The Aerosol Optical Properties over a Desert Industrial City Wuhai, Northwest China, During the 3-Year COVID-19 Pandemic

Aerosol optical depth (AOD) data from 2020 to 2022 during the COVID-19 pandemic in a typical desert industrial city, Wuhai, was analyzed to investigate aerosol optical properties, origins of different types of aerosols, and the impacts of the COVID-19 lockdown on desert pollution. Results show that annual AOD (500 nm) and Ångström exponent α were 0.36 ± 0.12 and 0.75 ± 0.22 in 2020, 0.30 ± 0.12 and 0.75 ± 0.14 in 2021, and 0.28 ± 0.09 and 0.74 ± 0.19 in 2022, respectively, representing a slightly polluted environment characterized by a mixture of coarse-mode dust aerosols and fine-mode anthropogenic aerosols. Seasonal analysis reveals that the highest AOD primarily occurred in spring due to frequent dust events, while the lowest AOD was observed in winter. Potential Source Contribution Function (PSCF) identified the Alxa Desert as a major potential source during the entire year, and anthropogenic industrial and mining activities in northern Ningxia and southern Inner Mongolia were also important contributors, particularly outside of the winter season. The prevailing wind direction in Wuhai was from the northwest (NW-quadrant), originating from the depopulated desert or Gobi area, accounting for 85.11% in spring, 61.45% in summer, 68.09% in autumn, and 100% in winter. The remaining air masses came from southeastern (SE-quadrant) densely populated areas. Despite the dominance of NW air flows, SE anthropogenic air masses resulted in the highest AOD of 0.47 ± 0.24 in spring, 0.38 ± 0.23 in summer, and 0.32 ± 0.17 in autumn, with corresponding finest particle sizes of 0.83 ± 0.31, 0.91 ± 0.30, and 1.02 ± 0.22 in α. This suggests that anthropogenic influence remains significant even under strict control measures during the COVID-19 lockdown. In winter, the northwest air masses contributed to the highest pollution of 0.49 ± 0.39 (AOD) and finest particle size of 0.90 ± 0.32 (α), likely associated with the coal/straw burning for winter heating. In addition, the particles leading to moderate pollution primarily ranged around 0.2–0.25 µm, and fine particle pollution persists throughout the year.