Abstract
Abstract. 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.
Highlights
With increasing efforts to mitigate anthropogenic greenhouse gas emissions, opportunities to reduce leaks from fossil fuel derived methane are of particular importance as they currently account for approximately 30 % of all anthropogenic methane emissions (Kirschke et al, 2013)
This study focuses on measurements of C2H6 contaminated methane sources by a cavity ringdown spectroscopy (CRDS) (Picarro G2201-i), with emphasis on correcting δ13CH4 and C2H6 for cross-interferences before calibration
Our extensive laboratory tests suggest that CRDS instruments of this model are all subject to similar interferences and that they can have a significant impact on reported concentrations and isotopic signatures if not accounted for properly when measuring industrial natural gas sources
Summary
With increasing efforts to mitigate anthropogenic greenhouse gas emissions, opportunities to reduce leaks from fossil fuel derived methane (ffCH4) are of particular importance as they currently account for approximately 30 % of all anthropogenic methane emissions (Kirschke et al, 2013). Technically feasible mitigation methods hold the potential to half future global anthropogenic CH4 emissions by 2030 Of this mitigation potential more than 60 % can be realized in the fossil fuel industry (Hoglund-Isaksson, 2012). Recent estimates of CH4 leaks vary widely (1–10 % of global production; Allen, 2014) and US inventories of natural gas CH4 emissions have uncertainties of up to 30 % (US EPA, 2016). In addressing this issue, the ability to distinguish between biogenic and different anthropogenic sources is of vital importance. The discrimination of sources with relatively close isotopic composition such as oil-associated gas and natural gas, which can have isotopic signatures separated by only ∼ 4 ‰ (Stevens and Engelkemeir, 1988), requires precise and reliable δ13CH4 measurements
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