## 3 Results and Discussion
### Comparison between EM27/SUN and Satellite-Retrieved XCO\({}_{2}\) and XCH\({}_{4}\)  
EM27/SUN observed XCO\({}_{2}\) and XCH\({}_{4}\) were compared with available satellite retrieves (Figure 4). Satellite retrieved XCO\({}_{2}\) were obtained from GOSAT, GOSAT-2, OCO-2, and OCO-3, while XCH\({}_{4}\) were acquired from GOSAT, GOSAT-2, and Sentinel-5 Precursor (S5-P). XCO\({}_{2}\) from GOSAT was approximately 0.34 ppm (0.30-0.35 ppm) lower than those observed from EM27/SUN. XCO\({}_{2}\) from GOSAT-2 were about 1.5 ppm (1.27-1.68 ppm) higher than the ground observations on 12, 16, and 17 July 2021, while on 11 July was \(-\)0.84 ppm lower than the ground observation (Table S3). XCO\({}_{2}\) from OCO-3 were higher (1.59 ppm, 26 June) than or close to surface observations (0.001 ppm, 28 June). XCO\({}_{2}\) from OCO-2 were all higher (1.77 ppm, 0.96-2.97 ppm) than ground-based observations (Figure 4A). XCH\({}_{4}\) from GOSAT on July 16 (five observations) was 2.62 ppb lower than that from EM27/SUN, while XCH\({}_{4}\) on 7 July (only 1 observation, 1850 ppb) was 33 ppb lower than the ground averaged value (Figure 4B). XCH\({}_{4}\) from GOSAT-2 were mostly lower (\(-\)0.2\(\leftarrow\)\(-\)18 ppb) than EM27/SUN observed values except on 11 July (4.5 ppb). Most XCH\({}_{4}\) observations from S5-P and EM27/SUN fluctuated around the 1:1 line within 10 ppb, except 14 and 15 July with differences of 43 and 39 ppb, respectively (Figure 4B). The huge differences on 14 and 15 July were attributed to some footprints having very low XCH\({}_{4}\) values (\(\sim\)300 ppb lower than the average values (\(\sim\)1900 ppb)) (Figure S5). In this case, the averaged XCH\({}_{4}\) from S5-P was lower than the ground observed values (\(-\)0.32%, (satellite--EM27)/EM27), close to the systematic difference between S5-P bias-corrected XCH\({}_{4}\) and TCCON data (\(-\)0.26%) [33]. XCO from GOSAT-2 was about 23 ppb (20-28 ppb) higher than the ground observations. The big differences in XCO observations might be originated from many factors and the most important of which is to confirm that the ground monitoring results are accurate in future work. The differences between different satellites and ground observations mainly originated from the differences in satellites payloads' technologies (grating and Fourier transform spectrometers) and their field of views (FOVs) (continuous and discrete footprints at different sizes in the across-track direction), as well as the differences retrieved methods (full physics algorithm from different institutes) [9; 53; 54; 55]. There are many studies validating satellite observations with ground observations at different locations, but the results were not the same. For example, [PERSON], et al. [20] found GOSAT and OCO-2 XCO\({}_{2}\) showed good consistency and the differences were 0.86 and 0.83 ppm after comparing with the TCCON Philippines site, while [PERSON], et al. [56] showed that GOSAT XCO\({}_{2}\) was underestimated by 1.70 ppm while OCO-2 overestimated by 1.33 ppm after comparing with in-situ aircraft measurements in China. [PERSON], et al. [57] showed the standard deviation of GOSAT observations was larger than that of OCO-2 at TCCON sites. [PERSON] et al. [12] compared EM27/SUN observations of GOSAT M- and H-gain retrievals and the results reflected that H-gain retrievals were close to EM27 observations in desert Australia. Most of the space- and ground-based observations are close to the 1:1 line with small differences (\(<\)0.1% for XCO\({}_{2}\) and \(<\)0.6% for XCH\({}_{4}\)), indicating a good consistency of ground and satellite observations in the Gobi Desert region. Furthermore, the temporal differences of the ground-based observations within 2 h of the satellite's transit were smaller than the spatial differences of satellite retrievals (Figure 4, Table S3), suggesting that spatially derived source-sink differences had a greater impact on the observed XCO\({}_{2}\) and XCH\({}_{4}\).