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

Abstract

Abstract. We investigated the interactions between the air pollutants and the structure of the 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 5 %, 24 % and 63 % in afternoon hours (from 12:00 to 14:00 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 3 %, 27 % and 68 %. 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 daytime on polluted days and even negative after sunrise (about 07:20 LST) till 14:00 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 vertical mixing was negatively related to 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) decreased about 44 % and 56 % on 2–3 December, when the average PM2.5 (PM1) concentrations in afternoon hours (from 12:00 to 14:00 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.

PM2.5 Pollution Modulates Wintertime Urban Heat Island Intensity in the Beijing‐Tianjin‐Hebei Megalopolis, China

“…In addition, some specific synoptic patterns might also cause noticeable changes in both UHII and PM 2.5 . Synoptic patterns can affect air quality episodes which are already expressed through the meteorological variables (e.g., temperature, wind speed, rainfall, and cloud cover), and thus, these variables can represent a certain synoptic situation over a given region (Kassomenos et al, ; Luo et al, ; Miao et al, , ; Ning et al, , ; Shahgedanova et al, ; Wang, Liu, et al, ; Yang Yim et al, ; Zheng et al, ). The possibly different associations between the UHII and PM 2.5 under different synoptic patterns will be the focus of a future study.…”

Using a coupled LES-aerosol radiation model to investigate urban haze: Sensitivity to aerosol loading and meteorological conditions

“…Many observations in Beijing found that meteorological conditions are a main driver on both the onset and longevity of haze. Large scale synoptic conditions such as southerly winds and low pressure often preempt pollution episodes which tend to occur every 4-7 days in Beijing wintertime (Liu et al, 2018a;Wang et al, 2019). These conditions are associated with the beginning of 'haze' as the switch in meteorological conditions from strong northwesterly to southerly winds advects pollution from surrounding provinces into Beijing.…”

“…conditions such as temperature inversions, increased humidity and decreased wind speed (Dou et al, 2015;Zhang et al, , 2017Wang et al, 2019;Zhong et al, 2019b).…”

Using a coupled large-eddy simulation–aerosol radiation model to investigate urban haze: sensitivity to aerosol loading and meteorological conditions

“…Research examining the feedback effect on Beijing haze episodes has thus far relied upon observations or regional modelling studies. Q. , Zhong et al (2018b), Gao et al (2015) and Wu et al (2019) performed model simulations of pollution episodes using the Weather Research and Forecasting model with added chemistry (WRF-Chem) to examine the feedback effect. Their results all confirm that aerosol-radiation interactions, aerosol hygroscopic growth and aqueous heterogeneous reactions are all factors in the suppression of boundary layer development and result in increased surface PM 2.5 concentrations during polluted episodes in the North China Plain.…”