Using Multiscale Ethane/Methane Observations to Attribute Coal Mine Vent Emissions in the San Juan Basin From 2013 to 2021

Source attribution of natural gas emissions from fossil fuels in New Mexico's San Juan Basin (SJB) is challenging due to source heterogeneity and emissions transience. We demonstrate that ethane (C2H6) to methane (CH4) mixing ratios can identify and separate sources over different scales using various measurement techniques. We report simultaneous CH4 and C2H6 observations near a coal mine vent and oil and gas (O&G) emission sources using ground-based in situ measurements in 2020/2021. During these campaigns, we observed a stable coal vent C2H6:CH4 ratio of 1.28% +/- 0.11%, discernibly different than nearby O&G source ratios ranging from 0.9% to 16.8%. We analyze airborne observations of the SJB taken in 2014/2015 that exhibit similar coal vent ratios and further show the region's heterogeneity. We identify episodic O&G sources, including a gas plant source detected in 2014/2015 that is absent in our 2020/2021 data. We examine total column observations of C2H6 and CH4 made in 2013 with a solar spectrometer and find a C2H6:CH4 ratio of 1.3% +/- 0.4% for the coal vent. The stable and unique coal vent ratio relative to other O&G sources in the region is used to demonstrate that consistent attribution is possible using various measurement methods at multiple scales across many years. Finally, we demonstrate that using C2H6 as a proxy for fossil CH4 inversions can inform detailed basin-scale inversions, provided we understand source specific changes in the C2H6:CH4 ratio like we report in the SJB.