Impacts of maritime shipping on air pollution along the U.S. East Coast

Abstract. Air pollution is considered a leading threat to human health in the United States (U.S.) and worldwide. An important source of air pollution in coastal areas is the globally increasing maritime shipping traffic. In this study, we take a high-resolution modeling approach to investigate the impacts of ship emissions on concentrations of various atmospheric pollutants, under the meteorological conditions and emissions of the year 2018. We utilize the Comprehensive Air Quality Model with extensions (CAMx) to simulate transport, diffusion, and chemical reactions, and the Weather Research and Forecasting (WRF) model to provide the meteorological inputs. We focus on four criteria pollutants – fine particulate matter with a diameter smaller than 2.5 μm (PM_2.5), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), and ozone (O₃) – as well as nitrogen oxide (NO), and we calculate their concentrations in the presence and absence of the ship emissions along the U.S. East Coast, particularly in the proximity of major ports.

We find that ship emissions increase the PM_2.5 concentrations over the ocean and sparse areas inland. The 98-th percentile of the 24-hour average PM_2.5 concentrations (the "design value'' used by the U.S. Environmental Protection Agency) increased by up to 3.2 μm m⁻³ in some coastal areas. In addition, ships contribute significantly to SO₂ concentrations, up to 95 % over the Atlantic and up to 90 % over land in coastal states, which represents a ~45 ppb increase in the SO₂ design values in some states. The 98-th percentile of the hourly NO₂ concentrations also increased by up to 15 ppb at the major ports and along the shore. In addition, we find that the impact of shipping emissions on O₃ concentrations is not uniform, meaning that ships affect ozone pollution in both positive and negative ways. Over the ocean, O₃ concentrations were significantly higher in the presence of ships, but in major coastal cities, O₃ concentrations decreased in the presence of ships. Our simulation results show that ships emit significant amounts of fresh NO in the atmosphere, which then helps scavenge O₃ in VOC-limited areas, such as major ports. By contrast, over the ocean (NO_x-limited regime), enhanced NO_x (NO₂ + NO) concentrations due to ships contribute to the formation of O₃ and therefore enhance O₃ concentrations. Overall, due to the dominant southwesterly wind direction in the region, the impacts of ships on air pollutants mainly remain offshore. However, in coastal states near major ports, the impacts are significantly important.