Significant reductions of urban daytime ozone by extremely high concentration NOₓ from ship's emissions: A case study

In this paper, a four nested Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model was applied to analyze ozone (O₃) concentration reductions under the influence of extremely high NO_x concentrations emitted from ships at the Hankou Jiangtan (HJ) station adjacent to the Yangtze River in Wuhan, China. By adding NO emissions along the waterway for the sensitivity experiments, the model successfully reproduced two cases of high NO_x concentrations on April 9 and April 28, 2020, and one case of persistent and extremely high NO_x concentrations on the night of May 3, 2020. During daytime, strong photochemical reactions generated new O₃, while the high concentration NO_x from ship emissions continuously reduced O₃ through catalytic cycling. This led to the O₃ concentrations at the HJ station being consistently lower than the surrounding stations in the afternoon, with the high NO_x plume dropping O₃ concentrations by 51 ppb in 1 h, and NO₂ concentrations up to 110 ppb. The presence of large amounts of fresh NO (up to 265 ppb) in the ship emission plumes rapidly reduces O₃ concentrations through titration reactions during night, resulting in a wide area of low O₃ concentrations (2–4 ppb) over a long period of time. This study demonstrates that the pollution and impact of emissions from ships traveling in waterways on the atmosphere of the surrounding area cannot be ignored.