Separation of methane emissions from agricultural and natural gas sources in the Colorado Front Range

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.