Find in Library

<p>Top-down greenhouse gas measurements can be used to independently assess the accuracy of bottom-up emission estimates. We report atmospheric methane (CH<span class="inline-formula">₄</span>) mole fractions and <span class="inline-formula"><i>δ</i>¹³</span>CH<span class="inline-formula">₄</span> 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 CH<span class="inline-formula">₄</span> mole fractions and <span class="inline-formula"><i>δ</i>¹³</span>CH<span class="inline-formula">₄</span> 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 <span class="inline-formula">∼</span> 2, <span class="inline-formula">∼</span> 10, and <span class="inline-formula">∼</span> 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 <span class="inline-formula"><i>δ</i>¹³</span>CH<span class="inline-formula">₄</span> 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 CH<span class="inline-formula">₄</span> 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 <span class="inline-formula">−</span>45 ‰, suggesting the primary CH<span class="inline-formula">₄</span> sources in London are actually natural gas leaks. The simulation–observation comparison of CH<span class="inline-formula">₄</span> mole fractions suggests that total emissions in London are much higher than the NAEI estimate (0.04 Tg CH<span class="inline-formula">₄</span> yr<span class="inline-formula">⁻¹</span>) but close to, or slightly lower than, the EDGAR estimate (0.10 Tg CH<span class="inline-formula">₄</span> yr<span class="inline-formula">⁻¹</span>). However, the simulation–observation comparison of <span class="inline-formula"><i>δ</i>¹³</span>CH<span class="inline-formula">₄</span> 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.</p>