Emissions from ocean-going ships may substantially boost acid rain on shore and account for more than a quarter of the ground-level ozone in some coastal areas.
Emissions from ocean-going ships may substantially boost acid rain on shore and account for more than a quarter of the ground-level ozone in some coastal areas, according to a new study. Many commercial vessels, especially long-haul cargo ships, spend much of their time on international waters. Nevertheless, exhaust from these ships contributes to onshore pollution. Studies have shown about 70 per cent of shipping occurs within 400km of land.
To assess the amount of pollution spewed by ships, as well as pinpoint where the emissions occur, Stig Dalsøren, an atmospheric scientist at the University of Oslo and his colleagues analysed data gathered worldwide during 2004.
In 2004, the world's merchant fleet included almost 91,000 ships that each weigh at least 100,000 tonnes, Dalsøren said. To tally the emissions from those vessels, the team divided the ships into 15 categories ? everything from tugboats and trawlers to cruise ships and supertankers ? subdivided each category into seven weight ranges, and estimated emissions that each type of ship would produce while cruising or while idling at ports.
Then, the researchers used information about the routes sailed by about 32,000 ships in 2004, including time spent at ports, to determine where emissions were produced. The team reported its findings online last week.
Together, ships worldwide burned about 217 million metric tonnes of fuel in 2004, about five per cent of which was consumed while in port, Dalsøren said.
Because much of that fuel was sulphur-rich diesel, emissions included more than 16 million tonnes of sulphurdioxide. That gas, plus the various nitrogen oxides, or NOx ? gases in the engine exhaust ? reacts with moisture in air to produce acid rain.
Worldwide, ships account for about 11 per cent of acid rain due to NOx emissions and about 4.5 per cent of acid rain due to sulphurdioxide emissions, the team estimates. In Singapore, the world's most visited port, about 15 per cent of the sulphurdioxide in air comes from ships.
In some coastal areas with little industrial activity but a lot of ships passing nearby, such as the northwestern coasts of North America and Scandinavia, about half of the acid rain there may stem from ship emissions.