Natural Gas and Biogenic CH ₄ Emissions from an Urban Center, Sakai, Japan, Based on Simultaneous Measurements of CH ₄ and C ₂ H ₆ fluxes Based on the Eddy Covariance Method

Urban areas are increasingly recognized as significant sources of methane (CH₄) emissions, with recent studies highlighting unexpectedly high emissions from fossil fuel-related sources. Here, we conducted quasi-continuous, one-year eddy covariance measurements of CH₄ and ethane (C₂H₆) fluxes in Sakai, a major metropolitan area in Japan. By leveraging the coemission of CH₄ and C₂H₆, we separated and quantified contributions from natural gas leakage and biogenic sources. Results show that 65-78% of the total annual CH₄ flux (10.7 g CH₄ m^-2 yr^-1) originated from natural gas, with emissions elevated during daytime and weekdays, indicating strong links to anthropogenic activity. Natural gas-related CH₄ fluxes remained high year-round and were further amplified in summer and winter, likely reflecting seasonal gas demand. Footprint analysis identified hotspots near steel and chemical industries, suggesting important point sources. Although biogenic CH₄ contributed less overall, fluxes increased in warmer months. Mobile surveys revealed multiple localized sources within the flux footprint, enabling identification of specific facilities. These findings indicate that much of the natural gas-related CH₄ emissions are not captured in local inventories, underscoring the need for improved quantification and mitigation. Eddy covariance and mobile monitoring are effective tools to support data-driven urban methane policies.