| Summary: | In order to investigate the abundance of and temporal variation in nitrogen dioxide (NO<sub>2</sub>) in the troposphere and validate the corresponding satellite products during a normal year and the lockdown period of coronavirus disease 2019 (COVID-19) in Lhasa, a city on the Tibetan Plateau (TP), ground-based remote-sensing measurements captured by applying multi-axis differential optical absorption spectroscopy (MAX-DOAS) were recorded from August 2021 to March 2023 at the Lhasa site (91.14°E, 29.66°N; 3552.5 m altitude). The NO<sub>2</sub> differential slant column densities (dSCDs) were retrieved from the spectra of scattered solar light at different elevation angles. Then, the tropospheric NO<sub>2</sub> vertical column densities (VCDs) were calculated with the geometric approximation method. Based on the retrieved tropospheric NO<sub>2</sub> VCDs, we found that the pattern of monthly variation in tropospheric NO<sub>2</sub> VCDs in Lhasa presented two peaks, one in winter and one around May. According to the monthly means of tropospheric NO<sub>2</sub> VCDs during the COVID-19 lockdown, the NO<sub>2</sub> background level in Lhasa was estimated to be 0.53 × 10<sup>15</sup> molecules·cm<sup>−2</sup>. For diurnal variations in tropospheric NO<sub>2</sub> VCDs, the morning and evening peaks disappeared during the COVID-19 lockdown period. The east–west direction (i.e., along the river valley) was the main path of NO<sub>2</sub> transport and dispersion in Lhasa, but the tropospheric NO<sub>2</sub> VCDs were little dependent on the wind direction or wind speed during the COVID-19 lockdown. The correlation coefficient of tropospheric NO<sub>2</sub> VCDs was R = 0.33 (R = 0.43), with the averaged relative deviation of −28% (99%) for the TROPOMI (GEMS) relative to ground-based MAX-DOAS. The monthly deviations of tropospheric NO<sub>2</sub> VCDs between ground-based MAX-DOAS and the satellite showed a dependence on NO<sub>2</sub> abundance, with the maxima of the monthly positive deviations during the COVID-19 lockdown period. The GEMS could not capture the strong and systematic diurnal variation in tropospheric NO<sub>2</sub> VCDs in the “normal” year well. During the COVID-19 lockdown, the GEMS (>2 × 10<sup>15</sup> molecules·cm<sup>−2</sup>) overestimated the hourly levels measured by ground-based MAX-DOAS (<1.6 × 10<sup>15</sup> molecules·cm<sup>−2</sup>). As a whole, these results are beneficial to understanding the influences of anthropogenic activities on NO<sub>2</sub> background levels in Lhasa and to learning the accuracy of satellite products over the TP, with its high altitude and complex terrain.
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