Previous studies have reported boundary layer features related to air pollution. However, few studies have comprehensively evaluated the characteristics and mechanisms of vertical wind in the formation and evolution of heavy particulate matter pollution episodes (EP) in Hong Kong. In this study, we analyzed the vertical characteristics of heavy particulate matter (PM) pollutions over Hong Kong and their relationships with vertical wind profiles using high‐time‐resolution Doppler lidar measurements and hourly meteorological and air quality observations. We identified nine EPs and show that the events were closely coupled to various vertical wind profiles in the planetary boundary layer (PBL). Our analysis suggests that strong vertical wind speed with wind shear at certain heights in the PBL had a positive correlation with surface PM during most superregional transboundary EPs. The maximum transport height extends from the surface to about 2.0 km or even higher; these transport heights differed among superregional and regional transboundary EPs. At peak surface pollution concentrations during the nine EPs, the surface PM10 had a significant negative correlation with PBL heights/mixing layer heights, while the averaged wind shear in the PBL was significantly positively correlated. These EPs with different mixing layer heights were mainly driven by different vertical wind shear conditions under various weather systems related to surface high pressure, cold fronts, dust storms, and typhoons. This work provides scientific evidence that surface PM pollutions were closely related to the characteristics of vertical profiles during the transboundary air pollutions.

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Turbulent mixing is critical in affecting urban climate and air pollution. Nevertheless, our understanding of it, especially in a cloud-topped boundary layer (CTBL), remains limited. High-temporal resolution observations provide sufficient information of vertical velocity profiles, which is essential for turbulence studies in the atmospheric boundary layer (ABL). We conducted Doppler Light Detection and Ranging (LiDAR) measurements in 2019 using the 3-Dimensional Real-time Atmospheric Monitoring System (3DREAMS) to reveal the characteristics of typical daytime turbulent mixing processes in CTBL over Hong Kong. We assessed the contribution of cloud radiative cooling on turbulent mixing and determined the altitudinal dependence of the contribution of surface heating and vertical wind shear to turbulent mixing. Our results show that more downdrafts and updrafts in spring and autumn were observed and positively associated with seasonal cloud fraction. These results reveal that cloud radiative cooling was the main source of downdraft, which was also confirmed by our detailed case study of vertical velocity. Compared to winter and autumn, cloud base heights were lower in spring and summer. Cloud radiative cooling contributed ~32% to turbulent mixing even near the surface, although the contribution was relatively weaker compared to surface heating and vertical wind shear. Surface heating and vertical wind shear together contributed to ~45% of turbulent mixing near the surface, but wind shear can affect up to ~1100 m while surface heating can only reach ~450 m. Despite the fact that more research is still needed to further understand the processes, our findings provide useful references for local weather forecast and air quality studies.

Transboundary‐local aerosols interaction requires to be comprehensively understood in urban air quality research. A year‐long intensive observation of the atmospheric boundary layer (ABL) at multiple sites in Hong Kong was conducted using a four‐Doppler Light Detection and Ranging (LiDAR) network with different scanning modes. Results show that heterogeneity of the ABL in terms of mixing layer height and wind shear was induced by orographic topography. Interaction between local and advected aerosol layers during a transboundary air pollution (TAP) episode was identified by the network. During TAP episode, downward transport of transboundary aerosol relied on small scale eddies with weak wind speed in nighttime, while the transport of surface local aerosol to upper level was the dominated process in daytime, but the heterogeneity of the ABL affected by terrain determined the capacity of the mixing, eventually resulting in the opposite transport direction of aerosols in the transboundary‐local aerosols interaction.

Aerosol particles, especially those from anthropogenic activities, can serve as cloud condensation nuclei (CCN), thus substantially influence the organization of cloud systems at various scales (Fan et al., 2008;van den Heever & Cotton, 2007;Wang et al., 2011). The number of CCN can alter the cloud properties by determining the rate of cloud droplet growth and size distribution, especially in convective clouds that are frequently initiated in the atmospheric boundary layer (ABL) (Cao et al., 2021;Twomey, 1977;Zhang et al., 2021). This interaction has been regarded as one of the largest uncertainties both in weather forecast and climate projections (IPCC, 2013). Hence, better understandings of the mechanisms of aerosol-cloud interaction are urgently required.Aerosol-cloud interaction substantially changes cloud microphysics, which influence the autoconversion process whereby cloud droplets grow to sizes beyond which precipitation occurs and can thus influence warm rain