Photograph courtesy David Valentine, UCSB

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Oil slicks color the sea surface near the Deepwater Horizon spill site in June 2010.

Photograph courtesy David Valentine, UCSB

Gulf Oil Spill Helps Explain Air Pollution Mystery

Source of harmful aerosols identified based on spill studies.

The Gulf oil spill may have helped crack an air-pollution mystery, according to a new study that shows how unexpected compounds can contribute to poor air quality.

While the air pollution produced by the spill itself was regional and relatively small, the study authors note, the secrets unlocked by studying it could have ramifications far beyond the Gulf.

Much of the fresh oil that ended up on the Gulf of Mexico's surface evaporated into the atmosphere and got broken down by sunlight.

But the evaporating oil also created tiny particles known as aerosols, which don't disappear so quickly.

Aerosols can impact human health, including lung and heart function. The particles can even influence climate by scattering incoming solar radiation—thus cooling Earth—and can promote water-vapor condensation and cloud formation.

Worldwide, many of these pollutant particles have mysterious origins, scientists say.

"In polluted urban air, a very large fraction of these aerosols are organic—more than half. But where they come from is very poorly understood," said study co-author Joost de Gouw, an atmospheric scientist at the Earth System Research Laboratory in Boulder, Colorado.

Now, thanks to the disastrous BP spill, de Gouw and colleagues may have found at least part of the answer.

Strange Pollution Plume Spotted

In June 2010 the scientists flew above the oil spill in a "hurricane hunter" research plane, taking measurements of the gases and aerosols in the air above the spill site. (See aerial views of the Gulf oil spill from late April.)

The team found a narrow air pollution plume extending downwind from the spill that contained particles made from the lightest hydrocarbons, so-called volatile organics that evaporate quickly. These hydrocarbons are known to contribute to air pollution.

But a strange, wider plume also appeared—one that was caused by heavier hydrocarbons, which evaporate slowly. These compounds had never been a proven contributor to aerosol pollution when emitted by cars or other sources, according to de Gouw.

Surprisingly, the heavier compounds contributed most to the downwind aerosol pollution from the Gulf spill, he said.

"What we saw was that the first 30 percent of the oil—the most volatile fraction that surfaced—evaporated within something like ten hours," he said.

"The next 10 to 20 percent evaporated on timescales of several days, and those were the compounds that formed this aerosol."

Gulf Oil Spill an Aerosol Experiment

Because water and wind spread out the oil over a large area, the various compounds could be detected as they evaporated over different time periods.

This created a kind of atmospheric map of which compounds contributed particle pollutants, when it happened, and in what quantities, according to the study, published tomorrow in the journal Science.

"Clearly what Joost and his group managed to do is serendipitously come upon an almost perfect natural experiment that spread out this volatility over the sea surface," said Hugh Coe of the U.K.'s University of Manchester.

"Essentially what happens most of the time, with vehicles for example, is that the very volatile material is co-emitted with the less volatile materials," said Coe, who was not involved in the research but who wrote a review of the paper also published in Science.

Because both are emitted at the same time and place, Coe added, "it's very hard to measure the differences between these different organic compounds."

Oil-Spill Study Could Improve Air Quality?

Most air-quality monitoring programs don't detect heavier compounds like those identified as Gulf pollutants, the scientists said.

Likewise, researchers have only recently theorized that heavier compounds could play a role in producing organic aerosol particles.

The new study suggests scientists may be closer to understanding where mysterious organic aerosols in the atmosphere originate worldwide. Such a discovery could eventually help people breathe a bit easier, de Gouw noted.

"The fuels we put into our vehicles—gasoline and diesel—are refined oil, and they contain many of the same compounds that were spilled in the Gulf of Mexico," de Gouw said.

"If further research confirms that these [less volatile hydrocarbon] compounds are important in the formation of aerosols, there may be things we can do to limit their presence in our fuels, as we've done with other harmful compounds."