## 3 Results
### Diurnal Development of the Urban Boundary Layer  
The results presented in the previous section suggest that the near surface WS has a significant effect on urban boundary layer variations in Beijing. Avoiding tall building disturbances around the 325-m tower, the daily average WS measured at the 47-m level was used to classify the different wind conditions. Accordingly, eight, fifteen, and eight days (marked in Figure 7) were categorized as low, medium, and high windy days based on the 25 th, 50 th, and 75 th percentiles of the WS, corresponding to 1.29 m s\({}^{-1}\), 1.54 m s\({}^{-1}\), and 2.23 m s\({}^{-1}\). For low and moderate wind classes, the diurnal changes of WS were not obvious, which indicates the thermal forcing dominates the evolution of the urban boundary layer. Notably, the growth of the boundary layer also influences the wind evolution in boundary layer. Using the measurements at 15 levels from the 325-m tower in 20 years, it was found  
Figure 6: Relationships between PM\({}_{2.5}\) concentration and urban BLH derived from threshold method combined with the Richardson number method over Beijing during daytime (**a**) and nighttime (**b**), with the dots indicating the accompanying RH. that the maximum WS often appeared at daytime at lower levels, while at higher levels the maximum WS usually occurred at nighttime [33], which was resulted from the surface stability of the urban boundary layer. Based on the case study (1-4 December 2016), it was also found that the stagnating urban boundary layer during heavy aerosol polluted daytime in winter seemed to act like an umbrella, preventing the air flow entrainment from the upper level to the lower level [29]. As shown in Figure 7a, different from the strong wind class, the WS showed a downward trend between sunrise and noon for these two wind classes, especially for the weak wind class, which may partly result from the evolution of urban boundary layer. The diurnal variations of the urban BLH were consistent with those of thermal forcing factors, e.g., solar radiation and sensible heat flux (Figure 7). Especially for weak wind class, with unapparent diurnal change of WS and TKE, the diurnal pattern of urban BLH was mainly modulated by thermal forcing (DSR and \(H\)). While under strong wind condition, the diurnal pattern of urban BLH was largely modulated both by dynamical forcing (WS and TKE) and thermal forcing (DSR and \(H\)). As shown in Figure 7b1, the DSR decreased with the decreasing wind speed level, which can be attributed to a combination of aerosol pollutants and cloud. These low or moderate wind days were mostly accompanied by high PM\({}_{2.5}\) concentrations, e.g., 200 \(\upmu\)g m\({}^{-3}\) and 100 \(\upmu\)g m\({}^{-3}\) median values, as illustrated in Figure 7b3. For the polluted days, particularly the heavily polluted low-wind days, they were usually covered with cloud, as shown in Figure 3, according to the cloud fraction data. However, it is hard to acquire accurate cloud information during heavily polluted periods. In addition, the existing clouds could be the result of polluted aerosols acting as nuclei for cloud formation, during heavy polluted days, more clouds were indeed found over Beijing in case studies [57]. Here, we considered the effects of both aerosol pollutants and cloud on the DSR. On this basis, reduced DSR and weak wind led to weak thermal and dynamic forcing, resulting in a smaller diurnal range of the urban boundary layer on low-wind days than on high-wind days. In other words, compared to strong wind classes as shown in Figure 7f1-f3, a reduction of approximately 400 m and 500 m to the maximum (median values) of the urban BLH for low and moderate classes, respectively, resulted from the combined changes in the wind, the aerosol pollutant concentration, and the cloud fraction. Therefore, it was considerably challenging to quantitatively analyze the radiative feedback effects of aerosol pollutants on the urban boundary layer in Beijing by only using the experimental measurements.