Influences of synoptic circulations on regional transport, local accumulation and chemical transformation for PM heavy pollution over Twain-Hu Basin, central China.

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 (T-mode Principal Component Analysis) objective synoptic pattern classification, Flexible Particle-Weather Research and Forecasting (FLEXPART-WRF) model, and Random Forest model, we investigate the influences of synoptic circulations on regional transport, local accumulation, and chemical transformation of PM 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 PM pollution, indicating that regional transport of PM dominates the THB's heavy air pollution. The PM/CO ratio is higher in the transport-type pattern and positively correlated with PM concentrations, reflecting a higher efficiency of chemical transformation to secondary PM in transport-type pollution compared with the accumulation-type pollution. Transport-type heavy PM 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 PM 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 PM for heavy air pollution, with implications for understanding changes of air quality in the receptor region of regional transport.