## 5 Conclusions  
In the present study, based on integrated observations from a 325-m meteorological tower and a Doppler lidar in urban Beijing, the entire diurnal cycle of the urban BLH in December 2016 was characterized. The highest daytime urban BLH observed in the measurement campaign was as high as 1000 m; however, under heavily polluted conditions with weak wind, the maximum daytime urban BLH reached up to 300 m only. The Doppler lidar is well suited to retrieve and monitor CBL evolution, while it trudges in monitoring the SBL process. A 325-m meteorological tower provided an important supplement on SBL observation from Doppler lidar, especially under NBL and heavily polluted conditions (urban BLH around or lower than 200 m, and even lower than 100 m). The variations of DSR, WS, \(H\), \(\mu^{*}\), and TKE correlated positively with the BLH, while there was a negative correlation between BLH and PM\({}_{2.5}\). Based on the full image of the boundary layer diurnal cycle, we have improved the knowledge of morning and afternoon transitions of the urban boundary layer under different weather and pollution conditions. That is, the D\({}_{\text{MT}}\) varied between 1 and 5 h, with the largest value occurring under conditions of weak wind and high PM\({}_{2.5}\) concentration, while D\({}_{\text{AT}}\) ranged from 3 to 6 h. In addition, D\({}_{\text{AT}}\) and \(r_{\text{AH}}\) presented positive correlations with WS but negative correlations with PM\({}_{2.5}\) concentration. Our work highlights that weak WS (weak dynamic motion) and high levels of aerosol pollutants (weak thermal forcing due to the cooling effect) can dramatically affect the development of the boundary layer. For the  
Figure 9: Mean diurnal cycles of WS, (**a1**,**a2**,**a3**) and vertical variance vertical profiles (**b1**,**b2**,**b3**), for weak, moderate, and strong wind classes, respectively. The shaded areas correspond to the interquartile range (25%–75%) during morning (blue) and afternoon transitions (red). diurnal cycle, under weak WS condition, the pattern of urban BLH was largely modulated by thermal forcing of solar radiation and might partly by WS; while for the strong wind condition, the pattern of urban BLH was largely modulated by dynamical forcing (TKE and WS), and as well by thermal forcing. In future work, many more examples need to be collected to study the impact of thermal and dynamic forcing on the diurnal development of the wintertime boundary layer and its association with air pollution in urban Beijing by using joint meteorological tower and Doppler lidar observations, especially under calm and heavily polluted conditions. The results reported also have important implications for other urban regions that frequently experience air pollution and may aid in designing effective regional mitigation strategies for the environment and human health at the diurnal scale. For instance, according to the different diurnal stages of boundary layer development, we can carry out staggered production strategy to modulate emissions under various urban boundary layer diffusion conditions. The following are available online at [[http://www.mdpi.com/2072-4292/12/23/3935/s1](http://www.mdpi.com/2072-4292/12/23/3935/s1)]([http://www.mdpi.com/2072-4292/12/23/3935/s1](http://www.mdpi.com/2072-4292/12/23/3935/s1)), Figure S1: Height dependence of \(Ri_{b}\) (calculated by Equation (1) with \(b=100\)) at 08:00 CST (purple line), 10:00 CST (orange line) and 20:00 CST (black line) on 19 December, 2016, Figure S2: Hourly variation of simulated PBLH derived by three different PBL parameterization schemes (detail as followings) in Weather Research and Forecast (WRF) model during 16-21, December 2016, Table S1: Properties of the three different boundary layer parameterization schemes. Conceptualization, L.W. and Y.Y. ; Methodology, S.F., Y.Z. and M.H. ; Software, S.F. ; Data curation, S.M. and H.Z. ; Validation, Z.G., Y.L., S.H.L.Y. and S.L. ; Formal analysis, L.W., Y.Y., S.H.L.Y. and S.L. ; Writing--original draft preparation Y.Y.