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This proof-of-concept study demonstrates that methane (CH4) emissions from natural gas (NG) and agriculture can be disentangled using the concept of excess column observations. A network of cost-effective sensors measured excess column-averaged dry-air mole fractions for CH4 (Delta XCH4), ethane (Delta XC2H6 as NG tracer), and ammonia (Delta XNH3 from agriculture) in the Denver-Julesburg Basin during March 2015. Delta XCH4 varied up to 17ppb and was >3 times higher with winds from directions where NG is produced. The Delta XCH4 variance is explained by variations in the C2H6-NH3 tracer pair, attributing 63 +/- 17% to NG, 25 +/- 10% to agriculture, and 12 +/- 12% to other sources. The ratios Delta XC2H6/Delta XCH4 (16 +/- 2%; indicates wet NG) and Delta XNH3/Delta XCH4 (43 +/- 12%) were compatible with in situ measured ratios. Excess columns are independent of boundary layer height, characterize gases in the open atmosphere, are inherently calibrated, average over extended spatial scales, and provide a complementary perspective to quantify and attribute CH4 emissions on regional scales.

Plain Language Summary Methane is the second most important anthropogenic greenhouse gas. Knowledge about methane sources is increasingly relevant as energy production continues to shift toward natural gas and becomes complicated by collocated emissions from natural gas production and agriculture due to shared land use. There is a need for methods to better decouple emissions from multiple sources that contribute to local enhancements in methane, which are small compared to the regional methane background concentrations, and depend on atmospheric transport and planetary boundary layer height. In this study, we show that the concept of collocated excess column measurements of methane and chemical tracers shows great promise as a viable approach to disentangle methane emissions from multiple sources by means of cost-effective networks of ground-based sensors. Excess columns are independent of boundary layer height, which makes quantification and source attribution of methane more straightforward.