Impacts of shipping emissions on ozone pollution in China

Abstract. With the Two Phases of Clean Air Actions in China, the shipping sector has emerged as a significant source with substantial emission reduction potential compared to land-based anthropogenic sectors. Therefore, understanding the contribution of shipping emissions to ozone (O3) pollution is therefore essential for advancing China's air pollution control efforts. In this study, a coupled framework including a chemical transport model with machine learning techniques was developed to systematically investigate the interannual and seasonal impacts of shipping emissions on O3 concentrations across China during the period from 2016 to 2020, and explore mechanisms by which shipping emissions influence O3 formation. Results indicate that shipping emissions increase O3 concentrations by a five-year average of 3.5 ppb nationwide, exhibiting significant spatial and temporal heterogeneity across different regions and seasons. Although significant differences exist between the emissions of ocean vessels and inland vessels, their contributions to O3 formation are becoming increasingly comparable. Solely controlling shipping emissions may has limited impact on O3 mitigation. Instead, coordinated reductions targeting both shipping and land-based anthropogenic sources, along with region-specific and targeted emission control strategies, are critical for achieving substantial improvements in O3 pollution mitigation.