The interactions of aerosol and planetary boundary layer over a large city in the Mongolian Plateau

The interactions of aerosol and planetary boundary layer (PBL) play a crucial role in deteriorating the air quality in vast urban agglomeration areas in eastern China. However, there remains a lacking of report regarding their performance in the hazy events in Mongolian Plateau cities in northern China. In this study, half-month (01-16 January 2020) physical and material datasets of the PBL measured by multi instrumentations mounted in downtown Hohhot, a largest Mongolian Plateau city, are statistically analyzed. Results demonstrate that the aerosol-PBL feedback is of particular importance in promoting the hazy outbreak and the statistical relationship follows PBLH =-76.14 x ln(PM2.5) + 820.61. The non-linear fitting implies that there exists a potential threshold of 76.14 mu gm 3 for PM2.5, below which the PBLH decrease rapidly along with the increasing of air pollutants, defined as strong aerosol-PBL interaction phase, while beyond which there is minimal decrease for PBLH even when PM reaches to a high value, i.e., the hazy accumulation phase. Using a large-eddy simulation model named as Dutch Atmospheric Large-Eddy Simulation (DALES) initialized with the synergetical observa-tions in a representative hazy process in 11 January 2020, we found that the DALES is effective to capture the diurnal PBLH in this region. The existence of atmospheric aerosols is essential for lowering PBLH by 51.4 % from 1090 m of clean scenario to 530 m of polluted condition, postponing the development time, and advancing the afternoon lapse time. The enhancement of aerosol absorption ability strengthens the aerosol heating rate, thereby weakening the sensible heat flux, and inhibiting the development of PBLH. While opposite elevation on PBLH is found for the scattering aerosols. These findings highlight the importance of aerosol-PBL interaction in moti-vating the hazy episodes in Mongolian Plateau cities, which provide scientific references for the local policy -making towards pollution reductions in future.