Methane and Ethane Emission Rates, Intensities, and Trends: Aircraft Mass Balance Insights Over the Denver‐Julesburg Basin, Fall 2021

Abstract Despite a 2.1‐fold increase in oil and gas (O&G) production from 2012 to 2021 in the Wattenberg Field (WF) area of the Denver‐Julesburg Basin (DJB), total methane (CH4) emissions have remained unchanged within measurement uncertainties (26.0 ± 6.8 t/hr in 2012, 24.0 ± 5 t/hr in 2015, and 21.4 ± 5.7 t/hr in 2021), while ethane (C 2 H 6 ) emissions have decreased by a factor of 2.7 since 2015, from 7.0 ± 1.1 to 2.6 ± 0.9 t/hr in 2021. We present airborne mass flux measurements of CH 4 and C 2 H 6 quantifying total emissions, their variability, and primary sources—O&G versus concentrated animal feeding operations (CAFOs). Based on four mass balance flights in September and October 2021, total CH 4 and C 2 H 6 emissions showed significant day‐to‐day variability, with standard deviations of 23% and 32%, respectively. Combining in situ and LiDAR measurements with a 3‐km atmospheric model improves our understanding of the reliability of flux determination. Using C 2 H 6 and vapor‐phase acetic acid (CH 3 COOH) as tracers, we attribute 63 ± 11% of CH 4 emissions to O&G. Emission intensities (EIs), calculated as thermogenic (TG‐ CH 4 ) and C 2 H 6 emissions per barrel of oil equivalent (BOE), show a declining trend, with TG‐ CH 4 EIs decreasing at an average annual rate of 4.4% from May 2012 to October 2021 and C 2 H 6 EIs declining at an average annual rate of 10.8% from April 2015 to October 2021. C 2 H 6 EIs fell at more than twice the rate of TG‐ CH 4 EIs.