A new look at methane and nonmethane hydrocarbon emissions from oil and natural gas operations in the Colorado Denver‐Julesburg Basin

Abstract Emissions of methane (CH 4 ) from oil and natural gas (O&G) operations in the most densely drilled area of the Denver‐Julesburg Basin in Weld County located in northeastern Colorado are estimated for 2 days in May 2012 using aircraft‐based CH 4 observations and planetary boundary layer height and ground‐based wind profile measurements. Total top‐down CH 4 emission estimates are 25.8 ± 8.4 and 26.2 ± 10.7 t CH 4 /h for the 29 and 31 May flights, respectively. Using inventory data, we estimate the total emissions of CH 4 from non‐O&G gas‐related sources at 7.1 ± 1.7 and 6.3 ± 1.0 t CH 4 /h for these 2 days. The difference in emissions is attributed to O&G sources in the study region, and their total emission is on average 19.3 ± 6.9 t/h, close to 3 times higher than an hourly emission estimate based on Environmental Protection Agency's Greenhouse Gas Reporting Program data for 2012. We derive top‐down emissions estimates for propane, n ‐butane, i ‐pentane, n ‐pentane, and benzene from our total top‐down CH 4 emission estimate and the relative hydrocarbon abundances in aircraft‐based discrete air samples. Emissions for these five nonmethane hydrocarbons alone total 25.4 ± 8.2 t/h. Assuming that these emissions are solely originating from O&G‐related activities in the study region, our results show that the state inventory for total volatile organic compounds emitted by O&G activities is at least a factor of 2 too low for May 2012. Our top‐down emission estimate of benzene emissions from O&G operations is 173 ± 64 kg/h, or 7 times larger than in the state inventory.