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

Abstract. Atmospheric methane (CH₄) concentrations have more than doubled since the beginning of the industrial age, making CH₄ the second most important anthropogenic greenhouse gas after carbon dioxide (CO₂). The oil and gas sector represent one of the major anthropogenic CH₄ emitters as it is estimated to account for 22 % of global anthropogenic CH₄ emissions. An airborne field campaign was conducted in April–May 2019 to study CH₄ emissions from offshore gas facilities in the Southern North Sea with the aim to derive emission estimates using a top-down (measurement-led) approach. We present CH₄ fluxes for six UK and five Dutch offshore platforms/platform complexes using the well-established mass balance flux method. We identify specific gas production 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 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 with operator-based reporting for individual Dutch facilities. In this study, we find that all 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 sampled Dutch installations together, we find that our estimated flux of (122.7 ± 9.7) kg h^-1 deviates by a factor 0.7 (0.35–12) from reported values (183.1 kg h^-1). Comparisons with aircraft observations in two other offshore regions (Norwegian Sea and 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 two orders of magnitude. Therefore, mitigation is warranted equally across geographies.