Methane Source Attribution in the UK Using Multi‐Year Records of CH4 and δ13C

Isotopic measurements of atmospheric methane are valuable for the verification of bottom-up atmospheric emissions inventories. The balance of sources in emissions inventories must be consistent with the δ13C-CH4 isotopic record in the air. Long-term records of both methane mole fraction and δ13C from five sites across the UK are presented, showing post-2007 growth in CH4 and negative trend in δ13C, consistent with global background sites. Miller-Tans analyses of atmospheric measurements identified that the δ13C signature of the methane source mix varied between −50.1 and −56.1‰, with less depleted δ13C signatures at sites receiving air from urban areas, consistent with an increased proportion of thermogenic sources. Isotopic signatures calculated for all sites are more enriched than those expected from the bottom-up emissions inventory, suggesting that inventories for the UK either underestimate contributions of thermogenic/pyrogenic emissions or overestimate biogenic sources

Measurement of the 13 C isotopic signature of methane emissions from Northern European wetlands

Isotopic data provide powerful constraints on regional and global methane emissions and their source profiles. However, inverse modeling of spatially-resolved methane flux is currently constrained by a lack of information on the variability of source isotopic signatures. In this study, isotopic signatures of emissions in the Fennoscandian Arctic have been determined in chambers over wetland, in the air 0.3 to 3 m above the wetland surface and by aircraft sampling from 100 m above wetlands up to the stratosphere. Overall the methane flux to atmosphere has a coherent δ13C isotopic signature of -71 ± 1‰, measured in situ on the ground in wetlands. This is in close agreement with δ13C isotopic signatures of local and regional methane increments measured by aircraft campaigns flying through air masses containing elevated methane mole fractions. In contrast results from wetlands in Canadian boreal forest further south gave isotopic signatures of -67 ± 1 ‰. Wetland emissions dominate the local methane source measured over the European Arctic in summer. Chamber measurements demonstrate a highly variably methane flux and isotopic signature, but the results from air sampling within wetland areas show that emissions mix rapidly immediately above the wetland surface and methane emissions reaching the wider atmosphere do indeed have strongly coherent C isotope signatures. The study suggests that for boreal wetlands (>60°N) global and regional modeling can use an isotopic signature of -71‰ to apportion sources more accurately, but there is much need for further measurements over other wetlands regions to verify this.UK Natural Environment Research Council (NERC). Grant Numbers: NE/I028874/1, NE/I014683/1, NE/F020937/1 European Community's Seventh Framework Programme. Grant Number: FP7/2007‐2013 InGOS. Grant Number: 28427