Measurement report: Mobile measurements to estimate urban methane emissions in Tokyo

Abstract. To investigate distributions and magnitudes of methane (CH4) emissions in Tokyo, the world's largest megacity, a vehicle-based mobile measurement was set up and 3-week measurement campaign was conducted in September to October 2023. As part of the campaign, we conducted a control release experiment to link downwind excess CH4 values to CH4 emission rate at the source. The empirical equation derived from the experiment was significantly different from those reported by previous studies, suggesting the limitation of such enhancement-to-emission rate conversion, which is a source of large uncertainty in estimating urban CH4 emissions based on street-level measurements. The uncertainty stems from different experiment settings and underlying assumptions (e.g., source distance and height) which do not always represent actual urban measurement environments. The mobile measurement campaign covered a large extent of the Greater Tokyo Area with total driving distance of over 2000 km. Locations of CH4 enhancement were identified and C2H6-to-CH4 enhancement ratios were determined for individual locations to categorize them into biogenic, fossil fuel and combustion CH4 sources. Among a total of 565 locations inferred as CH4 sources, 53 % and 42 % were considered as biogenic and fossil fuel origins, respectively, with the rest being minor contributions from combustion. Based on the statistics of measured CH4 excesses, CH4 emissions were estimated for the specific areas where relatively high measurement coverage was achieved. In the areas with biogenic facilities (landfill and wastewater treatment plants), our emission estimates are well correlated with local government reporting, indicating actual key contributions of the waste-sector facilities. On the other hand, in the residential areas, CH4 emissions were predominantly of fossil-fuel origin, with a magnitude comparable to the area with waste facilities. However, such fossil-fuel emissions are not accounted for in local government reporting. This result highlights the need for improved accounting of urban fossil fuel-related emissions.