Methane Source Attribution in the UK Using Multi-Year Records of CH₄ and ¹³C

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 delta C-13-CH4 isotopic record in the air. Long-term records of both methane mole fraction and delta C-13 from five sites across the UK are presented, showing post-2007 growth in CH4 and negative trend in delta C-13, consistent with global background sites. Miller-Tans analyses of atmospheric measurements identified that the delta C-13 signature of the methane source mix varied between -50.1 and -56.1 parts per thousand, with less depleted delta C-13 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.