Uncertainties in the Estimated Methane Emissions in Oil and Gas Production Regions Based on Aircraft Mass Balance Flights

Aircraft-based mass balance flights that estimate emissions in large regions, such as oil and gas production basins encompassing thousands of square kilometers, have been widely used to provide measurement based estimates of total regional emissions. The number and density of sources and the temporal and spatial variability in methane emissions from oil and gas facilities introduce uncertainties into the analyses of mass balance flights. To quantitatively characterize these uncertainties, mass balance flux calculations were conducted for simulated flight scenarios using a site-level emission inventory for >10,000 oil and gas sites and fine spatial scale (∼1 km) modeling of horizontal and vertical transport of emissions. Uncertainties due to varying wind directions, vertical mixing profile assumptions, boundary layer height assumptions, and temporal and spatial variability in emission sources were quantified. Some of the largest uncertainties evaluated in the simulations were due to temporal variations in large emission events. Other uncertainties, specific to the high density of sources in oil and gas production regions, are due to shifting wind directions which blur the boundaries of the domain measured. More precise definition of upwind source locations sampled by mass balance flights would reduce uncertainties in the use of mass balance flight data, and this work demonstrates that upwind domain footprints can be estimated using trajectory tools.