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 (ΔXCH₄), ethane (ΔXC₂H₆ as NG tracer), and ammonia (ΔXNH₃ from agriculture) in the Denver-Julesburg Basin during March 2015. ΔXCH₄ varied up to 17 ppb and was >3 times higher with winds from directions where NG is produced. The ΔXCH₄ 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 ΔXC₂H₆/ΔXCH₄ (16 ± 2%; indicates wet NG) and ΔXNH₃/ΔXCH₄ (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.