Changes in the impacts of ship emissions on PM _2.5 and its components in China under the staged fuel oil policies

Abstract. The issue of air pollution caused by ship emissions is becoming prominent with the increasing global shipping activities. China has carried out fuel oil policies in three stages in the past few years to meet the requirements of the global low sulfur regulation by the International Maritime Organization (IMO). However, the impacts of staged policies on air quality in China are not sufficiently understood. This study firstly updated the ship emission inventory including PM2.5 components based on field and on-board measurements under the staged fuel oil policies. Then, the impacts of ship emissions on PM2.5 and its gas precursors and primary and secondary components in China from 2017 to 2021 were revealed by using the Weather Research and Forecasting (WRF) model and the Community Multiscale Air Quality (CMAQ) model. In the model domain, the 99th percentile of the shipping-related PM2.5 concentrations was reduced by 19.5 % and then by 35.6 % due to the policy shifts. Ship emissions increased the PM2.5 concentrations up to 3.8 µg m−3 in 2017 and 2.6 µg m−3 in 2021. The areas with high concentration levels widely distributed over offshore waters in 2017 and shrunk to some parts of China's coast in 2021. The contributions of ship emissions to the PM2.5 concentrations over China's main port cities ranged from 3.0 % to 17.4 % in 2017 and 2.5 % to 10.3 % in 2021. In these cities, the change rates of the concentrations of PM2.5, SO42-, NO3-, NH4+, carbonaceous aerosols, V, and Ni related to ship emissions from 2017 to 2021 were −32.7 %, −74.0 %, +11.0 %, −27.5 %, −76.9 %, −90.3 %, and −38.4 %, respectively. NO3- constituted 54.6 % of the shipping-related PM2.5 in 2021. Our findings suggest that it is important to consider both transport pathways and secondary aerosol formation mechanisms to combat the PM2.5 pollution caused by shipping in different regions.