Do ship emission control areas in China reduce sulfur dioxide concentrations in local air? A study on causal effect using the difference-in-difference model.

As a global factory and trade power, China has paid a high ecological price. Heavy shipping traffic is observed around the nation's most highly populated areas, including the Yangtze River Delta (YRD), Pearl River Delta (PRD), and Bohai Rim (BR) regions. Among the various shipping pollutants, sulfur dioxide (SO2) has received increased attention because of its significant adverse health effects. China delimited three emission-control areas (ECAs) in three key waters of these regions to control the sulfur emissions from ships and improve the air-quality in China's coastal areas. Using the difference-in-difference model, this study determines that the ECA policy has a positive impact on reduction of SO2 concentrations in the YRD and BR regions. The results of this study show that the ECA policy has some time lag, possibly because enforcement has gradually become more stringent. Surprisingly, the ECA policy does not play a positive role in reducing the SO2 concentration in the PRD region. This lack of response could be caused by a series of measures that have been implemented before implementing the ECA policy in the PRD to reduce the impact of ship pollutant emissions on the local air quality.