Arctic wetlands and surrounding ecosystems are both a significant source of methane (CH₄) and a sink of carbon dioxide (CO₂) during summer months. However, precise quantification of this regional CH₄ source and CO₂ sink remains poorly characterized. A research flight using the UK Facility for Airborne Atmospheric Measurement was conducted in July 2019 over an area (approx. 78 000 km²) of mixed peatland and forest in northern Sweden and Finland. Area-averaged fluxes of CH₄ and carbon dioxide were calculated using an aircraft mass balance approach. Net CH₄ fluxes normalized to wetland area ranged between 5.93 ± 1.87 mg m^-2 h^-1 and 4.44 ± 0.64 mg m^-2 h^-1 (largest to smallest) over the region with a meridional gradient across three discrete areas enclosed by the flight survey. From largest to smallest, net CO₂ sinks ranged between -513 ± 74 mg m^-2 h^-1 and -284 ± 89 mg m^-2 h^-1 and result from net uptake of CO₂ by vegetation and soils in the biosphere. A clear gradient of decreasing bulk and area-averaged CH₄ flux was identified from north to south across the study region, correlated with decreasing peat bog land area from north to south identified from CORINE land cover classifications. While N₂O mole fraction was measured, no discernible gradient was measured over the flight track, but a minimum flux threshold using this mass balance method was calculated. Bulk (total area) CH₄ fluxes determined via mass balance were compared with area-weighted upscaled chamber fluxes from the same study area and were found to agree well within measurement uncertainty. The mass balance CH₄ fluxes were found to be significantly higher than the CH₄ fluxes reported by many land-surface process models compiled as part of the Global Carbon Project. There was high variability in both flux distribution and magnitude between the individual models. This further supports previous studies that suggest that land-surface models are currently ill-equipped to accurately capture carbon fluxes inthe region. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.

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