Methane’s missing emissions: The underestimated impact of small sources

Assessing methane emissions from various sources in urban areas
Equipment installed on a high-altitude tower and collecting information from the ground level using bikes accurately captured methane and ethane emissions in Osaka city.   
Image Credit: Osaka Metropolitan University

Scientific Frontline: Extended "At a Glance" Summary: Unaccounted Urban Methane Emissions

The Core Concept: Real-world, continuous environmental monitoring indicates that numerous small, localized sources—such as commercial facilities, private residences, and sewage infrastructure—contribute significantly more to urban methane emissions than is currently accounted for in official government inventories.

Key Distinction/Mechanism: Unlike traditional environmental monitoring that relies on sporadic spot checks and broad estimations, this approach utilizes continuous, integrated flux measurements. By tracking both methane and ethane simultaneously from high-altitude towers and ground-level mobile units, researchers can actively isolate human-driven fossil fuel leaks (often accompanied by ethane) from biogenic methane production.

Major Frameworks/Components:

• Eddy Covariance Method: A micrometeorological technique used to continuously measure and calculate vertical turbulent fluxes within atmospheric boundary layers.
• Multi-Elevation Monitoring: The deployment of atmospheric sensors on both high-altitude towers and ground-level tracking vehicles (bicycles) to capture a comprehensive, three-dimensional emission profile.
• Ethane Tracing: The simultaneous measurement of ethane (\(C_2H_6\)) alongside methane to accurately differentiate anthropogenic natural gas leaks from natural or biogenic biological processes.
• Temporal Pattern Analysis: The evaluation of weekday versus weekend outputs and diurnal (day-night) cycles to attribute emissions directly to human urban activity.

Branch of Science: Environmental Science, Atmospheric Chemistry, and Urban Climatology.

Future Application: The real-time monitoring methodology is designed to be scaled across global metropolitan areas. It will enable municipalities to pinpoint precise infrastructural leaks, optimize city gas distribution networks, and assess the direct effectiveness of localized greenhouse gas reduction measures.

Why It Matters: Methane is an exceptionally potent greenhouse gas, trapping roughly 80 times more heat than carbon dioxide over a 20-year period. Identifying and mitigating these overlooked "micro-emitters" is critical for formulating accurate climate models and executing effective, policy-driven emission reduction strategies in urban environments.

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.”

Resource material: What Is: Greenhouse Gas

Funding: Environmental Defense Fund, JSPS KAKENHI, grant number 24K03065

Published in journal: Environmental Science & Technology

Title: Natural Gas and Biogenic CH4 Emissions from an Urban Center, Sakai, Japan, Based on Simultaneous Measurements of CH4 and C2H6 fluxes Based on the Eddy Covariance Method

Authors: Masahito Ueyama, Akira Nakaoka, Taku Umezawa, Yukio Terao, and Mark Lunt

Source/Credit: Osaka Metropolitan University

Reference Number: env022426_01

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