Cross-regional PM2.5 sulfate transport to the Pearl River delta: dynamics and chemistry

Cross-regional transport significantly contributes to PM 2.5 sulfate (pSO 4 ) in the Pearl River Delta, South China, but its underlying processes remain insufficiently understood. Using WRF/CMAQ simulations, we investigated the dynamics and chemistry of pSO 4 transport during a polluted month. Source apportionment indicates high transport contributions (76–88%) to pSO 4 during pollution episodes. Vertical exchange across the boundary-layer top was the primary pathway of pSO 4 import. Notably, strong exchange occurred in two contrasting episodes, which were separately dominated by persistent inflow from more polluted North and Central China and stagnation of polluted parcels. However, the chemical pathways of pSO 4 formation during transport differed in these episodes, shifting from gas-phase OH oxidation within cold, dry, oxidant-rich plumes to aqueous-phase H 2 O 2 oxidation within warm, humid plumes. These findings reveal the complex interplay between weather systems, boundary-layer dynamics and chemistry in cross-regional pSO 4 transport, offering new insights into pSO 4 pollution mechanism and future PM 2.5 mitigation.