A stratified air mass enriched in methane (CH4) was sampled at ~600 m to ~2000 m altitude, between the north coast of Norway and Svalbard as part of the MAMM (Methane in the Arctic: Measurements and Modelling) campaign on board the UK’s BAe-146-301 Atmospheric Research Aircraft (ARA). The approach used here, which combines interpretation of multiple tracers with transport modeling, enables better understanding of the emission sources that contribute to the background mixing ratios of CH4 in the Arctic. Importantly, it allows constraints to be placed on the location and isotopic bulk signature of the emission source(s). Measurements of δ13C in CH4 in whole air samples taken whilst traversing the air mass identified that the source(s) had a strongly depleted combined δ13C CH4 isotopic signature of −70 (±2.1) ‰. Combined NAME (Numerical Atmospheric-dispersion Modelling Environment) and inventory analysis indicates that the air mass was recently in the planetary boundary layer over northwest Russia and the Barents Sea, with the dominant source of methane being from wetlands in that region.