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Smoke Smudges a Megacity's Air

Sulfate Lens Enhances Climate Warming Properities of Atmospheric Soot

Smoke from ships contributes much of the sulfate in fine particulate matter found in coastal California air.

By Kim McDonald

Ah, nothing like breathing clean coastal air, right? Think again.
Cargo ships burn bunker oil, the dregs that remain after refining cleaner fuels. Source: Treehugger

Chemists at UC San Diego have measured for the first time the impact that dirty smoke from ships cruising at sea and generating electricity in port can have on the air quality of coastal cities.

The scientists report in the September 2, 2008 issue of the journal Proceedings of the National Academy of Sciences that the impact of dirty smoke from ships burning high-sulfur fuel can be substantial, on some days accounting for nearly one-half of the fine, sulfur-rich particulate matter in the air known to be hazardous to human health.

Their results have particular significance for the state of California, which since July 2009 has required that all tankers, cargo and cruise ships sailing into a California port switch to more expensive, cleaner-burning fuels when they come within 24 miles of the coast. Similar international rules requiring clean-burning ship fuels are set to take effect in 2015.

While those regulations are intended to minimize the potential hazards dirty ship smoke may pose to human health and the environment, which some researchers have estimated may be responsible for as many as 60,000 deaths worldwide and a cost to the U.S. economy of $500 million a year, no one knows the actual impact of ship smoke. The reason is that air quality experts have been unable to quantify the specific contribution of ship smoke to the air pollution of coastal cities--until now.

"This is the first study that shows the contribution of ships to fine particulates in the atmosphere," said Mark Thiemens, Dean of the Division of Physical Sciences and a professor of chemistry and biochemistry at UCSD who headed the research team. "Ships are really unregulated when it comes to air pollution standards. What we wanted to find out was the contribution of ships to the air pollution in San Diego. And what we found was a surprise, because no one expected that the contribution from ships of solid sulfur-rich particles called primary sulfate would be so high."

Primary sulfate, or SO4, is produced when ships burn a cheap, sulfur-rich fuel called "bunker oil." Most of the sulfur emitted by ships burning bunker oil is released as sulfur dioxide, or SO2, a gaseous pollutant which is eventually converted to sulfate in the atmosphere. But although SO4 may be a smaller component in ship emissons, the scientists say, these primary sulfate particulates are particularly harmful to humans, because they are especially fine microscopic particles, less than 1.5 microns or millionth of a meter in size. As a result, they can travel extremely long distances because they stay in the atmosphere for longer periods and, unlike bigger dust grains and particles that are removed by the body when breathed, remain in the lungs.

"The importance of primary sulfate is usually ignored in assessments of the impact of ship emissions on air quality because less than 7 percent of all sulfur emitted by ships is found in primary sulfate particles," said Gerardo Dominguez, a postdoctoral researcher at UCSD and the first author of the paper. "But our results suggest that this component of ship emissions is important and should not be ignored in the future. Knowing how much sulfate from ships is in the air will also allow us to better understand what happens to the other 93 percent of sulfur emitted by ships."

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