Characterization of interferences to in situ observations of <i>δ</i><sup>13</sup>CH<sub>4</sub> and C<sub>2</sub>H<sub>6</sub> when using a cavity ring-down spectrometer at industrial sites

Abstract

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.

N<sub>2</sub>O isotopocule measurements using laser spectroscopy: analyzer characterization and intercomparison

“…For example, as highlighted in Table 9, the measured interference 25 effect on δ 15 N α measurements acquired using CRDS II for 10 ppm [CH4] at 330 ppb N2O was 24.9 ‰, while at 990 ppb N2O it was 8.1 ‰, resulting in a 16.8 ‰ difference. The scaling of interference effects from trace gases has been reported previously for CO2/CH4 laser spectrometers (Assan et al, 2017;Malowany et al, 2015). This underlines the usefulness of removing H2O, CO2 and CO with scrubbers prior to measurement, as this removes the need for correction equations to begin with and the scaling of corrections that can ensue.…”

Technical note: Interferences of volatile organic compounds (VOCs) on methane concentration measurements

“…In particular, the potential for biased measurements due to spectral interference with other gases still needs to be established for various environments and applications (e.g. Rella et al, 2015;Assan et al, 2017;Zhao et al, 2012;Zellweger et al, 2016).…”

“…Plants were recently identified as an important component of the natural cycles of CH 4 (Keppler et al, 2006;Nisbet et al, 2009;Carmichael et al, 2014). This has led to an increased interest in the role of trees in the CH 4 exchange of forests (e.g.…”

Novel Coalbed Methane (CBM) Origin Analysis and Source Apportionment Method Based on Carbon Isotope Ratio Using Infrared Dual‐Wavelength Laser Absorption Spectroscopy

“…Tuzson et al () obtained the 0.16‰ for [ 13 CO 2 / 12 CO 2 ] with an averaging time of about 500 s. Gagliardi et al () used the 2.008‐μm laser source to monitoring the volcanic gases, and the short‐term standard deviation (0.16‰) and a long‐term (1‰) were obtained. Assan et al () used CRDS to analyze the CH 4 in natural gas, and the calibration gas aliquots were measured with an average standard deviation of 0.03‰. Also, the distributed feedback (DFB) laser and a modified Herriot‐type cell were applied to analyzed the [ 13 CH 4 / 12 CH 4 ] by Yamamoto and Yoshida (), and the precision was 0.027‰.…”