## 3 Results and Discussion
### Comparison between EM27/SUN and Satellite-Retrieved XCO\({}_{2}\) and XCH\({}_{4}\)
The standard deviations (SD) for ground observed XCO\({}_{2}\) within 1 h before and after the satellite transit time were ranged from 0.07 to 0.30 ppm, which was lower than those of satellite observations (0.87-4.46 ppm) within the coincidence criteria of 10\({}^{\circ}\) (longitude) \(\times\) 5\({}^{\circ}\) (latitude) surrounding CRCS. The SDs of XCO\({}_{2}\) from GOSAT-2 (2.18-4.46 ppm) were also higher than OCO-2, OCO-3, and GOSAT (0.87-1.38 ppm) (Table S3). The SDs for XCH\({}_{4}\) from EM27/SUN observations (0.49-2.21 ppb) were also lower than satellites retrievals (2.30 ppb-58.88 ppb). Meanwhile, the average relative standard deviation (RSD) of EM27 observations was 0.03% (0.02%-0.07%) for XCO\({}_{2}\) and 0.05% (0.03%-0.12%) for XCH\({}_{4}\) (Table S3), suggesting the variations of XCO\({}_{2}\) and XCH\({}_{4}\) were very small, even less than the error of the equipment. The average RSD for satellite observations were 0.43% (0.21%-1.08%) for XCO\({}_{2}\) and 0.95%(0.12%-3.19%) for XCH\({}_{4}\) (Table S3). The differences between satellites and EM27/SUN observations ((satellite--EM27)/EM27) were 0.26% (\(-\)0.20%-0.72%) for XCO\({}_{2}\) and \(-\)0.38% (\(-\)2.31%-0.70%) for XCH\({}_{4}\). After deducting the influence of the two S5-P data (14 and 15 July, Figure S5), the difference of XCH\({}_{4}\) would reduce to \(-\)0.13% (\(-\)1.77%-0.70%). The satellites' footprints were also plotted on the map to show the differences between ground and satellite XCO\({}_{2}\) observations (Figure 5 and Figure S5). There was a footprint from GOSAT on 16 July close to the CRCS (Figure 5), XCO\({}_{2}\) from GOSAT (411.75 ppm) was approximately 1 ppm lower than the surface observation (412.73 ppm), and XCH\({}_{4}\) (1879.8 ppb) were close to EM27/SUN results (1880.7 ppb). This footprint was also observed by GOSAT-2 on 11, 16, and 17 July (Figure S6). XCO\({}_{2}\) from GOSAT-2 on these days were 413.34, 416.00, and 411.63 ppm, respectively, which were higher (2.96 ppm on 17 July) or lower (\(-\)0.68 ppm on 11 July and \(-\)1.54 ppm on 16 July) than ground observations on the same time. Meanwhile, the differences of XCH\({}_{4}\) among GOSAT-2 and ground observations (satellite minus surface observations) were less than 5 ppb (4.3, \(-\)3.2, and 3.4 ppb, respectively), while the differences in XCO varied bigger on different days (\(-\)3.1, 13.5, 1.48 ppb, respectively). The discrepancies between O  
Figure 4: Comparison of XCO\({}_{2}\) (**A**) and XCH\({}_{4}\) (**B**) observed from EM27/SUN and satellites (OCO-2, OCO-3, GOSAT, GOSAT-2, and S5-P) at CRCS in Dunhuang, western China. EM27/SUN observations in the east of the CRCS were narrower than those in the west side. For example, the differences between OCO-2 and EM27/SUN on 11 and 18 July were 0.96 ppm and 1.31 ppm, which were narrower than the difference with the tracks in the west of CRCS on 7 July (2.97 ppm) and 16 July (1.86 ppm). Furthermore, the difference between OCO-3 and EM27/SUN in the east of the CRCS on 28 June was 0.001 ppm, which was also narrower than the OCO-3 track on the west side on 26 June (1.59 ppm). Fortunately, both GOSAT, GOSAT-2, and OCO-2 passed through the CRCS on 16 July, and the average XCO\({}_{2}\) from OCO-2 (414.59 \(\pm\) 0.87 ppm) was close to GOSAT-2 retrieved (414.45 \(\pm\) 3.30 ppm), but higher than the results from GOSAT (412.40 \(\pm\) 1.10 ppm) and EM27/SUN (412.74 \(\pm\) 0.07 ppm).