Montreal's methane (CH4) emissions are unevenly distributed across the island, with the highest concentrations in the city's east end, McGill researchers have found. The worst polluters include the city's largest snow dump, which emits methane at levels comparable to the city's current and former landfills, and natural gas leaks.
The researchers identified more than 3,000 methane hotspots throughout the four-year mobile monitoring survey. They said this is fewer than comparably dense cities, but these potent emissions must be addressed.
"Though there's much less methane than carbon dioxide (CO2) in the atmosphere, every methane molecule will warm the planet by about 32 times as much as every CO2 molecule," said Peter Douglas, Associate Professor in the Department of Earth and Planetary Sciences and study co-author. "We need to know where these emissions are coming from to resolve them."
Waste management, infrastructure and snow removal at issue
Like most cities, Montreal publishes an annual emissions inventory, but the figures are only estimates. This mobile monitoring survey sought to gather robust data to help Quebec reach its goal to reduce GHG emissions by 37.5 per cent by 2030.
Inactive landfills - including urban renewal projects like Parc Frédéric-Back, pictured above - produced the most emissions. But the researchers were surprised to discover that the Francon Quarry, site of Montreal's largest snow dump, emits methane at a comparable rate.
"It's as large as some of the other landfills. A ton of stuff is dumped there, stuff that's picked up off the roads," Douglas said. As the snow melts, it creates a lake that potentially harbours microbes capable of releasing additional methane.
The highest methane concentrations were found in the city's east end, which, in addition to being the site of many former landfills, is a place where natural gas leaks are more common.
"Most gas leaks are concentrated where we use this older infrastructure," said Douglas. "But we really focused on population density. With more people, there are more natural gas lines and more leaks."
Methodology
The researchers conducted mobile surveys over four years (2019, 2022-2024) across 3,300 square kilometres of the city and at key off-island sites like the Saint-Sophie landfill. Air samples were collected using a device that measured carbon dioxide and methane levels every second.
Three fixed routes - two in densely populated areas, and one near the Lachine Canal - were surveyed weekly for 10 weeks. The repeated surveys helped researchers track how methane emissions changed over time, revealing which hotspots were persistent and which were more short-lived.
"We'd see an increase in concentration and then it would come back down, so we could figure out approximately how much gas was coming from that source and locate where it was, especially when we had wind data to help us triangulate the location," Douglas said.
Environment and Climate Change Canada conducted much of the testing by car. Local bikeshare service BIXI also contributed free memberships to help the scientists with their work.
The ongoing project will soon track seasonal changes in methane concentrations and explore the mitigating role of surface bacteria at landfills.
About this study
"Four years of mobile monitoring show that urban waste is the primary source of large methane emissions hotspots in Montreal, Canada," by Regina Gonzalez Moguel, Peter M J Douglas, Jacob Asomaning, Emilie Reid, Djordje Romanic, Felix Vogel, Sebastien Ars, Lawson Gillespie and Yi Huang, was published in Environmental Research Communications on Sept. 4, 2025. It was funded by Environment and Climate Change Canada.
