Evaluating urban methane emissions and their attributes in a megacity, Osaka, Japan, via mobile and eddy covariance measurements
Abstract. Urban areas are regions where we expect large amounts of greenhouse gas emissions, but in many urban areas, the sources and sinks have yet to be characterized with certainty. 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 of the 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 result in smaller natural gas emission estimates than bicycle measurements did. The estimated CH4 emissions were 10,021 ± 1,000 tCH4 yr-1 for Osaka city and 2,379 ± 480 tCH4 yr-1 for Sakai city, 18 times and 2.5 times greater, respectively, than those expected in the inventories. Coincident C2H6 observations indicated that natural gas emissions contributed 64 % of the total CH4 emissions in Osaka city and 47 % in Sakai city. The upscaled CH4 emissions were calibrated with the daytime CH4 fluxes via the eddy covariance method, as the empirical models significantly underestimated the regional fluxes. 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.