Aircraft-based Mass Balance Estimate of Methane Emissions from Offshore Gas Facilities in the Southern North Sea

Atmospheric methane (CH4) concentrations have more than doubled since the beginning of the industrial era, making methane the second most important anthropogenic greenhouse gas after carbon dioxide (CO2). Fossil fuel extraction is one of the major anthropogenic methane sources as it is estimated to account for 22 % of global anthropogenic CH4 emissions. However, studies indicate that inventories tend to underestimate emissions arising from the oil and gas industry. During two airborne field campaigns carried out in April 2018 and April-May 2019, atmospheric measurements near offshore oil and gas facilities in the Southern North Sea were conducted to derive CH4 emission estimates using a top-down approach. A total of nine research flights was conducted to characterize individual platform emissions. We present estimated CH4 fluxes for six UK and five Dutch offshore platforms/platform complexes using the well-established mass balance method. We identify fossil fuel CH4 emissions and emission processes (venting/fugitive or flaring/combustion) using observations of co-emitted ethane (C2H6) and CO2. We compare our top-down estimated fluxes with bottom-up estimates from UK national annual point-source inventories, a globally gridded annual inventory, and with operator-based reporting for individual Dutch facilities for the specific survey date. Annual inventories underestimate emissions from both UK and Dutch sites. Individual facility reporting shows the most accurate agreement with the measurement-based estimates.