Vertical-distribution Characteristics of Atmospheric Aerosols under Different Thermodynamic Conditions in Beijing

摘要

Understanding the vertical distribution of atmospheric aerosols is crucial to elucidating their spatial distribution and the formation of extreme air pollution events. Based on multisource data from specialized aircraft, lidar, and conventional surface observations with meteorological reanalysis, the vertical distribution of atmospheric aerosols and related changes during two air pollution cases in Beijing in the spring of 2012 were analyzed and compared. The results indicated that temperature inversion occurred in the atmospheric boundary layer in both cases. Aerosols accumulated considerably within the inversion layer, and the vertical distribution of the aerosol concentration was consistent with the relative humidity. However, the vertical distributions of the pollution layer thickness, aerosol concentration, and particle size differed significantly under different temperature-inversion conditions, primarily because of differences in the air pollution-diffusing abilities, which depended on vertical changes in the atmospheric thermodynamic structure. When radiation inversion occurred, the diurnal variation in the vertical distributions of aerosol and relative humidity was notable: The air pollution layer was thin in the morning, but the aerosol and particle size became more vertically uniform during the day, and the aerosol concentration and relative humidity near the surface decreased sharply because of enhanced pollutant vertical diffusion, which was influenced by newly developed unstable thermodynamic stratification. During the temperature inversion resulting from coupled subsidence and advection, the inversion layer was higher and more stable, allowing it to inhibit the vertical diffusion of pollutants, which subsequently caused long-lasting and thick pollution with a higher aerosol concentration and relative humidity at lower levels alongside a relatively unchanging vertical distribution of particle sizes. Moreover, in the daytime, as the southerly airflow strengthened below the inversion layer and the mixed-layer height increased, the pollution and wet layers thickened, and the aerosol concentration increased rapidly because of pollutant transport to the Beijing region.