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

Abstract

We have conducted simultaneous measurements of the mole fraction and carbon and hydrogen isotope ratios (δ13C and δD) of atmospheric methane (CH4) 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), CH4 mole fraction is generally higher and δ13C and δD are lower at Churchill due to regional biogenic CH4 emissions. Clear seasonal cycles in the CH4 mole fraction, δ13C, 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 CH4 emissions from boreal wetlands on the two sites. Short‐term CH4 variations are also observed at Churchill throughout the year. The analysis of the observed isotopic signatures of atmospheric CH4 confirmed that the short‐term CH4 variations are mainly produced by biogenic CH4 released from the HBL wetlands in summer and by fossil fuel CH4 transported over the Arctic in winter. Forward simulations of an atmospheric chemistry‐transport model, with wetland CH4 fluxes prescribed by a process‐based model, show unrealistically high CH4 mole fractions at Churchill in summer, suggesting that CH4 emissions assigned to the HBL wetlands are overestimated. Our best estimate of the HBL CH4 emissions is 2.7 ± 0.3 Tg CH4 yr−1 as an average of 2007–2013, consistent with recent estimations by inverse modeling studies.

Emissions from the Oil and Gas Sectors, Coal Mining and Ruminant Farming Drive Methane Growth over the Past Three Decades

“…Only a part (region: 36 -42.8°N, 136 -141.7°E) of the whole data from the commercial aircraft observation are used in the analysis. The aircraft data are available on the Tohoku University CH 4 scale (TU1987 scale), which is converted into the World Meteorological Organization scale by subtracting 0.5 ppb (Fujita et al 2018;Aoki et al 1992). It is emphasized that the observation location is downstream of continental East Asia in terms of atmospheric transport, thereby the dataset being advantageous in addressing emission changes in the region.…”

Advancing Scientific Understanding of the Global Methane Budget in Support of the Paris Agreement

“…For δD‐CH 4 , detecting trends is even more challenged by current analytical precisions and the technical capability to make measurements. Reported trends in δD‐CH 4 have not been statistically significant (Fujita et al, ), with +0.2 ± 1.4‰/yr reported between 2000 and 2006 (Rice et al, ). Typical analytical precisions for δD‐CH 4 by IRMS are >1‰.…”

“…Isotopic measurements are more straightforward to interpret on regional scales rather than global scales as pollution episodes have a direct and linear impact on the observed isotopic composition, which can be related directly to the changes in mole fraction Fujita et al, 2018;Röckmann et al, 2016;Warwick et al, 2016). However, inadequate spatial and temporal coverage of measurements has meant that measurements have so far only been used to rule in or out a particular source type contribution rather to than make flux estimates Umezawa et al, 2012;Warwick et al, 2016).…”

“…δD‐CH 4 could therefore be of particular use in quantifying changes in OH concentration ([OH]), one of the most significant challenges in interpreting atmospheric CH 4 (section ). Only few studies have used measurements of δD‐CH 4 (Fujita et al, ; Rice et al, ; Tyler et al, ; Umezawa et al, ; Warwick et al, ) owing to their sparsity (measurements of δD‐CH 4 in the NOAA network ceased some years ago, and currently, there are very few continuing long‐term time series), and information on δD‐CH 4 source signatures is comparatively even more limited than for δ 13 C‐CH 4 . Collecting representative measurements on δD‐CH 4 source signatures will become important if this valuable and important constraint is to be used in model studies.…”

“…Isotopic measurements are more straightforward to interpret on regional scales rather than global scales as pollution episodes have a direct and linear impact on the observed isotopic composition, which can be related directly to the changes in mole fraction (Fisher et al, ; Fujita et al, ; Röckmann et al, ; Warwick et al, ). However, inadequate spatial and temporal coverage of measurements has meant that measurements have so far only been used to rule in or out a particular source type contribution rather to than make flux estimates (Fujita et al, ; Umezawa et al, ; Warwick et al, ). Deploying field instruments to measure near‐continuous and high‐precision δ 13 C‐CH 4 and δD‐CH 4 alongside mole fraction measurements will drive development in regional flux estimation over the coming years (Rigby et al, ; Röckmann et al, ).…”