spectroscopy (CRDS). To achieve precise results a CRDS analyser, which measures methane (CH 4 ), carbon dioxide (CO 2 ) and their 13 C-to- 12 C 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 δ 13 CH 4 are water vapour (H 2 O) and ethane (C 2 H 6 ). To avoid the cross sensitivity with H 2 O, the air is dried with a Nafion dryer during mobile measurements. C 2 H 6 is typically abundant in natural gases and thus in methane plumes or samples originating from natural gas. A C 2 H 6 correction and calibration are essential to obtain accurate δ 13 CH 4 results, which can deviate by up to 3% depending on whether a C 2 H 6 correction is applied. The isotopic signature is determined with the Miller-Tans approach and the York fitting method. During 21 field campaigns the mean δ 13 CH 4 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 δ 13 CH 4 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.