Detection and quantification of large fugitive methane emissions in the Madrid region using ambient concentration measurements and dispersion modeling

Methane (CH 4 ) is a potent greenhouse gas and a key target for near-term climate mitigation. Accurate quantification of urban methane sources is difficult due to strong spatiotemporal variability and limitations in inventories and satellite-based retrievals. This study applies a combination of continuous ground-based CH 4 monitoring and atmospheric dispersion modeling with satellite observations to identify and attribute significant methane plumes in the Madrid air basin. These plumes were observed crossing the metropolitan area and linked to major landfills of Madrid region. Continuous CH 4 measurements were conducted in 2024 at the CIEMAT-GHG station (Madrid) using a Picarro G2301 analyzer. Three immission events, under different meteorological conditions, were analyzed using HYSPLIT simulations with ERA5 reanalysis data as input. Satellite retrievals of Sentinel-5P TROPOMI and SRON plume detections were used for cross-validation. Immission peaks were attributed to emissions from the Colmenar Viejo, Pinto, and Valdemingómez landfills, with event-specific hourly emission rates up to 3-6 times higher than the hourly mean values implied by official annual inventories. The results mark the temporal variability of landfill emissions and reveal that inventories can significantly underestimate emission events. Our work demonstrates the strength of integrating in-situ methane observations with dispersion models for source attribution, even with limitations of satellite detection. The integrated top-down framework facilitates emissions assessments and underscores the need for continuous high-resolution monitoring to inform methane mitigation. • Continuous CH 4 data measurement reveal strong peaks from Madrid landfills • TROPOMI and SRON satellite data help identify large methane plumes • HYSPLIT modeling allows precise landfill source attribution in real time • In key events, methane emissions estimates is 6 times higher than hourly inventories