## 3 Results and Discussion
### Temporal Variation of XCO\({}_{2}\) and XCH\({}_{4}\)
However, along with the rise of ambient temperature (which could exceed 30 \({}^{\circ}\)C at 11:00) (Figure S1), plants from nearby oases (Dunhuang city and nearby, Figure 1B,C) could close their stomata to reduce water evaporation and decrease their photosynthetic efficiency [35], resulting in an increase of XCO\({}_{2}\) in the atmosphere. After noontime, XCO\({}_{2}\) decreased again along with the decrease in anthropogenic activities (due to high temperature) and recovery of plant photosynthesis in the nearby oasis. The XCO\({}_{2}\) increases again in the late afternoon due to the recovery of anthropogenic activities and the decrease in plant photosynthesis. The hourly XCH\({}_{4}\) showed unimodal distributions in the daytime (e.g., 16, 12, 14, and 11 July), which were different from those of XCO\({}_{2}\) (Figure 3B). XCH\({}_{4}\) increased continuously in the morning due to biological and anthropogenic emissions from the surrounding area, especially from nearby Dunhuang city (20 km east). It is expected that the diurnal OH radical variation in Dunhuang is similar to that in other regions with the highest values around local noontime, such as in Beijing, Shanghai, and Pearl River Delta regions [46; 47; 48; 49]. Thus, the XCH\({}_{4}\) could increase in the morning due to the absence or lack of OH radicals. Along with the production of OH radicals under sunshine in the daytime, the increase in XCH\({}_{4}\) slowed down and reversed at local noon (Figure 3B). Unlike XCO\({}_{2}\) and XCH\({}_{4}\), XCO did not show a significant diurnal variation, although its mole fraction decreased slightly in the afternoon (Figure 3C). The decreasing trend in the afternoon, especially on 12, 14, 16, and 18 July, is probably due to the removal of the reaction with OH radical [50]. XCO varied among 78-90 ppb (Figure 3C), which was about 30 ppb lower than that observed at Hefei city in eastern China [51]. Furthermore, the differences in the diurnal variations on most days were narrower than the differences among different days suggesting the emissions and the sinks of CO were relatively balanced at CRCS sites. XCO\({}_{2}\)/XCH\({}_{4}\) were close to 221 on most days with lower ratios in the nomtime but higher ratios in the morning and later afternoon (Figure 3D). Average XCO\({}_{2}\)/XCH\({}_{4}\) (220 \(\pm\) 1) was higher than the corresponding ratios of surface mole fractions of CO\({}_{2}\) and CH\({}_{4}\) (220 \(\pm\) 1) (Table 1). XCO\({}_{2}\)/XCO and XCH\({}_{4}\)/XCO were averaged at 4.9 \(\pm\) 0.2 (\(\times 10^{3}\)) and 22.5 \(\pm\) 0.9 with small amplitude (Figure 3, Table 1). In general, hourly XCO\({}_{2}\), XCH\({}_{4}\), XCO, and their ratios show discrepancies on different days (Figure 3), mainly due to the changes in anthropogenic emissions and synoptic weather conditions [52]. Figure 3: Diurnal variation in EM27/SUN observed XCO\({}_{2}\) (**A**), XCH\({}_{4}\) (**B**), XCO (**C**), XCO\({}_{2}\)/XCH\({}_{4}\) (**D**), XCO\({}_{2}\)/XCO (**E**), and XCH\({}_{4}\)/XCO (**F**) on different days. The format of the date is mm-dd on the top of the Figure.