Abstract
The diurnal evolution of the atmospheric boundary layer—the lowermost part of the atmosphere where the majority of human activity and meteorological phenomena take place—is described by its depth. Additionally, the boundary layer height (BLH) and the turbulence intensity strongly impact the pollutant diffusion, especially during transition periods. Based on integrated observations from a 325-m meteorological tower and a Doppler Wind lidar in the center of Beijing, the entire diurnal cycle of urban BLH in December 2016 was characterized. Results highlight that the Doppler lidar exhibited it is well suited for monitoring convective BLH while it trudges in monitoring stable BLH, while a 325-m meteorological tower provided an important supplement for Doppler lidar under nocturnal boundary layer and heavily polluted conditions. For the diurnal cycle, under light wind condition, the pattern of urban BLH was largely modulated by thermal forcing of solar radiation and may partly be affected by wind speed. While under strong wind condition, the pattern of urban BLH was largely modulated both by thermal forcing and dynamical forcing. The present work also presented evidence for several new features in the morning and afternoon transitions of the urban boundary layer, showing the duration of the morning transition varied between 1 and 5 h, with the largest value occurring under weak wind with high PM2.5 concentration; while the afternoon transition ranged from 3 to 6 h, which was positively (negatively) correlated to wind speed (PM2.5 concentration). Our work highlights that weak wind speed (weak dynamic motion) and heavy aerosol pollution (weak thermal forcing due to the effect of cooling) can dramatically affect the evolution of the boundary layer.
Highlights
The atmospheric boundary layer (ABL) height is an important factor in diluting pollutants emitted at the surface, in which layered pollutants vertically disperse through the action of turbulence
When the height could be retrieved from both the two methods, the urban boundary layer height (BLH) retrieved from the Richardson number (Rib) method was considered as reference (4% of the total values) if the urban BLHs from threshold method were lower than 200 m, and the averaged values were chosen if the urban BLHs from threshold method were larger than or equal to 200 m, and lower than 325 m (3% of the total values)
About 1% of the urban BLHs retrievals from threshold method were higher than 325 m and than the values from Rib method, which happened during the clean noon time with relatively larger turbulence
Summary
The atmospheric boundary layer (ABL) height is an important factor in diluting pollutants emitted at the surface, in which layered pollutants vertically disperse through the action of turbulence. The most common way to determine the boundary layer height (BLH) is by using vertical profiles of meteorological variables, such as the potential temperature, wind speed (WS), and bulk Richardson number (Rib), which are generally obtained from radiosonde launches or aircraft observations [8]. Radio soundings collected at rural sites nearby cities are used to assist in the analysis of the urban boundary layer [10,11]. The BLH estimated from radio soundings launched at rural sites is not representative of urban areas. I.e., during field campaigns and densified observation, radio-sounding are launched three times a day and cannot capture the diurnal development of the urban boundary layer
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