Aircraft-based mass balance estimate of methane emissions from offshore gas facilities in the southern North Sea

Atmospheric methane (CH 4 ) concentrations have more than doubled since the beginning of the industrial age, making CH 4 the second most important anthropogenic greenhouse gas after carbon dioxide (CO 2 ). The oil and gas sector represents one of the major anthropogenic CH 4 emitters as it is estimated to account for 22 % of global anthropogenic CH 4 emissions. An airborne field campaign was conducted in April-May 2019 to study CH 4 emissions from offshore gas facilities in the southern North Sea with the aim of deriving emission estimates using a top-down (measurement-led) approach. We present CH 4 fluxes for six UK and five Dutch offshore platforms or platform complexes using the well-established mass balance flux method. We identify specific gas production emissions and emission processes (venting and fugitive or flaring and combustion) using observations of co-emitted ethane (C 2 H 6 ) and CO 2 . We compare our top-down estimated fluxes with a ship-based top-down study in the Dutch sector and with bottom-up estimates from a globally gridded annual inventory, UK national annual point-source inventories, and operator-based reporting for individual Dutch facilities. In this study, we find that all the inventories, except for the operator-based facility-level reporting, underestimate measured emissions, with the largest discrepancy observed with the globally gridded inventory. Individual facility reporting, as available for Dutch sites for the specific survey date, shows better agreement with our measurement-based estimates. For all the sampled Dutch installations together, we find that our estimated flux of (122.9 +/- 36.8) kg h - 1 deviates by a factor of 0.64 (0.33-12) from reported values (192.8 kg h - 1 ). Comparisons with aircraft observations in two other offshore regions (the Norwegian Sea and the Gulf of Mexico) show that measured, absolute facility-level emission rates agree with the general distribution found in other offshore basins despite different production types (oil, gas) and gas production rates, which vary by 2 orders of magnitude. Therefore, mitigation is warranted equally across geographies.