Sudden decrease of airborne sulfates in summer at sites in western Japan prior to the enforcement of the MARPOL Treaty

Ship emissions are a dominant contributor of airborne sulfates. We established two sites at either end of an enclosed water area—the Seto Inland Sea in Japan—to identify the influence of ship emissions and the effectiveness of countermeasures against sulfur emissions from ships from 2017 to 2021, i.e., from before to after the start of the MARPOL Treaty's enforcement. As a result, a sudden decrease in airborne sulfates in the summer prior to the enforcement of the MARPOL Treaty was observed. In the process/analysis of identifying the cause of this sudden decrease out of synchronization with the MARPOL Treaty, the effectiveness of the MARPOL Treaty in reducing the sulfur content in marine fuel oil from 2020 was distinctly and successfully detected, as was that of individual regulations regarding requirements for the emission control area in Chinese territorial waters from 2019. The establishment of observation sites at either end of an enclosed water area made it possible for us to evaluate the effectiveness of the two reductions by the MARPOL Treaty and individual Chinese regulations prior to the MARPOL Treaty. The sudden decrease at the site on the western edge of the enclosed water area in 2019 out of synchronization with the MARPOL Treaty could be strongly related to the pathway of the air mass arriving at the site and strongly due to individual Chinese regulations prior to the MARPOL Treaty.