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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 15 Jan 2019

Research article | 15 Jan 2019

Review status
This discussion paper is a preprint. A revision of the manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP).

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

Linlin Wang1,2, Junkai Liu1,5, Zhiqiu Gao1, Yubin Li2, Meng Huang2, Sihui Fan2, Xiaoye Zhang3, Yuanjian Yang2, Shiguang Miao4, Han Zou1, Yele Sun1, Yong Chen1, and Ting Yang1 Linlin Wang et al.
  • 1State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry (LAPC), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
  • 2Collaborative Innovation Centre on Forecast and Evaluation of Meteorological Disasters, School of Atmospheric Physics, Nanjing University of Information Science and Technology, Nanjing, 210044, China
  • 3Chinese Academy of Meteorological Sciences, Beijing, 100081, China
  • 4Institute of Urban Meteorology, China Meteorological Administration, Beijing, 100081, China
  • 5University of Chinese Academy of Sciences, Beijing 100049, China

Abstract. We investigated the interactions between the air pollutants and the structure of urban boundary layer (UBL) over Beijing by using the data mainly obtained from the 325-m meteorological tower and a Doppler wind lidar during 1–4 December, 2016. Results showed that the pollution episodes in this period could be characterized by low surface pressure, high relative humidity, weak wind, and temperature inversion. Compared with a clean daytime episode that took place on 1 December, results also showed that the attenuation ratio of downward shortwave radiation was about 4 %, 23 % and 78 % at 1200 local standard time (LST) on 2–4 December respectively, while for the net radiation (Rn) attenuation ratio at the 140-m level of the 325-m tower was 2 %, 24 %, and 86 %. The large reduction in Rn on 4 December was not only the result of the aerosols, but also clouds. Based on analysis of the surface energy balance at the 140-m level, we found that the sensible heat flux was remarkably diminished during daytimes on polluted days, and even negative after sunrise (about 0720 LST) till 1400 LST on 4 December. We also found that heat storage in the urban surface layer played an important role in the exchange of the sensible heat flux. Owing to the advantages 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 weak vertical mixing caused the concentrating of pollutants, and that vertical mixing would also be weakened by a certain quantity of pollutants, and then in turn worsened the pollution further. Compared to the clean daytime on 1 December, the maximums of the boundary layer height (BLH) reduced about 44 % and 56 % on 2–3 December, when the average PM2.5 (PM1) concentrations in afternoon hours (from 1200 to 1400 LST) were 44 (48) µg m−3 and 150 (120) µg m−3 . Part of these reductions of the BLH was also contributed by the effect of the heat storage in the urban canopy.

Linlin Wang et al.
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Linlin Wang et al.
Linlin Wang et al.
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Short summary
Urban boundary layer (UBL) affects the physical and chemical processes of the pollutants, and UBL structure can also be altered by pollutants. This paper presents the interactions between the air pollution and the UBL structure by using the field data mainly collected from a 325-m meteorology tower, as well as from a Doppler wind lidar, during a severe heavy pollution event occurred during 1–4 December, 2016 in Beijing.
Urban boundary layer (UBL) affects the physical and chemical processes of the pollutants, and...