Importance of regional PM2.5 transport and precipitation washout in heavy air pollution in the Twain-Hu Basin over Central China: Observational analysis and WRF-Chem simulation.

With observational analysis and WRF-Chem simulation on a heavy air pollution event in January 2019 over the Twain-Hu Basin (THB) in Central China, this study characterized the regional transport of PM2.5 emitted from the North China Plain (NCP) to the THB region in Central China and quantitatively assessed the influence of the regional PM2.5 transport and precipitation washout on PM2.5 change in the wintertime heavy air pollution over the THB. It was found that the THB's heavy air pollution event was exacerbated by the strong northeasterly winds driving a quasi 2-day time lag of regional PM2.5 transport from the NCP to the THB. The multi-scale atmospheric circulations of cold air invasion influenced by East Asian winter monsoon and the terrain block of THB altered the structures of regional PM2.5 transport in deteriorating air quality to the THB. It was assessed for the THB region that the enhancing contribution of regional PM2.5 transport to the high air pollution level reached up to 70.5% in the heavy air pollution, and the precipitation washout could contribute the 55.3% PM2.5 removal to dissipating the PM2.5 pollution over the THB with frequent precipitation and wet environment, distinguishing from the dominance of wind-cleaning air pollution in the other regions in China.