Ocean-going ships emit large amounts of air pollutants such as nitrogen oxide (NO.sub.x) and particulate matter. NO.sub.x emitted from ships can be converted to nitrous acid (HONO) and nitryl chloride (ClNO.sub.2 ), which produce hydroxyl (OH) and chlorine (Cl) radicals and recycle NO.sub.x, thereby affecting the oxidative capacity and production of secondary pollutants. However, these effects have not been quantified in previous investigations of the impacts of ship emissions. In this study, a regional transport model (WRF-Chem) revised to incorporate the latest HONO and ClNO.sub.2 processes was used to investigate their effects on the concentrations of RO.sub.x (RO.sub.2 + HO.sub.2 + OH) radicals, ozone (O.sub.3 ), and fine particulate matter (PM.sub.2.5) in Asia during summer. The results show that the ship-derived HONO and ClNO.sub.2 increased the concentration of RO.sub.x radicals by approximately 2-3 times in the marine boundary layer. The enhanced radicals then increased the O.sub.3 and PM.sub.2.5 concentrations in marine areas, with the ship contributions increasing from 9 % to 21 % and from 7 % to 10 % respectively. The largest RO.sub.x enhancement was simulated over the remote ocean with the ship contribution increasing from 29 % to 50 %, which led to increases in ship-contributed O.sub.3 and PM.sub.2.5 from 21 % to 38 % and from 13 % to 19 % respectively. In coastal cities, the enhanced levels of radicals also increased the maximum O.sub.3 and averaged PM.sub.2.5 concentrations from 5 % to 11 % and from 4 %-8 % to 4 %-12 % respectively. These findings indicate that modelling studies that do not consider HONO and ClNO.sub.2 can significantly underestimate the impact of ship emissions on radicals and secondary pollutants. Therefore, it is important that these nitrogen compounds be included in future models of the impact of ship emissions on air quality.