A new study has determined that commercial ships in the world emit almost half as much particulate pollution into the air as the total amount released by cars.
The study is the first to provide a global estimate of maritime shipping's total contribution to air particle pollution based on direct measurements of emissions.
The researchers estimated that worldwide, ships emit 0.9 teragrams, or about 2.2 million pounds, of particulate pollution each year.
Shipping also contributes almost 30 percent of smog-forming nitrogen oxide gases.
"Since more than 70 percent of shipping traffic takes place within 250 miles of the coastline, this is a significant health concern for coastal communities," said lead author Daniel Lack, a researcher at the National Oceanic and Atmospheric Administration (NOAA)'s Earth System Research Laboratory in Boulder, Colorado.
Commercial ships emit both particulate pollution and carbon dioxide. Carbon dioxide from ships makes up roughly three percent of all human-caused emissions of the gas.
During the summer of 2006, Lack and colleagues, aboard the NOAA ship Ronald H. Brown, analyzed the exhaust from over 200 commercial vessels, including cargo ships, tankers and cruise ships, in the Gulf of Mexico, Galveston Bay, and the Houston Ship Channel.
The researchers also examined the chemistry of particles in ship exhaust to understand what makes ships such hefty polluters.
Ships emit sulfates - the same polluting particles associated with diesel-engine cars and trucks that prompted improvements in on-road vehicle fuel standards.
Sulfate emissions from ships vary with the concentration of sulfur in ship fuel.
As a result of the cap, some ships use "cleaner," low-sulfur fuels, while others continue to use the high-sulfur counterparts.
Lack and colleagues find that the organic and black carbon portion of ship exhaust is less likely to form cloud droplets.
As a result, these particles remain suspended for longer periods of time before being washed to the ground through precipitation.