## 3 Results and Discussion
### Comparison with Surface In Situ and Column CO\({}_{2}\) and CH\({}_{4}\) Hole Fractions  
The column mole fractions were also compared with in-situ observations at CRCS. Because ground EM27/SUN tracks the sun and observes its near-infrared spectrum to further retrieve XCO\({}_{2}\) and XCH\({}_{4}\), only GHGs in the daytime on cloudless days can be observed and retrieved. The XCO\({}_{2}\) and XCH\({}_{4}\) were 0.90 ppm and 72 ppb lower than their corresponding in situ CO\({}_{2}\) and CH\({}_{4}\) mole fractions observed at the same time, while their ratio was higher (Table 1). Hourly XCO\({}_{2}\) was close to the daily and hourly averaged CO\({}_{2}\) mole fractions on different days and showed similar trends (Figures 1A and S2A). However, hourly XCH\({}_{4}\) exhibited large gaps with CH\({}_{4}\) mole fractions (Figures 1B and S2B), probably due to photoreaction removal of CH\({}_{4}\) in the upper atmosphere. The discrepancies of column and in-situ mole fractions were also reflected by the significant correlation between CO\({}_{2}\) and XCO\({}_{2}\) (\(r\) = 0.51, \(p\) < 0.01, \(n\) = 68) as well as XCO (\(r\) = 0.53, \(p\) < 0.01), but not for CH\({}_{4}\) and XCH\({}_{4}\) (\(r\) = 0.057, \(p\) = 0.645).  
XCO\({}_{2}\) and XCH\({}_{4}\) showed different diurnal variation patterns with the corresponding CO\({}_{2}\) and CH\({}_{4}\) mole fractions (Figure 6). Hourly XCO\({}_{2}\) and XCH\({}_{4}\) (Figure 6) were calculated from the four days (11, 12, 14, and 16 July) because there were continuous observations in good weather conditions. Hourly XCO\({}_{2}\) and XCH\({}_{4}\) showed a curve like a dome from 11:00 to 17:00: their mole fractions started to increase in the morning and achieved their peak at 13:00 (XCH\({}_{4}\)) or 14:00 (XCO\({}_{2}\)), but XCH\({}_{4}\) decreased faster than XCO\({}_{2}\) because XCH\({}_{4}\) at 17:00 was lower than that at 11:00 (Figure 6). The hourly averaged CO\({}_{2}\) mole fraction in the four days decreased from 414.60 ppm at 11:00 to 414.03 ppm at 12:00 CST, and then showed a trend similar to that of XCO\({}_{2}\) (Figure 6A). The gap between CO\({}_{2}\) and XCO\({}_{2}\) was lowest  
Figure 5: Footprints of XCO\({}_{2}\) observations from OCO-2, OCO-3, and GOSAT near the EM27/SUN observation site from 26 June to 19 July 2021. The observation data from different satellites were selected as a region spanning 5\({}^{\circ}\) latitude and 10\({}^{\circ}\) longitude of the center of the EM27/SUN site.  
around the local mid-noon (0.76 ppm at 14:00 CST) but highest in the morning (1.82 ppm at 11:00 CST). Meanwhile, the hourly averaged CH\({}_{4}\) mole fraction decreased monotonically from 11:00 to 17:00 CST due to photochemical removal and showed a similar trend to XCH\({}_{4}\) after 13:00 (Figure 6B). The gap between CH\({}_{4}\) and XCH\({}_{4}\) was also largest at 11:00 (72 ppb), narrowed to \(\sim\)65 ppb at 13:00, and remained there until 16:00. The mole fraction ratio of XCO\({}_{2}\)/XCH\({}_{4}\) and CO\({}_{2}\)/CH\({}_{4}\) exhibited a similar growth trend with a gap of approximately 7 (Figure 6C). Many factors could affect the diel mole fraction gaps between column and surface values. One probable reason might be the differences in sink processes because CO\({}_{2}\) is mainly absorbed by ground vegetation, while CH\({}_{4}\) can be removed by OH radical in the whole atmosphere. Another possible reason could be the influence of the changes in the planetary boundary layer (PBL) height. The PBL height could increase in the morning due to the heating effect of the ground after sunrise and thus dilute the near-surface GHG mole fractions under the PBL.