Evaluating urban methane emissions and their attributes in a megacity, Osaka, Japan, via mobile and eddy covariance measurements

Abstract. Urban areas are regions where large greenhouse gas emissions are expected, but in many urban areas, the sources and sinks remain uncertain. In this study, we conducted mobile and eddy covariance measurements to evaluate CH4 emissions in the megacity Osaka, Japan. Based on the mobile measurements, several elevated CH4 concentrations were observed. Most locations were not related to CH4 sources identified by emission inventories reported by local governments. Two platforms for mobile measurements, vehicle and bicycle, showed good consistency for estimating total CH4 emissions, but vehicle measurements tended to yield smaller natural gas emission estimates than bicycle measurements. CH4 emissions from bicycle measurements were adjusted using daytime CH4 fluxes via the eddy covariance method for flux footprint areas and used to upscale to Sakai and Osaka cities. Using these upscaled emissions from vehicle measurements as a proxy for total area fluxes the estimated CH4 emissions were 10 021 ± 1000 t CH4 yr−1 for Osaka and 2379 ± 480 t CH4 yr−1 for Sakai, 18 times and 2.5 times greater, respectively, than inventory estimates. Coincident C2H6 observations indicated that natural gas emissions contributed 64 % of the total CH4 emissions in Osaka city and 47 % in Sakai city. From these snapshots, the CH4 emissions from the metropolitan areas in Japan may be considerably greater than the emission inventories, and most CH4 sources are not well characterized in those inventories. These unaccounted sources need to be better characterized to improve the Japanese CH4 inventory and assess whether these emissions can be mitigated.