| 29 Jul 2025
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.
The manuscript presents a methane source characterization in the Tokyo area using mobile measurements of CH4 and C2H6. Although this technique is not novel, this is the first study of its kind in the Tokyo area. It is very interesting to see a comparison with other major cities, where most CH4 sources are of fossil origin, while in Tokyo biogenic sources seem to be more important. The manuscript is generally well written, and the instrument characterization is well detailed. However, I find the emission quantification method not sound, due to the issues related to the height of the emission release, the distance from the source and different wind conditions, which affect gas dispersion. I am not sure that, given the different meteorological and sampling conditions during the sampling campaigns, it is possible to quantify areal emissions by using the equation yielded during the control release experiment. I see this procedure as a general assessment of emissions rather than a tool for comparing the estimated fluxes to reported emissions, because the level of uncertainty associated with such estimates is simply too high. In the manuscript all these biases are described, but it is hard to reach a conclusion whether emission inventories underestimate methane sources or not, and I would give way less weight to the emission quantification part.
Therefore, I suggest publication after addressing this issue and the following minor points:
Line 117: the repeatability of..
Line 128: explain here why you chose the 1 ppm threshold. You explained that later, but I feel that we need more explanation at this stage
130: why do the C2H6 values by MIRA Ultra vary so much?
137: “The mole fractions here are uncorrected as..”. This sentence is not clear, please rephrase.
Figure 165: change the y axis label. Should this be “difference from the nominal value (ppm)..?”. Please clarify
Line 244-249: I would move these sentences to the result section
Figure 4 (b): could you change the colors of red lines according to the source category (e.g. fossil, biogenic and combustion)?
Line 270-272: Move to the result section
Line 307: I would explain the concept of LP density here
Table 2: I am not very convinced about reporting these emission estimates, see my previous comment