Surface ozone pollution in subtropical urban regions exhibits strong seasonal differences in its driving mechanisms, but it is still unclear at present. Here, based on integrated ground-based and vertical measurements of ozone and its precursors in the Pearl River Delta region, this study finds that ozone exceedances in the wet season are primarily driven by intense local photochemistry, linked to higher temperatures and biogenic emissions. In contrast, the observed dry-season period pollution episodes are dominated by the downward entrainment of ozone within the planetary boundary layer (PBL), where the stored ozone in aloft is transported downward as the PBL expands. During these events, downward transport (including regional transport) contributes 58 %–90 % of the boundary-layer ozone enhancement (08:00–14:00 LT), whereas local photochemistry plays a minor role (3 %–31 %). These results imply that short-term emission controls are less effective in the dry season. Proactive reduction of precursors ahead of stable weather conditions may therefore help alleviate the buildup of ozone in the high atmosphere.