Satellite-based global monitoring of urban-scale methane emissions using TROPOMI

Quantifying and understanding methane emissions of cities are of great importance since cities are a key focus of current and future mitigation efforts to combat climate change. However, it remains challenging to routinely characterize and verify city scale emission inventories.  Previous studies of urban methane emissions have employed a range of flux quantification methods, leading to inconsistent estimates and different associated uncertainties. As a result, a robust and widely applicable framework for quantifying urban emissions remains lacking. In this study, we have developed and tested an advanced emissions calculation method that uses mass balance accounting and satellite observations from TROPOMI to estimate net bulk (city-level) methane emissions and corresponding emissions uncertainties due to the method. We test and validate the method and demonstrate that the novel integration of boundary layer height and hourly-resolved wind data enables a more robust assessment of methane emissions from urban areas compared with methods that do not take such factors into account. Initial assessments with this approach were tested for three megacities (London, Los Angeles and New York) from 2021 to 2023. Our results suggest that emission inventories generally underestimate methane emissions, but by widely varying proportions, and with substantial differences year-to-year. The estimates range from 0.3 to 9.2 times higher than spatially gridded bottom-up inventories. For example, in New York city in 2021, the estimated CH₄ emission rate is 43.54 ± 17.77 t h^-1, compared with the reported inventory value of 7.18 t h^-1 from the U.S. Environmental Protection Agency. In London, the emission estimate is only slightly higher than the NAEI inventory. We also find generally lower but overall consistent emissions when compared with previous top-down studies that use different quantification methods. Our results provide evidence that satellites can serve as a promising technology for ongoing city emissions monitoring, reconciliation and reporting through long-term monitoring across the globe, which can be used to help build methane emission characteristics and track whether stated emission targets are being met.