| Summary: | Cities have multiple fugitive emission sources of methane (CH<sub>4</sub>) and policies adopted by China on replacing coal with natural gas in recent years can cause fine spatial heterogeneities at the range of kilometers within a city and also contribute to the CH<sub>4</sub> inventory. In this study, a mobile observatory was used to monitor the real-time CH<sub>4</sub> concentrations at fine spatial and temporal resolutions in Beijing, the most important pilot city of energy transition. Results showed that: several point sources, such as a liquefied natural gas (LNG) power plant which has not been included in the Chinese national greenhouse gas inventory yet, can be identified; the ratio “fingerprints” (CH<sub>4</sub>:CO<sub>2</sub>) for an LNG carrier, LNG filling station, and LNG power plant show a shape of “L”; for city observations, the distribution of CH<sub>4</sub> concentration, in the range of 1940−2370 ppbv, had small variations while that in the rural area had a much higher concentration gradient; significant correlations between CO<sub>2</sub> and CH<sub>4</sub> concentrations were found in the rural area but in the urban area there were no such significant correlations; a shape of “L” of CH<sub>4</sub>:CO<sub>2</sub> ratios is obtained in the urban area in wintertime and it is assigned to fugitive emissions from LNG sources. This mobile measurement methodology is capable of monitoring point and non-point CH<sub>4</sub> sources in Beijing and the observation results could improve the CH<sub>4</sub> inventory and inform relevant policy-making on emission reduction in China.
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