## 3 Results and Discussion  
### Temporal Variation of CO\({}_{2}\) and CH\({}_{4}\) Mole Fractions  
The average mole fractions of CO\({}_{2}\) and CH\({}_{4}\) at CRCS between 27 June and 19 July 2021 were 412.62 \(\pm\) 2.06 ppm (average \(\pm\) standard deviation) and 1959 \(\pm\) 19 ppb, respectively (Table 1). Observations on 26 June and 20 July were excluded to calculate daily average values because the observation time was less than 18 h on the two days. These values are 5.93 ppm and 9 ppb higher than the monthly CO\({}_{2}\) and CH\({}_{4}\) mole fractions observed in July 2020 (406.69 ppm for CO\({}_{2}\) and 1950 ppb for CH\({}_{4}\), data accessed from the World Data Centre for Greenhouse gases (WDCGG)) at the Global background Waliguan Observatory, about 700 km southeast of CRCS. The CO\({}_{2}\) mole fraction at CRCS was about 4.34 and 0.87 ppm lower than that observed at Mauna Loa Observatory and the global monthly average value in July 2021. Meanwhile, the CH\({}_{4}\) mole fraction was 73 ppb higher than the global mean average value in July 2021 (monthly averaged CO\({}_{2}\) and CH\({}_{4}\) mole fractions were obtained from [[https://gml.noaa.gov/ccgg/trends/data.html](https://gml.noaa.gov/ccgg/trends/data.html)]([https://gml.noaa.gov/ccgg/trends/data.html](https://gml.noaa.gov/ccgg/trends/data.html)), accessed on 27 March 2022). In-situ observations provide a clear diel variation of GHGs because they can measure GHGs continuously in both daytime and nighttime. Diurnal CO\({}_{2}\) and CH\({}_{4}\) patterns at CRCS appeared in different modes (Figure 2). The diurnal CO\({}_{2}\) mole fraction showed an M-mode curve (Figure 2B), which was different from the unimodal curve observed at Megacity Shanghai [31] and the nearby global background observatory Waliguan [34]. The CO\({}_{2}\) mole fraction generally increased to the daily maximum value at 8:00 CST (Figure 2B) due to the accumulation of CO\({}_{2}\) and the decrease in the PBL height. The CO\({}_{2}\) mole fraction decreased to a trough at noon along with the increase in PBL height and photosynthesis effect after sunrise. However, the CO\({}_{2}\) mole fraction stopped falling and rebounded to ~412.80 ppm until 19:00 and then decreased again to the daily lowest value at 23:00 (Figure 2B). One probable reason is that the vegetation in nearby desert oases (e.g., Dunhuang city in the east of CRCS) (Figure 1B,C) reduces their respiration and photosynthesis rate to adapt to high temperatures (>30 \({}^{\circ}\)C) [35] after 12:00 (Figure S1). Another possible reason is the influence of valley winds caused by the difference in the diurnal variation in the temperature between the basin and the mountain (Figure 1) [36]. The airmass with higher CO\({}_{2}\) mole fractions from Dunhuang city passed by CRCS, and therefore, a constant CO\({}_{2}\) mole fraction was monitored in the afternoon at a wind speed of ~1.5 m s\({}^{-1}\) (Figures 2 and S1). However, the CO\({}_{2}\) mole fraction decreased in the evening (19:00-23:00) due to the changed wind direction from the city (~50\({}^{\circ}\) direction) to the mountains (~200\({}^{\circ}\) direction) (Figures 2 and S1). The diurnal CH\({}_{4}\) pattern (Figure 2D) was different from CO\({}_{2}\) but similar to the summer pattern observed at the nearby Waliguan observatory except for morning time [37]. CH\({}_{4}\) mole fraction peaked at 2:00 with a value of 1988 ppb, probably due to some anthropogenic emissions nearby. The original data were checked, and some rapidly rising and falling CH\({}_{4}\) peaks occurred in the morning (Figure S2). The CH\({}_{4}\) mole fraction at 21:00 on 5 July was also high (2150 ppb) and introduced a high hourly average mole fraction (Figure 2D). At the same time, the wind speed and direction were 1.8 m s\({}^{-1}\) and 309\({}^{\circ}\) suggesting the probable emissions from the northwest direction.