Entrainment is critical to the development of the atmospheric convective boundary layers (CBLs) and for the exchange of energy and substances between the boundary layer and free atmosphere. In this study, entrainment relationships are systematically evaluated from large‐eddy simulations (LESs) of shear‐free and sheared CBLs under a broad variety of atmospheric conditions. A total of 36 LES runs are conducted for varying free‐atmosphere stratifications, surface heat fluxes, and height‐constant geostrophic winds. It has been found that entrainment flux ratios have a common value of 0.2 regardless of the stratification of free atmosphere and the surface heat‐flux value when analysed within the zero‐order‐model (ZOM) framework for the shear‐free CBLs, whereas the actual (LES) entrainment ratios are not only stratification‐dependent, but also surface heat flux‐dependent. For the sheared CBLs, the entrainment‐flux ratios are substantially increased when the geostrophic winds are strong while they have a slight reduction under weak wind conditions. In the free‐encroachment regime, the dimensionless entrainment rates have been found to be approximately constant and surface heat‐flux independent, with the value in the range from 0.4 to 0.7 depending on the method used to evaluate the CBL depth.

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Entrainment is critical to the development of the atmospheric convective boundary layer (CBL), but little is known about how entrainment is impacted by the aerosol radiative effect. An aerosol radiation transfer model is used in conjunction with large-eddy simulation (LES) to quantify the impact of aerosol shortwave radiative heating on entrainment and thermodynamics of an idealized dry CBL under aerosol-loading conditions. An entrainment equation is derived within the framework of a zero-order model (ZOM) with the aerosol radiative heating effect included; the equation is then examined against the LES outputs for varying aerosol optical depths (AODs) and free-atmosphere stratification scenarios. The results show that the heat flux profiles become more nonlinear in shape as compared to the case of the clean (no aerosol pollution) CBL, with the degree of nonlinearity being highly dependent on the AOD of the layer for the given type of radiation-absorbing aerosols. As AOD increases, less solar radiation reaches the surface and thus the surface heat flux becomes smaller, and both actual (LES) and ZOM-derived entrainment flux ratios decrease. This trend is opposite to the clean CBL where the LES-predicted flux ratios show an increasing trend with diminishing surface heat flux, while the ZOM-calculated flux ratio remains constant. The modified dimensionless entrainment rate closely follows the −1 power law with a modified Richardson number. The study suggests that including the aerosol radiative effect may improve numerical air quality predictions for heavy-air-pollution events.

With the ground-level ozone pollution problem increasingly prominent in recent years in China, it is particularly important in basic researches on ozone contamination characteristics. In this study, 13 cities in Beijing-Tianjin-Hebei (BTH) area were examined to determine the characteristics of surface ozone (O3) from 2015 to 2018. Due to the photochemical oxidation of ozone precursors (such as nitrogen oxides and carbon monoxide) along with the presence of sunlight and characteristics of local emission sources, the O3 and oxidant (OX) concentrations showed obvious seasonal variation and daily variation. It implicated that the O3 concentrations reached the maximum during summer. The concentrations of O3 were higher at daytime than those measured at nighttime. The ozone weekend effect was estimated by the difference and deviation, which exhibited that the difference between weekday and weekend were related to the concentrations of ozone precursors and PM, vehicle emissions, and solar radiation. Moreover, the O3 concentrations decreased with the increase of other air pollutants by correlation analysis. The ozone pollution was easily formed at light and moderate polluted periods when compared to other air quality levels.