Numerical simulation and cause analysis for a severe mixed pollution of smog and haze in the Pearl River Delta in Winter

Abstract

Based on the simulation results of the Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ), the meteorological elements, boundary layer characteristics and temporal and spatial distribution characteristics of particulate matter during the heavy haze-dense fog-haze pollution process from 1 to 12 January 2017 were analyzed with the aid of the data from conventional meteorological observation and environmental monitoring station. Also, the causes of this pollution have been presented considering the terrain effects. The results show that, during the first process (day 1 to day 3), the upwind transports pollutants to Shaoguan, Qingyuan and other provinces, and the local surface wind velocity is lower than 3 m·s−1, which increased concentration of pollutants in the Pearl River Delta region and formed moderate smog and haze pollution. During the second process (day 4 to day 6), the average surface wind speed is lower, and the static wind is lower than 2 m·s−1 with the influence of terrain effect (even lasted for 5-16 hours). Besides, the influence of the strong convergence field of small wind increases the accumulation and convergence of pollutants on the ground, and simultaneously, the thickness of the ground inversion layer is further intensified and the height of the boundary layer is as low as 100 meters. At the same time, the relative humidity increases to nearly 100%, resulting in continuous visibility of fewer than 500 meters for 10 hours. Additionally, it was found that on the day of heavy pollution in Guangzhou, the main source is coarse nuclear particles, and the corresponding actual situation is that the concentration of NO2 was higher than that in other places, which reflects the influence of high concentration of human activities on air pollution. As the northeasterly winds continued to strengthen, pollutants across the region continued to be transported to the Pearl River Delta. Through the convergence of wind farms in Zhaoqing, Foshan, and Guangzhou, cross-regional pollutants and local pollutants emitted by the Pearl River Delta are again accumulated in the pearl river delta central area, forming a third moderate haze pollution process. It is concluded that the influence of strong convergence field of small wind under 2 m·s−1 (even 5-16 hours static wind), the increase of ground temperature inversion, the height of boundary layer as low as 100 m, the increase of near-ground humidity are superposed, and the combined effect of topography are the main meteorological causes of mixed pollution of fog and haze in the Pearl River Delta in winter.