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Commercial ships emit almost half as much particulate matter pollutants into the air as the total amount released by the world’s cars, according to a new study led by the National Oceanic and Atmospheric Administration (NOAA) and the University of Colorado.

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 authors estimate that globally, ships emit 0.9 teragrams, or about 2.2 million pounds, of particle pollution each year.

“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 study lead-author Daniel Lack, a researcher with the NOAA-supported CU Cooperative Institute for Research in Environmental Sciences, based at NOAA’s Earth System Research Laboratory.

Earlier research by one of the study’s authors, James Corbett, of the University of Delaware, linked particle pollution to premature deaths among coastal populations.

During the summer of 2006, Lack and colleagues, aboard the NOAA ship Ronald H. Brown [pictured], analyzed the exhaust from more than 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 sulphates, the same particles associated with diesel-engine cars and trucks which motivated improvements in on-road vehicle fuel standards. Sulphate emissions from ships vary with the concentration of sulphur in ship fuel, the authors found.

Globally, fuel sulphur content is regulated under the International Convention for the Prevention of Pollution from Ships. As a result, some ships use “cleaner”, low-sulphur fuels, whilst others continue to use the high-sulphur counterparts.

Yet, sulphates make up just under half of shipping’s total particle emissions, according to the NOAA-CU study. Organic pollutants and sooty, black carbon, which make up the other half of emissions, are not directly targeted by today’s regulations. A 2008 study by Lack’s team focused exclusively on soot.

Emissions of these non-sulphate particles, the study found, depend on the operating speed of the engine and the amount of lubricating oil needed to deal with wear and tear from burning less-refined fuels.

“Fortunately, engines burning ‘cleaner,’ low-sulfur fuels tend to require less complex lubricants. So the sulphur fuel regulations have the indirect effect of reducing the organic particles emitted,” said Corbett.

One surprising result of burning low-sulphur fuels: while total particle emissions diminish, the time the remaining particles spend in the air appears to increase. It’s while they’re airborne that particles pose a risk to human health and affect climate.

Lack and colleagues found 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.