Synoptic pattern and planetary boundary layer structure associated with aerosol pollution during winter in Beijing, China.

The day-to-day variations in the planetary boundary layer (PBL) structure and air quality are closely governed by large-scale synoptic forcings. Partly due to the lack of long-term PBL observations during the winter in Beijing, the complex relationships between the large-scale synoptic patterns, local PBL structures/processes, and PM _2.5 pollution have not been fully understood. Thus, this study systematically investigated these linkages by combining aerosol measurements, surface meteorological observations, radiosonde data, reanalysis, long-term three-dimensional meteorological simulations, and idealized meteorology-chemistry coupled simulations. Based on the validated long-term simulation results, the boundary layer height (BLH) in Beijing during two winters from 2013 to 2015 was calculated and compared with PM _2.5 measurements. A significant anti-correlation was found between the daily BLH and PM _2.5 concentration in Beijing, indicating the importance of the PBL structure on the variations in the aerosol pollution levels. Those days with low BLHs are often accompanied by a strong elevated thermal inversion layer. Based on the daily 900-hPa geopotential height fields, seven synoptic patterns were identified using an objective approach, in which two types were found to be associated with heavy PM _2.5 pollution in Beijing. One pattern was characterized by weak northwesterly prevailing winds and a strong elevated thermal inversion layer over Beijing, and the local emissions of aerosols played a decisive role in the formation of heavy pollution. The other pattern was associated with southerly prevailing winds, which could transport the pollutants emitted from southern cities to Beijing. According to the meteorology-chemistry coupled simulations, southerly regional transportation can contribute approximately 56% of the PM _2.5 in Beijing. The results of this study have important implications for understanding the crucial roles that multiscale meteorological factors play in modulating the aerosol pollution in Beijing during the winter.
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