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Black cloud over estimates of tugboat, cargo ship soot

July 9, 2008 |  2:00 pm

Tugpic1

Tugboats and cargo ships alike are pumping out far more soot than previously thought, according to new findings released today by federal and private scientists.

Stubby tugboats plying busy harbors puff out more sooty black carbon than any other commercial vessels, and large cargo ships emit more than twice as much soot as previously estimated, according to the first broad study of commercial vessel emissions, say researchers from the National Oceanic and Atmospheric Administration and the University of Colorado. The findings are laid out in the July 11 issue of the journal Geophysical Research Letters.

"Commercial shipping emissions have been one of the least studied areas of all combustion emissions," said lead author Daniel Lack, of NOAA's Earth System Research Laboratory. "The two previous studies of soot emissions examined a total of three ships. We reviewed plumes from 96 different vessels."

Lack and his colleagues observed and measured black carbon plumes emitted by tankers, cargo and container ships, large fishing boats, tug boats and ferries in open ocean waters, channels and ports along the southeast United States and Texas during the summer of 2006.

The ports of Los Angeles and Long Beach, the nation's busiest, along with nearby Santa Barbara Channel, are regularly used by hundreds of polluting vessels, and local and state air regulators say marine vessel emissions are a major piece of Southern California's continuing air pollution woes.

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A major culprit is the dirty "bunker" fuel used by such vessels, literally the dregs of other, cleaner fuels, which can contain levels of harmful pollutants thousands of times higher than legal limits inside the U.S. Major cargo shipping companies say they want to clean up their act, but have fought efforts at domestic regulation, saying they are only subject to international standards. The U.N.'s International Maritime organization is inching toward tightening up allowable amounts but has yet to do so.

A separate 2007 study by American and German scientists linked particle pollution from shipping to tens of thousands of premature deaths each year. Soot makes up a quarter of that pollution, said Lack.

Tugboats have a disproportionate impact on air quality because they travel within ports, emitting potentially harmful particles near populous urban areas, according to the authors.

"Tugboats are a huge source of black carbon that may be under-reported or not reported at all in emissions inventories compiled by ports," said Lack.

On a global scale, soot's small, dark particles absorb sunlight, create haze and affect how clouds form and make rain, further altering a region's heat balance, according to the new study.  In the Arctic, an increase in soot may contribute to climate change if shipping routes expand through waters there, according to the study.

-- Janet Wilson

Photo: Tugboat emitting sooty smoke in San Francisco Bay. Credit: Michael Macor, AP Photo/San Francisco Chronicle

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