The WRF-CMAQ Simulation of a Complex Pollution Episode with High-Level O3 and PM2.5 over the North China Plain: Pollution Characteristics and Causes

The problem of atmospheric complex pollution led by PM2.5 and O3 has become an important factor restricting the improvement of air quality in China. In drawing on observations and Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model simulations, this study analyzed the characteristics and causes of a regional PM2.5-O3 complex pollution episode in North China Plain, in the period from 3 to 5 April 2019. The results showed that in static and stable weather conditions with high temperature and low wind speed, despite photochemical reactions of O3 near the ground being weakened by high PM2.5 concentrations, a large amount of O3 generated through gas-phase chemical reactions at high altitudes was transported downwards and increased the O3 concentrations at the ground level. The high ground-level O3 could facilitate both the conversion of SO2 and NO2 into secondary inorganic salts and volatile organic compounds into secondary organic aerosols, thereby amplifying PM2.5 concentrations and exacerbating air pollution. The contributions of transport from outside sources to PM2.5 (above 60%) and O3 (above 46%) increased significantly during the episode. This study will play an instrumental role in helping researchers to comprehend the factors that contribute to complex pollution in China, and also offers valuable references for air pollution management.