## 3 Results and Discussion
### Temporal Variation of XCO\({}_{2}\) and XCH\({}_{4}\)  
XCO\({}_{2}\), XCH\({}_{4}\), and XCO were averaged at 413.00 \(\pm\) 1.09 ppm, 1876 \(\pm\) 6 ppb, and 84 \(\pm\) 3 ppb, respectively (Table 1). XCO\({}_{2}\), XCH\({}_{4}\), and XCO varied on different days and showed different diurnal patterns (Figure 3). The daily averaged XCO\({}_{2}\) was highest on 26 June (414.63 \(\pm\) 0.11 ppm) and decreased to 412.02 ppm on 28 June. XCO\({}_{2}\) started to increase from the lowest value on 7 July (411.06 \(\pm\) 0.30 ppm) to the second peak on 12 July (413.97 \(\pm\) 0.40 ppm) and generally decreased thereafter (Figure 2, Table S1). Although XCO\({}_{2}\) fluctuated on these days, the gap between the maximum and minimum daily average values was less than 1% (3.57 ppm). The PROFFAST software had already done airmass and independent airmass correction during the spectral processing process. The retrieved  
Figure 2: Daily and hourly average mole fractions of surface CO\({}_{2}\) (**A,B**) and CH\({}_{4}\) (**C,D**), as well as hourly average column mole fractions of CO\({}_{2}\) (**A**) and CH\({}_{4}\) (**C**), observed at CRCS in Dunhuang, China. results may be affected by the potential airmass-dependent effects, because the XCO\({}_{2}\) and XCH\({}_{4}\) mole fractions were highest at noon and decreased along with an increase in solar zenith angle (SZA) (Figure S4) [10; 44]. The daily averaged XCH\({}_{4}\) remained at approximately 1880 ppb on the first four observed days but decreased to 1870 ppb on 11 July. XCH\({}_{4}\) rebounded to a value >1880 ppb again on 15 July but decreased continuously thereafter to the lowest value on 19 July (1868 \(\pm\) 1 ppb) (Figure 2, Table S1). Similar to XCO\({}_{2}\), the concentration gap of maximum and minimum XCH\({}_{4}\) values was also less than 1% (14 ppb). XCO was also high on 26 and 28 June, started to increase from 7 July to 15 July, and remained constant at around 85 ppb thereafter (Table S1). XCO\({}_{2}\) was significantly correlated with CO\({}_{2}\) (\(r\) = 0.51, \(p\) < 0.001), but XCH\({}_{4}\) was not correlated with CH\({}_{4}\) (\(r\) = 0.057, \(p\) = 0.645), suggesting the different behaviors of surface and column mole fractions of greenhouse gases, especially CH\({}_{4}\). XCO was also significantly correlated with CO\({}_{2}\) (\(r\) = 0.53, \(p\) < 0.001), and the correlation coefficient was higher than the coefficients between XCO and XCO\({}_{2}\) (\(r\) = 0.33, \(p\) < 0.001) and XCH\({}_{4}\) (\(r\) = 0.29, \(p\) < 0.001). Both XCO\({}_{2}\) and XCH\({}_{4}\) showed diurnal variations. Diurnal variations of XCO\({}_{2}\), XCH\({}_{4}\), XCO, and their ratios on different days (observation time \(\geq\)5 h) are plotted together using the hourly average values (Figure 3). The observation time was not consistent on each day due to the differences in arrival and departure times at CRCS to set up EM27/SUN, as well as the weather conditions (e.g., enough sunshine). Diurnal XCO\({}_{2}\) seemed to show sinusoidal mode curves (Figure 3A), similar to the observations (especially on 26 and 28 March 2008) at the Jet Propulsion Laboratory (JPL) in the South Coast Air Basin (SCB) of California, USA [45]. XCO\({}_{2}\) decreased slightly from 8:00 to 10:00 (e.g., 11, 18, and 16 July) or 9:00 (17 July) in the morning, inversed after that to a noon peak at 13:00 or 14:00 (Solar noontime is \(\sim\)13:45 at Dunhuang), and decreased again to the late afternoon continuously (26 June, 11, 26 July) or to an afternoon valley at 17:00 and reverse after that (12, 14 July). Sunshine and temperature could be the two factors that affected diurnal XCO\({}_{2}\) variation in addition to anthropogenic emissions. After sunrise, plants in the surrounding oasis conduct photosynthesis and reduce the ambient XCO\({}_{2}\) accumulated throughout the whole night.