Continuous CH4 and delta(CH4)-C-13 measurements in London demonstrate under-reported natural gas leakage

Top-down greenhouse gas measurements can be used to independently assess the accuracy of bottomup emission estimates. We report atmospheric methane (CH4) mole fractions and delta(CH4)-C-13 measurements from Imperial College London from early 2018 onwards using a Picarro G2201-i analyser. Measurements from March 2018 to October 2020 were compared to simulations of CH4 mole fractions and ffi delta(CH4)-C-13 produced using the NAME (Numerical Atmospheric-dispersion Modelling Environment) dispersion model coupled with the UK National Atmospheric Emissions Inventory, UK NAEI, and a global inventory, the Emissions Database for Global Atmospheric Research (EDGAR), with model spatial resolutions of similar to 2, similar to 10, and similar to 25 km. Simulation-measurement comparisons are used to evaluate London emissions and the source apportionment in the global (EDGAR) and UK national (NAEI) emission inventories. Observed mole fractions were underestimated by 30 %-35% in the NAEI simulations. In contrast, a good correspondence between observations and EDGAR simulations was seen. There was no correlation between the measured and simulated delta(CH4)-C-13 values for either NAEI or EDGAR, however, suggesting the inventories' sectoral attributions are incorrect. On average, natural gas sources accounted for 20%-28% of the above background CH4 in the NAEI simulations and only 6%-9% in the EDGAR simulations. In contrast, nearly 84% of isotopic source values calculated by Keeling plot analysis (using measurement data from the afternoon) of individual pollution events were higher than -45 parts per thousand, suggesting the primary CH4 sources in London are actually natural gas leaks. The simulation-observation comparison of CH4 mole fractions suggests that total emissions in London are much higher than the NAEI estimate (0.04 TgCH(4) yr(-1)) but close to, or slightly lower than, the EDGAR estimate (0.10 TgCH(4) yr(-1)). However, the simulation-observation comparison of delta(CH4)-C-13 and the Keeling plot results indicate that emissions due to natural gas leaks in London are being underestimated in both the UK NAEI and EDGAR.