When the US government dispatched a special aircraft to the site of the Gulf oil spill last summer, scientists arrived to find a broad cloud of particles, in levels mirroring air pollution in US cities.

But the bulk of the pollutants they detected - organic aerosols linked to both climate change and health hazards - showed up in a wider area than expected from the spill site, where the Deepwater Horizon drilling rig exploded on April 20, 2010.

That meant the compounds that formed those particles had more time to spread out over the water before evaporating, leading the team to a startling conclusion: "Heavier" chemicals in the oil - long thought to take a backseat to more volatile compounds in causing pollution - likely gave rise to the large plume of aerosols over the Gulf.

Because these less-volatile components have often been disregarded in air quality surveys, scientists say this could be the missing link in explaining the presence of high levels of organic aerosols in the atmosphere.

Led by experts from the National Oceanic and Atmospheric Administration (NOAA), the team presented its findings in a paper published Friday in the journal Science.

Joost de Gouw, a NOAA scientist and lead author of the report, told Deutsche Welle that several researchers have highlighted the difficulties in understanding how much organic aerosol is formed in urban environments.

"These heavier molecular weights that aren't measured routinely could provide an explanation for that," he said. "It could be an important mechanism in urban air."

Mission to the Mississippi coast

As of June 2010, crude had been spilling into the Gulf of Mexico for 41 days, despite oil giant BP's efforts to plug the underwater wellhead.

That month, NOAA diverted one of its special aircrafts, the WP-3D Orion fondly dubbed a "high-tech flying chemistry laboratory," to take air quality samples around the site - both near the spill, farther downwind, and along the shorelines of Florida, Alabama and Mississippi.

The goal was monitoring the pollutant effects of both the oil crisis itself and efforts to clean up the spill, including smoke from controlled burns of surface crude.

The team expected to see a narrow cloud of pollutants in close proximity to the spill, formed from highly volatile, fast-evaporating hydrocarbons in the oil.

Instead, they found two clouds. The first plume was located, as expected, near the site of the spill, as the compounds had little time to spread out before evaporating within two hours. But this cloud of particles accounted for less than a third of the components in the oil - leaving the team wondering: what happened to the rest?

A second, larger cloud of organic aerosols provided the answer.

"The aerosol that we saw seemed to come from a much broader area," de Gouw told Deutsche Welle. "So even though we didn't measure the compounds they were formed from, the only logical conclusion was that they were formed by the heavier molecular weights."

De Gouw said the compounds in the Gulf that gave rise to that aerosol are also present in vehicle exhaust from gasoline and diesel trucks - key sources of urban pollution. "We saw up to 25 micrograms per meter cubed, and that's comparable to what you have in polluted air," he said.

The science behind the spill

When the NOAA-led team arrived in the Gulf region for their first flight on June 8, BP was still trying to plug the leaking wellhead, and oil was still spilling into the water.

NOAA research chemist Ann Middlebrook, who took part in the first expedition, recalled seeing smoke from surface burns, and said the team could see the oil from up above in the aircraft.

Meanwhile the spill provided scientists with a unique opportunity to study how air pollutant particles are formed.

In urban areas, pollutants are released all at once from sources like diesel engines, making it difficult to measure the different compounds - which may be more or less volatile - that cause particles to form.

Yet in the Gulf, oil that rose to the surface evaporated at different rates, making for a "distribution of volatilities in different areas," Middlebrook told Deutsche Welle.

"This is the first time that we've been able to attribute organic aerosol forming from compounds that are not typically measured," she said.

In turn, the evidence on how less volatile chemicals created those particles "can help close the gap on understanding how organic aerosol is formed in urban atmospheres," according to Middlebrook.

Building on a theory

The team's discovery bolsters previous research into heavier hydrocarbons' role in pollution. A 2007 study speculated that less volatile compounds could be responsible for the production of organic aerosols, but noted the difficulty of testing that hypothesis.

Neil M. Donahue of Carnegie Mellon University in Pittsburgh was involved in that research four years ago. His colleague, Allen Robinson, also took part in NOAA's monitoring of the Gulf in 2010.

Donahue told Deutsche Welle that people have been accustomed to thinking of particles emitted from sources like diesel engines as "little lumps of non-volatile things," when in reality, many of these heavier compounds wind up as gases.

"The gist of this is that a lot of these things that you wouldn't really think of as being volatile, they actually do evaporate," he said.

And when it comes to the Gulf oil spill, Donahue said that knowledge is also useful from a "bookkeeping" perspective: as a way of accounting for what actually happened to the millions of barrels of crude in the water.

Author: Amanda Price
Editor: Nathan Witkop