Continuous CH<sub>4</sub> and <i>δ</i><sup>13</sup>CH<sub>4</sub> measurements in London demonstrate under-reported natural gas leakage

Abstract. Top-down greenhouse gas measurements can be used to independently assess the accuracy of bottom-up emission estimates. We report atmospheric methane (CH4) mole fractions and δ13CH4 from Imperial College London early 2018 onwards using a Picarro G2201-i analyser. Measurements March October 2020 were compared simulations CH4 produced NAME (Numerical Atmospheric-dispersion Modelling Environment) dispersion model coupled with UK National Atmospheric Emissions Inventory, NAEI, global inventory, Database for Global Research (EDGAR), spatial resolutions ∼ 2, 10, 25 km. Simulation–measurement comparisons are evaluate emissions source apportionment in (EDGAR) national (NAEI) inventories. Observed underestimated by 30 %–35 % NAEI simulations. In contrast, good correspondence between observations EDGAR was seen. There no correlation measured simulated values either or EDGAR, however, suggesting inventories' sectoral attributions incorrect. On average, natural sources accounted 20 %–28 above background only 6 %–9 nearly 84 isotopic calculated Keeling plot analysis (using measurement data afternoon) individual pollution events higher than −45 ‰, primary actually leaks. The simulation–observation comparison suggests that total much estimate (0.04 Tg yr−1) but close to, slightly lower than, (0.10 yr−1). However, results indicate due leaks being both EDGAR.