Understanding the sources and variability of methane emissions in Madrid with Sentinel-5P TROPOMI, GHGSat, and aircraft observations

Abstract. Routine, passive monitoring using satellite sensors offers an effective means of understanding the location, frequency and intensity of industrial methane emissions. Satellites have been used largely to identify fugitive point source emissions in sectors such as oil and gas, but their efficacy for more diffuse sources such as landfills is less established, and formal integration into official reporting and compliance frameworks has yet to be achieved. A persistent methane hotspot originating from Madrid was identified using TROPOMI, where it remains one of the largest signals in Europe. Two landfills, Las Dehesas and Pinto, were identified as significant contributors based on GHGSat observations.

This study presents a comprehensive analysis of the TROPOMI and GHGSat observational record up to 2024 for Madrid, alongside aircraft observations, all contextualised against previous short-term studies, facility-scale reporting, and gridded inventories. Plume-based city-scale emissions averaged 13 ± 9 t/hr from TROPOMI compared to ~8 t/hr from the gridded inventories. Facility-scale emissions ranged from 2.5 ± 1.3 t/hr (Las Dehesas) and 3.8 ± 2.0 t/hr (Pinto) from GHGSat, and 3.0 ± 1.8 to 4.6 ± 2.7 t/hr respectively from aircraft, compared to reported averages of 0.4 and 1.0 t/hr between 2018–2024. Measurement-derived rates exceeded reported estimates by a factor of 0.5–6, highlighting a severe gap between bottom-up inventory and top-down observational approaches. We discuss challenges associated with satellite interpretation and meteorology, connect emissions to on-site management practices, and make recommendations towards operational use of these datasets.