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. We investigated the interactions between the air pollutants andthe structure of the urban boundary layer (UBL) over Beijing by using the datamainly obtained from the 325 m meteorological tower and a Doppler wind lidarduring 1–4 December 2016. Results showed that the pollution episodes inthis period could be characterized by low surface pressure, high relativehumidity, weak wind, and temperature inversion. Compared with a clean daytimeepisode that took place on 1 December, results also showed that theattenuation ratio of downward shortwave radiation was about 5 %, 24 %and 63 % in afternoon hours (from 12:00 to 14:00 local standardtime, LST) on 2–4 December, respectively, while for the net radiation( Rn ) attenuation ratio at the 140 m level of the 325 m tower was3 %, 27 % and 68 %. The large reduction in Rn on4 December was not only the result of the aerosols, but also clouds. Based onanalysis of the surface energy balance at the 140 m level, we found that thesensible heat flux was remarkably diminished during daytime on polluteddays and even negative after sunrise (about 07:20 LST) till 14:00 LST on 4December. We also found that heat storage in the urban surface layer playedan important role in the exchange of the sensible heat flux. Owing to theadvantages of the wind lidar having superior spatial and temporal resolution,the vertical velocity variance could capture the evolution of the UBL well.It clearly showed that vertical mixing was negatively related to theconcentrating of pollutants, and that vertical mixing would also be weakenedby a certain quantity of pollutants, and then in turn worsened the pollutionfurther. Compared to the clean daytime on 1 December, the maximums of theboundary layer height (BLH) decreased about 44 % and 56 % on2–3 December, when the average PM 2.5 (PM 1 ) concentrations inafternoon hours (from 12:00 to 14:00 LST) were44 (48)  µ g m −3 and 150 (120)  µ g m −3 . Partof these reductions of the BLH was also contributed by the effect of the heatstorage in the urban canopy.

Vertical observations of the atmospheric boundary layer structure over Beijing urban area during air pollution episodes