Meteorological impacts on interannual anomalies of O₃ import over Twain-Hu Basin

Prominent O₃ pollution over Twain-Hu Basin (THB) is observed during 2014-2020, with the annual concentrations of near-surface O₃ in the range of 49-65 μg·m^-3, larger than those in Sichuan Basin (SCB) and Pearl River Delta (PRD) in China. The increasing rate of O₃ over THB (1.9 μg·m^-3·yr^-1) is also higher than the counterparts in Yangtze River Delta (YRD), SCB and PRD. Besides, the exceeding rate of O₃ levels in THB rises from 3.9 % in 2014 to 11.5 % in 2019, greater than those in SCB and PRD as well. Based on the GEOS-Chem simulations in summer over 2013-2020, we find that nonlocal O₃ contributes dominantly to THB, with YRD being its principle source region, during regional O₃ transport over the central and eastern China. Here, the imported O₃ in THB is found to be mainly driven by the wind fields and the windward topography. The East Asia Summer Monsoon (EASM) circulations significantly regulate the interannual anomalies of imported O₃ over THB. In the years with abnormally higher O₃ import over THB, the EASM weakens, and the location of Western Pacific Subtropical High tends to more eastward compared with the years with abnormally lower O₃ import. Particularly, an abnormal easterly winds at the surface of YRD region effectively favor the delivery of O₃ from YRD to THB. In addition, the weak EASM promotes and undermines the regional transport of O₃ from NCP and PRD to THB, respectively. Correspondingly, the O₃ concentrations over THB can be fluctuated greatly depending on the degree of regional O₃ transport regulated by the EASM circulations, which indicates a complex relationship between sources and receptors of O₃ transport for air quality improvement.