Detection of Local-Source Contributions in Ground-Level Atmospheric Methane in Japan

Methane (CH 4 ) is a potent greenhouse gas, and managing its emissions is critical for climate change mitigation. This study analyzes ground-level CH 4 concentrations from a nationwide monitoring network in Japan over 2011–2021, along with an anthropogenic emissions inventory to isolate local-source contributions from background methane levels. We developed a method to separate local CH 4 enhancements attributable to nearby sources from the baseline background concentration. The results show that local CH 4 enhancements have slightly declined over the past decade, suggesting that Japan’s methane mitigation efforts are yielding measurable benefits. The results also show distinct spatial and seasonal patterns. For example, urban monitoring sites tend to exhibit higher local methane contributions in winter, whereas rural sites peak in summer, dominated by agricultural emission during the growing season. Overall, this study provides an integrated view of Japan’s ground-level methane trends and confirms that local source contributions are generally decreasing. These findings underscore the importance of sustained nationwide monitoring and targeted emission control measures to further reduce methane and support Japan’s climate goals. • A new detection method isolates local methane sources from global background levels. • Local methane contributions in Japan fell from 2011–2021 as global baseline rose. • Trends in local methane contributions align with EDGAR emissions data for Japan. • Rising global methane levels can obscure the effects of national emission reductions.