The fine particulate matter pollution in winter that still exists under the emission reduction situation has a more complex formation mechanism. The effects of boundary layer dynamics and chemistry on air pollution are not sufficiently understood due to the lack of vertical observations. Therefore, this study collected comprehensive observations, including the vertical distribution of PBL dynamics, aerosol optical properties, and PM_2.5 chemical composition, in a semi-arid city in western China from 4 Jan to 31 Jan 2021. The air pollution during the field campaign was divided into three levels: excellent (20%), good (56%), and polluted (24%). The results showed as follows: the aerosol optical depth (AOD) was the smallest in the excellent level, while AOD was comparable in the good and polluted levels due to the difference in the vertical distribution of aerosols, i.e., the extinction lapse rate in the good level (0.12 km⁻²) was smaller than that in the polluted level (0.19 km⁻²). The boundary layer stability structures reflected by gradient Richardson number (Ri) profiles with high vertical resolution can significantly influence the vertical distributions of aerosols. Amounts of pollutants were removed from the basin when the synoptic weather systems and boundary layer were unstable (Ri < 0.25) causing the excellent level, while the very stable boundary layer (Ri > 1) decoupled the pollutants with clean air out of the basin and trapped them in the lower layer causing the polluted level. However, the pollution mechanism under the good level was more complex due to the effect of the coupled boundary layer dynamics and chemistry. The precursors were vertical mixing to the higher layer by the unstable lower and middle boundary layer (Ri < 0.25), and thus the photochemistry was enhanced with the stronger solar radiation, especially in the 0.42 km height. This phenomenon was also indicated by the difference in daytime and nighttime extinction coefficient profiles and the proportion of secondary inorganic aerosols (SIAs) at different heights. This paper provides observational evidence for the significant influence of the coupled atmospheric physical and chemical processes on the vertical distribution of aerosol and particulate pollution levels at the surface.

Particulate pollution, vertical distribution, boundary layer dynamics, photochemistry, in situ observations.