Four Years of Mobile Monitoring Show that Urban Waste is the Primary Source of Large Methane Emissions Hotspots in Montreal, Canada

Abstract Urban centers contribute significantly to anthropogenic methane (CH4) emissions, making them key targets for mitigation. This study aimed to map the spatial distribution of CH4 hotspots in Montreal, Canada, identify potential sources, and quantify emissions from key sectors. In over four years, we surveyed over 3,300 km with our mobile monitoring system and detected 3,045 CH4 hotspots, defined as mole fractions exceeding a baseline. Most hotspots were smaller than 1 ppm (85%), while larger hotspots (>1 ppm) were linked to landfills. Three routes were surveyed 10 times each, and within this subset of hotspots, most (89%) were observed only once. Among all detected hotspots, 487 were classified as leak indications, defined as hotspots with narrow widths (<160 m) and distant from known CH4 sources. Leak indications occurred more frequently in densely populated neighborhoods (R² = 0.48, p = 5.22×10⁻⁶), with an estimated emission rate of 250–507 kg day⁻¹. Emissions from four major landfills, calculated through a Gaussian plume inversion, were estimated at 10,064–36,410 kg CH₄ day⁻¹, with historical landfills alone contributing 6,641–18,467 kg CH₄ day⁻¹. These findings confirm the dominant role of landfills to Montreal CH4 emissions and highlight the importance of targeting waste management sites for urban methane mitigation.