Temporal Variations of the Mole Fraction, Carbon, and Hydrogen Isotope Ratios of Atmospheric Methane in the Hudson Bay Lowlands, Canada

We have conducted simultaneous measurements of the mole fraction and carbon and hydrogen isotope ratios (δ 13 C and δD) of atmospheric methane (CH 4 ) at Churchill (58°44′N, 93°49′W) in the northern part of the Hudson Bay Lowlands (HBL), Canada, since 2007. Compared with the measurements at an Arctic baseline monitoring station, Ny‐Ålesund, Svalbard (78°55′N, 11°56′E), CH 4 mole fraction is generally higher and δ 13 C and δD are lower at Churchill due to regional biogenic CH 4 emissions. Clear seasonal cycles in the CH 4 mole fraction, δ 13 C, and δD are observable at Churchill, and their seasonal phases in summer are earlier by approximately 2 weeks than those at Ny‐Ålesund. Using the one‐box model analysis, the phase difference is ascribed to the different seasonal influence of CH 4 emissions from boreal wetlands on the two sites. Short‐term CH 4 variations are also observed at Churchill throughout the year. The analysis of the observed isotopic signatures of atmospheric CH 4 confirmed that the short‐term CH 4 variations are mainly produced by biogenic CH 4 released from the HBL wetlands in summer and by fossil fuel CH 4 transported over the Arctic in winter. Forward simulations of an atmospheric chemistry‐transport model, with wetland CH 4 fluxes prescribed by a process‐based model, show unrealistically high CH 4 mole fractions at Churchill in summer, suggesting that CH 4 emissions assigned to the HBL wetlands are overestimated. Our best estimate of the HBL CH 4 emissions is 2.7 ± 0.3 Tg CH 4  yr −1 as an average of 2007–2013, consistent with recent estimations by inverse modeling studies.