Methane is a potent greenhouse gas, with an impact estimated as 80 times that of CO₂. Although efforts are being made to reduce the contribution of big polluters to methane in Japan, new research from Osaka Metropolitan University suggests that smaller sources are vastly underestimated in the Osaka metropolitan area.

The discovery was made by an international collaborative research team led by Associate Professor Masahito Ueyama of the Graduate School of Agriculture who used a tower for high-altitude readings and a bike for ground-level readings of methane and ethane. Instead of spot checks, the measurements were continuous and integrated over the city center, giving a more complete overview of their output.

When the researchers compared their findings with government inventories, they found large differences. As well as the well-known large emitters of greenhouse gases, especially chemical and industrial plants, they found unaccounted emissions from numerous small sources, including restaurants, commercial facilities, and private residences.

Because emissions were higher on weekdays, followed a clear day–night pattern, and included ethane—a gas linked to human activity—the researchers concluded that people, not natural processes, were the main source. Even so, methane produced by biological processes was also underestimated, probably due to small but widespread sources, like sewage manholes and the production of fermented foods common in Japanese cuisine.

Ultimately, the study highlights hidden sources of methane that could be fixed with technology and policy. “By clarifying the existence of methane emissions originating from city gas that had previously been overlooked, our research is expected to aid in identifying these unaccounted emission sources within urban areas,” Professor Ueyama explained.

“This research establishes a method for real-time monitoring of methane emissions by source, which is expected to be utilized in assessments evaluating the effectiveness of emission reduction measures,” he added. He believes that the group’s technique is useful for separating human fossil fuel leakage from biological emissions. “Going forward, it is hoped that this approach will be expanded to other cities and utilized for methane emission management and policy formulation in a wider range of urban areas.”

The findings were published in Environmental Science & Technology.

Competing interests
The authors declare no competing financial interest.

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