Informing methane emissions inventories using facility aerial measurements at midstream natural gas facilities

Increased interest in greenhouse gas (GHG) emissions, including recent legislative action and voluntary programs, has increased attention on quantifying, and ultimately reducing, methane emissions from the natural gas supply chain. While inventories used for public or corporate GHG policies have traditionally utilized bottom-up (BU) methods to estimate emissions, the validity of such inventories has been questioned, growing interest in utilizing full-facility measurements using airborne, satellite or drone (top- down (TD)) techniques to inform, improve, or validate inventories. This work utilized full-facility estimates from aerial measurements to evaluate and improve emissions inventories from 15 midstream natural gas facilities in the U.S.A. and represents one of the first systematic studies of measurement-informed inventory methods in this supply chain sector. At each facility, two independent, full-facility estimates were made and compared with a contemporaneous inventory assembled by the facility operator, employing comprehensive inventory methods. Estimates from the two TD methods statistically agreed in 2 of 28 paired measurements. Operator inventories, which include extensions to capture more sources and integrate local measurements, estimated significantly lower emissions than the TD estimates for 40 of 43 paired comparisons. Significant disagreement at most facilities, both between the two TD methods and between the TD estimates and enhanced inventory, indicates that using TD estimates to inform inventories (a) will require additional diagnostic screening and measurement of all sources to improve inventory estimates, and (b) that TD full-facility measurement methods require additional testing, characterization, and potentially improvement, for use in complex midstream facilities.