Airborne quantification of net methane and carbon dioxide fluxes from European Arctic wetlands in Summer 2019

Arctic wetlands and surrounding ecosystems are both a significant source of methane (CH 4) and a sink of carbon dioxide (CO 2) during summer months. However, precise quantification of this regional CH 4 source and CO 2 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 km 2) of mixed peatland and forest in northern Sweden and Finland. Area-averaged fluxes of CH 4 and carbon dioxide were calculated using an aircraft mass balance approach. Net CH 4 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 CO 2 sinks ranged between -513 ± 74 mg m -2 h -1 and -284 ± 89 mg m -2 h -1 and result from net uptake of CO 2 by vegetation and soils in the biosphere. A clear gradient of decreasing bulk and area-averaged CH 4 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 N 2 O 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) CH 4 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 CH 4 fluxes were found to be significantly higher than the CH 4 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)'.