Transport-dominated ozone pollution suppresses heat-ozone hotspots in a receptor basin

Abstract. The Twain-Hu Basin in central China is a relative “cold spot” for heat-ozone compound extremes, despite being surrounded by compound hotspots and experiencing both extreme summer heat and severe ozone pollution. This paradox arises because synoptic-scale regional transport, rather than local thermal forcing, governs severe ozone episodes, as diagnosed by PCA typing, FLEXPART-WRF, and WRF-Chem source tagging. Non-local sources contribute ~66% of basin-mean ozone. Two transport regimes (P1: 52.7%; P3: 12.9%) together explain 65.6% of the MDA8 O₃ variance. P1 (deep advective subsidence) features an East China Sea cyclone driving deep northerlies from North China, with western-flank subsidence injecting O₃ and precursors into the boundary layer. P3 (boundary-layer convergence trapping) involves low-level easterly advection from East China driven by the continental high, trapping pollutants via convergence over the basin. This transport dominance sustains elevated ozone throughout the diurnal cycle with bimodal peaks (late-morning and nighttime) reaching ~30% above climatology. Correspondingly, the September peak of ozone exceedance lags the summer temperature maximum, decoupling the most polluted month from the hottest months. These findings explain how a receptor basin escapes surrounding heat-ozone compound hotspots and suggest that this transport-dominated pattern may be a common feature of receptor basins surrounded by multiple source regions.