The dynamic-thermal structures of the planetary boundary layer dominated by synoptic circulations and the regular effect on air pollution in Beijing

Abstract. Synoptic circulations play important roles in meteorological conditions and air quality within the planetary boundary layer (PBL). Based on Lamb-Jenkinson weather typing and multiple field measurements, this study reveals the mechanism of how the coupling effects of multiscale circulations influence PBL structure and pollution. Due to the topographic blocking in the daytime, pollutants accumulate in the plain areas horizontally. The sinking divergent flows overlying on the rising convergent flows within the PBL inhibit the continuously upward dispersion of aerosols vertically. At night, the horizontal and vertical coupling mechanisms synergistically worsen the pollution. The large-scale environmental winds and regional-scale breezes affect the pollution directly via the horizontal coupling effect, which generates a pollution convergent zone of different directional flows. The relative strength of flows causes the severely polluted area to move around horizontally from 39° N to 41° N. In addition, the multiscale circulations regulate the mixing and diffusion of pollutants indirectly via the vertical coupling effect, which changes the PBL dynamic-thermal structure. The warm advection transported by the upper environmental winds overlies the cold advection transported by the lower regional breezes, generating strong wind direction shear and advective inversion. The capping inversion and the convergent sinking motion within the PBL suppress massive pollutants below the zero speed zone. The multilayer PBL under cyclonic circulation has no diurnal variation. Weak ambient winds strengthen the mountain breezes observably at night, the temperature inversion can reach 900 m. The nocturnal shallower PBL, consistent with the zero velocity zone between ambient and mountain winds, can reach 600 m. By contrast, the PBL under southwesterly circulation is a mono-layer with obvious diurnal variation, reaching 2000 m in the daytime. The strong winds circulations restrain the development of regional breezes, the zero speed zone is located at 400 m and the inversion is lower than 200 m at night. The PBL under westerly circulation has a hybrid structure with both multiple aerosol layers and diurnal variation. The inversion is generated by the vertical shear of zonal winds. Clean and strong north winds are dominated under anticyclone circulation, the vertical shear and the diurnal variation of thermal field disappear because of strong turbulent mixing, and there is no significant PBL structure. Our results imply that the algorithm of atmospheric environmental capacity under synoptic circulations, such as the cyclonic type, with a multilayer PBL needs to be improved.