[Analysis of Characteristics and Causes of a Typical Haze Pollution in Beijing in the Winter of 2019].

In order to study the pollution characteristics and causes of winter haze pollution in Beijing, a typical PM2.5 pollution process in Beijing in December 2019 was used as the analysis object using aerosol vertical detection data, boundary layer meteorological field and near-ground turbulence data, and the difference in haze. The characteristics of the pollution stage and the evolution of the physical and chemical characteristics of the boundary layer were comprehensively analyzed. The results showed that ① the pollution process in Beijing during the observation period lasted 5 d and experienced two generations and eliminations. The maximum hourly PM2.5 concentration was 220 µg·m-3 and the time exceeding the severe pollution standard was 64 h, thereby accounting for 53% of the total time. ② The aerosol optical properties and meteorological field observation data showed that the pollution originated from the regional transmission of aerosols and water vapor on the surface of the southwest urban agglomeration in Beijing, which accounted for 48% of the total pollution transmission, followed by a stable high-altitude situation and ground pressure field configuration. The near-surface layer maintained weak southerly winds (wind speed: 1-2 m·s-1), a strong inversion temperature close to the ground ï¼»0.8 K·(100 m)-1ï¼½, high humidity (relative humidity above 80%), and other unfavorable diffusion weather conditions, thereby promoting the accumulation of pollutants and the conversion of moisture absorption. Superimposing local pollution emissions were the main reasons for the maintenance of haze days. In addition, the near-ground extinction coefficient increased from 0.070 km-1 to 5.954 km-1, and the depolarization ratio decreased from 0.05 to 0.02 during the two pollution generation and disappearance processes, thereby indicating that the spherical characteristics of aerosols gradually became significant as the pollution increased. ③ The analysis of the turbulence observation data showed that the characteristic quantities of different pollution stages were significantly different and negatively correlated with the pollutant concentration. Before the occurrence of heavy pollution, the turbulence statistics (turbulence intensity, friction velocity, and turbulent kinetic energy) suddenly decreased from high values (the hourly variation rate was 77%, thereby far exceeding the daily fluctuation of 33%), and the turbulence intensity responded first. During the pollution accumulation stage, the friction velocity (0.04-0.21 m·s-1), turbulence intensity (average: 0.678 m2·s-2), and turbulence energy (average: 0.643 m2·s-2) were maintained at a low level, and the bottom atmosphere had a poor mixing and diffusion ability, which is important for continuous pollution accumulation. Four hours before the end of the pollution event, the turbulence intensity again showed a sharp increase (increment of more than one order of magnitude); thus, the turbulence intensity can be used as a predictive indicator of the occurrence and end of a heavy pollution event, and the response time is the same as the continuous turbulence intensity after the turbulence peak. In addition, the sensible heat fluxes on sunny days and haze days were both transported from the ground to the atmosphere, and showed clear daily single-peak changes. The sensible heat flux on haze days (20 W·m-2) was smaller than that on sunny days (60 W·m-2). The latent heat flux was approximately 0 W·m-2 in the whole process. ④ There was a feedback effect between the meteorological conditions of the pollution layer and the boundary layer. On the one hand, unfavorable diffusion of the meteorological conditions was conducive to the accumulation of pollution. On the other hand, the aerosol layer and water vapor cooling effect that accumulated near the ground were worse than the night cooling radiation on the inversion layer The contribution was greater, thereby further inhibiting the development of turbulent motion and ultimately resulting in increased pollution.