Analysis on meteorological causes of heavy pollution in Beijing-Tianjin-Hebei Region during the prevention and control of COVID-19

A continuous regional heavy pollution process occurred in Beijing, Tianjin and Hebei from February 8 to 13, 2020, during the period of COVID-19 prevention and control. Based on the surface and upper air meteorological observation data and ERA5 data of European center, the meteorological causes of the heavy pollution event were analyzed from the circulation background, temperature advection, surface meteorological elements and boundary layer wind field distribution. The results showed: (1) During this process, the atmospheric circulation was stable. In the early stage of pollution, the upper air was dominated by northwest airflow. From February 10, the shortwave trough moved eastward one after another. South wind occurred at 850 hPa and below. There were many closed low pressure on the ground, the pressure field was weak in the whole, and the weather situation was stable. (2) The cold air activity in the central region was not significant, and the warm advection or weak cold advection occurred in the middle and low levels in most periods. The inversion times were obviously more, and the inversion layer thickness and strength were greater than the south. The pollutants were not easy to diffuse. (3) The ground wind in the central area was relatively small, and the wind direction below 1 000 m of the boundary layer was unstable, and the ventilation performance was poor. In addition, the transportation of pollutants in the south led to the strengthening of heavy pollution in this area and its surrounding areas. (4) The distribution of heavy pollution area was consistent with that of surface convergence line, which appeared in the north of surface convergence line and wet area. (5) Compared with the heavy pollution weather in Beijing, Tianjin and Hebei in February, 2014, the meteorological elements in this process were more unfavorable to the diffusion of pollutants, but the prevention and control of emission reduction had weakened the degree of pollution to a certain extent.