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 that such conversion is not straightforward and is a source of large uncertainty in estimating urban CH₄ emissions based on street-level measurements. The mobile measurement campaign covered 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 total 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. The results were consistent in emission tendency with the local government reporting for areas with the waste-sector facilities, but not for residential areas, suggesting need of improved accounting of fossil fuel-related emissions.