Characterisation of interferences to in-situ observations of δ13CH4 and C2H6 when using a Cavity Ring Down Spectrometer at industrial sites
Sabina Assan, Alexia Baudic, Ali Guemri, Philippe Ciais, Valerie Gros, and Felix R. Vogel
Laboratoire des Sciences du Climat et de l'Environnement, Chaire BridGES, UMR CNRS-CEA-UVSQ, Gif-sur-Yvette, Ile-de-France, 91191, France
Received: 02 Aug 2016 – Accepted for review: 10 Dec 2016 – Discussion started: 19 Dec 2016
Abstract. The increase of atmospheric methane (CH4) is the second largest contributor to the increased radiative forcing since the industrial revolution. Natural gas extraction and distribution is associated with CH4 leaks of uncertain magnitude that has spurred interest for developing new methods to measure them. Using a CRDS instrument we evaluate two methane identification methods commonly used to better constrain emission estimates from natural gas leaks, namely stable isotope analysis and the ethane: methane ratio (C2H6 : CH4). Recently it has come to light that CRDS measurements of δ13CH4 and C2H6 in the near infrared spectral domain are subject to cross sensitivities due to absorption from multiple gases. These sensitivities translate into biases in the retrieval of δ13CH4 and C2H6 concentrations in air samples, and should thus be accounted for during data processing. Here we present extensive laboratory tests using two CRDS instruments to characterize their cross sensitivities and propose corrections to calculate unbiased δ13CH4 and C2H6. Methane isotopic measurements were found to be subject to interference from elevated C2H6 concentrations (a secondary component in many natural gas types) 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, the predominant being H2O (with an average linear sensitivity of 0.9 ppm C2H6 per % H2O in ambient conditions, meaning that the presence of H2O causes the inference of too high C2H6 mixing ratios if no correction is applied). Yet, this sensitivity was found to be discontinuous with a strong hysteresis effect. Throughout the range of C2H6 concentrations measured in this study (0–5 ppm C2H6), both CRDS instruments consistently measure concentrations double that reported by a GC, thus we have calculated a calibration factor of 0.5. To demonstrate the significance of the corrections we test the source identification methods on air measured at a natural gas compressor station. The presence of C2H6 in gas emissions at an average level of 0.3 ppm was found to shift the isotopic signature by 2.5 ‰. We find that after correction and calibration 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.
Assan, S., Baudic, A., Guemri, A., Ciais, P., Gros, V., and Vogel, F. R.: Characterisation of interferences to in-situ observations of δ13CH4 and C2H6 when using a Cavity Ring Down Spectrometer at industrial sites, Atmos. Meas. Tech. Discuss., doi:10.5194/amt-2016-261, in review, 2016.