## 4 Results
### High-Frequency Monitoring of Methane Point Sources
#### 4.2.1 Sanyuan Nan Yao Coal Mine in Shanxi  
The point source in Changzhi, Shanxi, is associated with a coal mining facility. We began observing emissions from this source in January 2020, while TROPOMI has detected plumes dating back to November 2017 [42]. The GF5-02-AHSI first identified emissions from this source on 18 October 2022. The Sanyuan Nan Yao Coal Mine point source in Shanxi exhibited intermittent activity during Sentinel-2 observations from 1 January 2020 to 30 August 2024. Out of 271 satellite overflights, 52 plumes were detected: 91 were obscured by cloud cover, 126 showed no detectable plumes, and there were 2 instances of missing data. Consequently, the persistence rate for the Sanyuan Nan Yao Coal Mine point source, excluding cloud-covered observations, was 19%. Over the four-year measurement period, Sentinel-2 monitored the Sanyuan Nan Yao Coal Mine point source approximately every five days on average. For each satellite overflight with a detectable plume, methane was retrieved within a 1 \(\times\) 1 kmarea centered on the point source. Figure 7 illustrates some of the methane plumes detected at the Sanyuan Nan Yao Coal Mine point source. The time series of source rates for the Sanyuan Nan Yao Coal Mine in Shanxi, as shown in Figure 8, were derived from the MBMP method. Our detection of the Sanyuan Nan Yao Coal Mine's emissions revealed that its emission intensity was more than double that of the EOG Shale Gas Well in Eddy County, with an average emission rate of 14.5 \(\pm\) 7.2 t/h (ranging from 7.1 to 38.9 t/h), and it fluctuated throughout the 4-year observation period. [PERSON] devised a novel interpolation algorithm based on high-resolution satellite observations (including Gaofen5-01A/02, Ziyuan-1 02D, PRISMA, GHGSat-C1 to C5, EnMAP, and EMIT) estimating the annual average coal mine methane emissions in Shanxi Province from 2019 to 2023 to be 8.9 \(\pm\) 0.5 t/h [56]. From January 2022 to mid-February 2022, plumes were detected in 66% of cloud-free observations, after which emissions seemed to cease for three months due to the mine's suspension for illegal overproduction. Emissions resumed in mid-May 2022, but at a lower persistence rate, with plumes detected in only 20% of cloud-free observations. In June 2023, we observed a temporary halt in emissions for nearly a month, likely due to flooding, geological disasters, and other calamities in Shanxi that led to the shutdown of 60 coal mines. The extended shutdown in 2022 reflected the facility operator's intervention to control methane emissions during coal mining. The reduced frequency of plume detection in June 2023 illustrated how satellite observations can aid in methane emission reduction, while the resumption in May 2022 highlighted the importance of continuous monitoring. Using the 19% persistence rate from the entire record and assuming no daily variations, we estimated the total methane emissions from the mine from 1 January 2020 to 1 October 2024 to be 11.2 kilotons, or approximately 2.4 kilotons per year. This represents 1.1% of the 238.5 thousand tons (Tg) of national methane emissions reported by the Chinese government to the United Nations Framework Convention on Climate Change (unifCCC) for the coal industry in 2023. Figure 7: Methane plume masks detected at methane point sources located in the Changzhi Sanyuan Nan Yao Coal Mine according to Sentinel-2 observations, with red circles indicating the emission sources and the image in the second row, fifth column showing the located emission source. The white arrow indicates the direction of the wind, the background is from ©Google Earth imagery. The dates in the figure are (**a**) 10 January 2022; (**b**) 19 February 2023; (**c**) 10 April 2022; (**d**) 2 May 2022; (**e**) 24 June 2022; (**f**) 7 September 2022; (**g**) 19 October 2022; (**h**) 8 December 2022; (**i**) 7 January 2023.