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.