Soot From Ships Worse Than Expected

Sea Technology, Aug 2008

Large cargo ships emit more than twice as much soot as previously estimated, and tugboats puff out more soot for the amount of fuel used than other commercial vessels, according to the first extensive study of commercial vessel emissions.

In the Arctic, an increase in soot may contribute to climate change if shipping routes expand, according to the study.

Oceangoing tankers and container ships emit half a gram of soot per kilogram of fuel burned when at dock and slightly less when traveling, according to scientists from the National Oceanic and Atmospheric Administration (NOAA) and the University of Colorado (CU), who conducted the new study. Tugs emit nearly a gram of soot per kilogram of fuel burned-twice as much as any other vessel type, the authors found.

"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 the study's lead author, Daniel Lack of NOAA's Earth System Research Laboratory and the NOAA-CU Cooperative Institute for Research in Environmental Sciences.

Commercial shipping releases roughly 130,000 metric tons of soot per year, or 1.7 percent of the global total, much of it near highly populated coastlines, the authors estimate. In the coming years, global shipping is expected to grow two to six percent annually.

Exceptionally high soot levels from tugboats point to their low-quality fuel - a thick, black tar left over from crude oil after the gasoline and kerosene have been removed. Engine age and maintenance also play a role. 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.

To investigate ship emissions, the researchers observed plumes from commercial vessels in open ocean waters, channels and ports along the U.S. Gulf coastline during the summer of 2006. From the NOAA research vessel Ronald H. Brown, the team measured black carbon emitted by tankers, cargo and container ships; large fishing boats; tugboats; and ferries, many of them in the Houston Ship Channel.

"Commercial shipping emissions have been one of the least studied areas of all combustion emissions," said Lack. "The two previous studies of soot emissions examined a total of three ships. We reviewed plumes from 96 different vessels."