article in Nature
Each year tankers, container ships and trawlers emit a quantity of nitrogen oxides (NOx) similar to that released by the entire United States, the study finds1. "A single industry's emissions rival an entire nation," says marine-policy researcher James Corbett of the University of Delaware in Newark.Nitrogen oxides are potent pollutants. Produced in large amounts by ships' burning of heavy diesel fuel, called bunker, they can release complex cocktails of reactive gases, such as ozone, into the atmosphere.
Bunker fuel is nasty stuff - consistancy of tar. It has to be kept hot in order for it to flow.