According to the scientists, 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.
International rules requiring clean-burning ship fuels are set to take effect in 2015, intended to minimize the potential hazards dirty ship smoke may pose to human health and the environment.
Some researchers have estimated that these hazards may be responsible for as many as 60,000 deaths worldwide and a cost to the US economy of 500 million dollars 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," he added.
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, according to scientists, although SO4 may be a smaller component in ship emissions, 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.
According to Gerardo Dominguez, a postdoctoral researcher at UCSD, 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.
"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," he said.