The impact of port throughput and throughput growth on sulphur-oxide concentrations in China's coastal ports: boosters or brakes

This study investigates how port throughput scale and throughput growth relate to sulphur-oxides (SOX) concentrations at 18 major coastal ports in China. Using a port-year panel dataset over 2005-2019 and combining satellite-based SOX observations with a consistent set of port operational indicators, we apply quantile regression to examine nonlinearities and distributional heterogeneity. The results suggest an EKC-type pattern in which expanding throughput can increase SOX in earlier stages, whereas further expansion can coincide with reduced SOX as ports modernise and compliance capacity strengthens. We also document pronounced regional and scale heterogeneity: northern and eastern ports exhibit an inverted-U relationship between throughput and SOX, while southern ports show a U-shaped pattern. Overall, high-throughput ports tend to exert stronger impacts on SOX outcomes, underscoring the role of operational intensity. These findings imply that mitigation policies should be tailored to port-specific characteristics, including development stage, scale and regional context.