Impact of China's ECA policies on air pollution in coastal cities: Empirical analysis based on synthetic-DID model

Emissions from ships stand as one of the principal contributors to air pollution in coastal areas. China has implemented the Domestic Ship Emission Control Area (DECA) policy progressively since 2016 to alleviate sulfur oxide and particulate matter pollution in coastal cities by limiting the sulfur content in marine fuel oil. This study utilizes Synthetic Difference-in-Differences model to construct a counterfactual framework for evaluating the DECA policy effect on air quality of cities in the Yangtze River Delta, the Pearl River Delta, and the Bohai Rim region. Results revealed that DECA policy significantly reduced the urban sulfur dioxide (SO2) concentrations of YRD, BR, and PRD regions by 9.93 %, 8.64 %, and 7.09 %, respectively. However, the policy's efficacy in restraining fine particulate matter (PM2.5) concentrations was found to be non-significant. This study further estimated the dynamic changes in SO2 concentration in coastal cities of different regions in the DECA coverage. The results showed that the DECA policy generally began to significantly reduce SO2 concentration levels in the second year after implementation, and with the tightening of the DECA policy, the reduction effect followed a growing-and-then-falling trend. Finally, the paper offers policy recommendations to further reduce ship emissions, aligned with recent emission reduction advancements.