Influences of Synoptic Circulations on Regional Transport, Accumulation And Chemical Transformation for Pm2.5 Heavy Pollution Over Twain-Hu Basin, Central China

Abstract

Abstract: The Twain-Hu Basin (THB), located in Central China, serves as a key juncture where the northerly "polluted" airflows of the East Asian winter monsoon meet the southerly warm and humid airflows. Using the T-PCA objective synoptic pattern classification, FLEXPART-WRF, and Random Forest model, we investigate the influences of synoptic circulations on regional transport, local accumulation, and chemical transformation of PM2.5 during heavy air pollution over the THB in January of 2015-2022. The results show that the transport-type synoptic pattern accounts for 65.16% of heavy PM2.5 pollution, indicating that regional transport of PM2.5 dominates the THB’s heavy air pollution. The PM2.5/CO ratio is higher in the transport-type pattern and positively correlated with PM2.5 concentrations, reflecting a higher efficiency of chemical transformation to secondary PM2.5 in transport-type pollution compared with the accumulation-type pollution. Transport-type heavy PM2.5 pollution is predominantly influenced by upstream anomalous northerly and easterly airflows at the bottom of the high-pressure system, converging with the southern wind in the receptor area over the THB. Accumulation-type heavy pollution exhibits weak wind anomalies in central and eastern China under the control of a uniform pressure field. Furthermore, thermally-induced vertical circulations with sinking airflows in the middle and lower troposphere suppress the vertical air pollutant dispersions. The relative contributions of atmospheric factors for transport-type PM2.5 heavy pollution events are 38.0% for dynamical driver, 26.8% for thermal driver, and 35.1% for chemical transformation, while in accumulation-type, the contribution rates are 33.9%, 36.3%, and 29.7%, respectively. This study elucidates the influences of synoptic patterns on regional transport, local accumulation, and chemical transformation of PM2.5 for heavy air pollution, with implications for understanding changes of air quality in the receptor region of regional transport.

Keywords: Central China, PM2.5 pollution, regional transport, Circulation classification, FLEXPART-WRF, Random Forest