Ocean freighters spew twice as much soot into the air as previously believed and tugboats are among the worst maritime offenders when it comes to air pollution, according to a new government study.
Soot is comprised of tiny particles of black carbon, which become airborne during the burning of diesel and other fossil fuels, wildfires and the burning of vegetation for agricultural purposes. Soot can settle in human lungs, causing asthma and premature deaths; researchers also believe soot emissions may contribute to global warming.
The study by scientists at the National Oceanic and Atmospheric Administration and the University of Colorado estimated that commercial ships release 130,000 metric tons of soot per year, 1.7 percent of the global total, into the air. They said that figure could rise as global shipping traffic increases.
"Commercial shipping emissions have been one of the least studied areas of all combustion emissions," said Daniel Lack, a NOAA scientist who led the study.
Researchers studied plumes of black smoke from 96 different ships, including cargo freighters, tankers, ferries, large fishing boats and tugs. Most of the boats were sampled in the Houston Ship Channel.
The results were significant for the Great Lakes for two reasons: Hundreds of diesel-powered freighters, tugboats and ferries ply the lakes; and some coastal communities in the region, including Ottawa County, violate the federal air quality standard for soot.
James Weakley, executive director of the Lake Carriers Association, said lake freighters that operate exclusively within the Great Lakes emit less soot than ocean ships. The reason: Lake freighters burn a lighter grade of diesel fuel, which produces less soot than the thicker fuel ocean freighters use.
"A straight comparison between an oceangoing ship and a laker is not a fair comparison," said Weakley, whose group represents 63 U.S.-flagged ships on the Great Lakes.
Tugboats emit nearly a gram of soot per kilogram of fuel burned -- twice as much as any other vessel type, according to the researchers. They attributed the disproportionate amount of soot emissions from tugs to the thick, tar-like fuel the small but mighty vessels burn in their engines.
"Tugboats are a huge source of black carbon that may be underreported or not reported at all in emissions inventories compiled by ports," Lack said in a press release.
Ocean freighters emit a half-gram of soot per kilogram of fuel burned while docked and slightly less when traveling. Lack said that figure was twice the amount of soot emissions researchers previously assigned to ships.
Results of the study were published in the July 11 edition of the journal Geophysical Research Letters.
Weakley said lake freighters, which are equipped with bow thrusters to improve maneuverability, use tugboats less often than ocean freighters. He said freighters are the cleanest way to move large volumes of cargo; freighters haul about 200 million tons of cargo on the Great Lakes annually.
"If you take into account the economies of scale and the tons of cargo moved per mile of fuel consumption, our ships move a ton of cargo from Duluth to Detroit on one gallon of fuel," Weakley said. "That's a distance of about 800 miles on one gallon of fuel -- you couldn't get that kind of mileage in a Toyota Prius."
John Taylor, a marketing professor and transportation expert at Grand Valley State University, said studies have shown that freighters emit more soot, sulfur dioxide, carbon dioxide and volatile organic compounds than trains, per ton of cargo carried. Semi-trucks emit more pollutants, per ton of cargo, than freighters or trains, according to 2004 study in the journal Marine Economics and Logistics.
"Ships are pretty big air polluters on an emissions-per ton basis," Taylor said. "For sulfur dioxide and soot emissions, ships are way worse than trains."
The state of California is leading national efforts to reduce air pollution from ships, which can contribute to ground level ozone -- known commonly as smog -- in coastal areas. Other sources of soot emissions include vehicles, power plants and factories that burn gasoline, diesel or coal.
The U.S. Environmental Protection Agency recently adopted new air emission standards for ships and trains. Those rules will reduce soot emissions by 90 percent and nitrogen oxide emissions 80 percent by 2030.