Characterization of interferences to in situ observations of δ13CH4 and C2H6 when using a cavity ring-down spectrometer at industrial sites

[摘要] Due to increased demand for an understanding of CH₄ emissions from industrial sites, the subject of cross sensitivities caused by absorption from multiple gases on δ¹³CH₄ and C₂H₆ 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 C₂H₆ concentrations resulting in heavier δ¹³CH₄ by +23.5 ‰ per ppm C₂H₆ ∕ ppm CH₄. Measured C₂H₆ is subject to absorption interference from a number of other trace gases, predominantly H₂O (with an average linear sensitivity of 0.9 ppm C₂H₆ per  % H₂O in ambient conditions). Yet, this sensitivity was found to be discontinuous with a strong hysteresis effect and we suggest removing H₂O from gas samples prior to analysis. The C₂H₆ calibration factor was calculated using a GC and measured as 0.5 (confirmed up to 5 ppm C₂H₆). Field tests at a natural gas compressor station demonstrated that the presence of C₂H₆ 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 C₂H₆  CH₄ ratio shifts by +0.06. These results indicate that, when using such a CRDS instrument in conditions of elevated C₂H₆ for CH₄ source determination, it is imperative to account for the biases discussed within this study.

[学科分类] 大气科学