International audienc

Abstract. Due to increased demand for an understanding of CH 4 emissions from industrial sites, the subject of cross sensitivities caused by absorption from multiple gases on δ 13 CH 4 and C 2 H 6 measured in the near-infrared spectral domain using CRDS has become increasingly important. Extensive laboratory tests are presented here, which characterize these cross sensitivities and propose corrections for the biases they induce. We found methane isotopic measurements to be subject to interference from elevated C 2 H 6 concentrations resulting in heavier δ 13 CH 4 by +23.5 ‰ per ppm C 2 H 6 / ppm CH 4 . Measured C 2 H 6 is subject to absorption interference from a number of other trace gases, predominantly H 2 O (with an average linear sensitivity of 0.9 ppm C 2 H 6 per % H 2 O in ambient conditions). Yet, this sensitivity was found to be discontinuous with a strong hysteresis effect and we suggest removing H 2 O from gas samples prior to analysis. The C 2 H 6 calibration factor was calculated using a GC and measured as 0.5 (confirmed up to 5 ppm C 2 H 6 ). Field tests at a natural gas compressor station demonstrated that the presence of C 2 H 6 in gas emissions at an average level of 0.3 ppm shifted the isotopic signature by 2.5 ‰, whilst after calibration we find that the average C 2 H 6 : CH 4 ratio shifts by +0.06. These results indicate that, when using such a CRDS instrument in conditions of elevated C 2 H 6 for CH 4 source determination, it is imperative to account for the biases discussed within this study.