## 3 Results
### Morning and Afternoon Transitions of the Urban Boundary Layer  
#### 3.3.1 Durations  
The CBL starts to grow slowly after sunrise, and a transition feature of both the CBL and SBL, named \"morning transition\", is the first stage of the development of the ABL. Examination of the transition periods (e.g., passing from night to day or day to night) has important implications for air quality [58; 59]. In previous studies, different algorithms were used to define the morning transition (MT) from observations [57; 60; 61]. It was defined as the period from the time when the surface buoyancy flux first becomes positive (crossover) to the time at which the BLH reaches about 200 m [57] based on the observations from meteorological tower. Considering the positive heat flux also occurred in nighttime and the urban BLH was often greater than 200 m for urban site during summertime, the beginning of the MT was marked as the sunrise, and the end of MT was defined as the rapid onset of the rapid-growth phase in [58] by using the measurements from a tall tower in London. Given that the positive \(H\) occasionally occurred during nighttime in our study period, we defined the sunrise as the starting time of MT. We chose the time when the urban BLH reaches about 200 m as the end of the MT after the sunrise in this wintertime period. The period between the maximum \(H\) at midday and \(H\) < 0 around sunset is usually defined as the afternoon transition (AT) [8; 62]. In this paper, we adopted these two methods to estimate the durations of MT and AT during wintertime over horizontally homogeneous Beijing urban surface. Note that the fixed height (200 m) method for MT employed in the present study may not be robust to investigate precise absolute change characteristics of the urban boundary layer. However, at least, it can help us to study the relative change characteristics of the morning development of the urban boundary layer under different conditions  
Figure 7: Mean diurnal cycles of downward shortwave radiation (DSR), sensible heat flux, PM\({}_{2.5}\) concentration, and urban BLH under low (**a1,b1,c1,d1,e1,f1**), moderate (**a2,b2,c2,d2,e2,f2**), and strong (**a3,b3,c3,d3,e3,f3**) wind classes. The shaded areas, blue solid line, red dashed line, and pink lines correspond to the interquartile range (25%–75%), median, 200 m (definition of onset the convective boundary layer), and the slopes of growth rate of the median value of urban BLH from 09:00 CST to the peak time, respectively. (e.g., wind, solar radiation, pollutants) quantitatively. Similarly, AT is also employed to show relative change characteristics of convective urban boundary layer at daytime. Excluding strong-wind nights (e.g., 5, 15, and 22 December), the MT duration (D\({}_{\rm MT}\)) ranged from 1 to 5 h, and the average value was 2.4 h. The maximum D\({}_{\rm MT}\) occurred on the heavily polluted days, such as 4 and 18 December, with weak morning-averaged winds of 0.6 m s\({}^{-1}\) and 0.8 m s\({}^{-1}\), respectively. The variation of D\({}_{\rm MT}\) was negatively correlated (\(r=-0.4\) for both the 8-m level and 47-m level) with the change in WS, which is consistent with previous studies [57; 58; 63], and implies that weak wind prolongs the D\({}_{\rm MT}\). However, this correlation was weaker than that in [58] (\(r=-0.7\)), possibly because of the complexity of the aerosol loading. A positive correlation (\(r=0.5\)) was found between D\({}_{\rm MT}\) and PM\({}_{2.5}\) concentration, which suggests that the significant loading of the aerosol pollutants had large effects on D\({}_{\rm MT}\) by reducing the net radiation reaching the surface. This can be proved by model results from a large-eddy simulation in which aerosols delayed the CBL onset in the morning by up to 4.5 h [64]. Note that weak wind is usually associated with high concentrations of PM\({}_{2.5}\) over Beijing; and besides, many other contributing factors, such as stability, advection, the state of the previous day's RL [8], and the entrainment [65], can affect the D\({}_{\rm MT}\).