Abstract
Abstract. To investigate the impacts of multiscale circulations on the planetary boundary layer (PBL), we have carried out the PBL thermodynamic structure field experiment with a Doppler wind profile lidar, a microwave radiometer, and a ceilometer from January 2018 to December 2019 in Beijing. We found that the direct regulatory effect of synoptic circulation worked through transporting and accumulating pollutants in front of mountains in the daytime, while the indirect effect of multiscale circulations worked through coupling mechanisms in the nighttime. The horizontal coupling of different-direction winds produced a severe pollution convergent zone. The vertical coupling of upper environmental winds and lower regional breezes regulated the mixing and diffusion of pollutants by generating dynamic wind shear and advective temperature inversion. We also found that the dominating synoptic circulations led to great differences in PBL thermodynamic structure and pollution. The cyclonic circulation resulted in a typical multilayer PBL characterized by high vertical shear (600 m), temperature inversion (900 m), and an inhomogeneous stratification. Meanwhile, strong regional breezes pushed the pollution convergent zone to the south of Beijing. The southwesterly circulation resulted in a mono-layer PBL characterized by low vertical shear (400 m) and inversion (200 m). The westerly circulation led to a hybrid-structure PBL, and the advective inversion was generated by the vertical shear of zonal winds. Strong environmental winds of southwesterly and westerly circulations pushed the severe-pollution zone to the front of mountains. There was no distinct PBL structure under the anticyclone circulation. The study systematically revealed the appreciable effects of synoptic and regional circulations on PBL structure and air quality, which enriched the prediction theory of atmospheric pollution in the complex terrain.
Highlights
The megacity of Beijing is the political, economic, and cultural center of China
According to the pollution intensity, three pollution types and one clean type occurring most frequently in the North China Plain (NCP) were selected as the studied circulation patterns
This paper explores the direct regulatory effect and indirect coupling effect of synoptic circulations by choosing the most frequent pollution types and clean type classified by the Lamb–Jenkinson weather typing (LWT) approach
Summary
The megacity of Beijing is the political, economic, and cultural center of China. With the recent economic development and acceleration of urbanization, an increasing number of air pollution episodes have emerged and pose a direct threat to human health (Quan et al, 2014; Fu et al, 2014; Cheng et al, 2016; Song et al, 2017). Miao et al (2017) specially targeted summertime synoptic types, indicating that the horizontal transport of pollutants induced by the synoptic forcing is the most important factor affecting the air quality of Beijing in summer. They found that synoptic patterns with high-pressure systems located to the east or southeast of Beijing are the most favorable types for heavy-aerosol-pollution events. We adopt an objective Lamb– Jenkinson classification scheme to categorize the large-scale atmospheric circulations centered on Beijing. The Lamb– Jenkinson approach, which confirms that the categorization results have a clear physical understanding, has been applied widely in previous studies (Huang et al, 2016; Liao et al, 2017; Yu et al, 2017)
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