Synoptic control on the dynamics and chemistry of regional PM2.5 sulfate transport in South China

The local abundance of PM2.5 sulfate, an aerosol component with important health and environmental impacts, is often influenced by cross-regional transport. However, the associated dynamic and chemical processes governing PM2.5 sulfate transport under different synoptic conditions remain insufficiently understood. Improving this process-level understanding is essential for interpreting sulfate pollution in regions downwind of major emission sources. To this end, this study introduces a process-based framework to investigate how synoptic systems regulate PM2.5 sulfate transport.Based on WRF/CMAQ simulations, we diagnosed the relative importance of horizontal transport and vertical exchange, as well as various in-plume sulfate production pathways, during two distinct PM2.5 sulfate pollution episodes in South China during autumn 2015. These episodes were linked to contrasting synoptic influences, namely the typhoon periphery and the subtropical high, and were characterized by strong and weak effects of cross-regional transport, respectively.Our analyses show that vertical exchange across the boundary-layer top served as the major process of PM2.5 sulfate import in both episodes. Interestingly, pronounced vertical exchange occurred under both strong inflow and stagnant conditions, suggesting that they could independently intensify vertical PM2.5 sulfate exchange. Meanwhile, contrasting meteorological conditions and chemical environments in the two episodes resulted in different contributions of in-plume sulfate production pathways: gas-phase OH oxidation dominated within dry, cold and oxidant-rich plumes under typhoon periphery, whereas aqueous-phase H2O2 oxidation prevailed within wet and humid plumes under relatively stable conditions.Overall, these results highlight the complex coupling between synoptic forcing, atmospheric dynamics and chemistry in cross-regional PM2.5 sulfate transport, providing new perspectives into sulfate pollution mechanisms and implications for future PM2.5 mitigation.