Deepwater Air Pollution Study Yields Surprising Results

Scientists from the University of Miami Rosenstiel School of Marine & Atmospheric Science worked with National Oceanic & Atmospheric Administration (NOAA) researchers to find two plumes of oil-based pollutants downwind of the BP Deepwater Horizon oil spill.

In a study published in the journal Science this week, the team of researchers discovered a new mechanism by which the crude oil traveled from the sea surface to the atmosphere. Although the mechanism was predicted four years ago, the discovery now confirms the importance of this mechanism and could change the way urban air quality is understood and predicted.

“We were able to confirm a theory that a major portion of particulate air pollution is formed from chemicals that few are measuring, and which we once assumed were not abundant enough to cause harm,” Joost de Gouw, a scientist at the NOAA and a co-author of the study, told Reuters in a statement.

As scientists expected, the lightest chemicals in the oil evaporated within hours. But what surprised them was that heavier compounds — ones with more carbon atoms per molecule — in the oil took longer to evaporate, spread out further and contributed to most of the formation of air pollution particles.

The NOAA-led team collected data of atmospheric gas and aerosol concentrations during two flights, one on June 8 and one on June 10, aboard a specially equipped WP3 Orion aircraft.

“By having such a well-defined source of the evaporating oil we were able to investigate how aerosols form in the atmosphere,” said UM Rosenstiel School Professor of Marine and Atmospheric Chemistry Elliot Atlas, a co-author of the paper.

The data revealed that two plumes of hydrocarbons were released into the air by the surface oil and from smoke from the burning of oil associated with the cleanup. The first was a narrower 1.8-mile-wide hydrocarbon plume downwind from the spill site. This was the result of “direct evaporation of fresh oil on the sea surface,” the team suggested.

The second, a larger 24-mile-wide plume, contained higher concentrations of organic aerosols and was “formed from vapors released from the oil and the condensation of their atmospheric oxidation products onto existing particles,” according to the study’s authors.

The researchers observed that methane and other light hydrocarbons dissolved in the water column, while other, less volatile components of crude oil, made their way to the surface and into the atmosphere.

Claire Paris, a UM Rosenstiel School assistant professor of Applied Marine Physics, working with another team of researchers, produced numerical simulations of the oil spill during and following the airborne measurements by the NOAA-led team.

“These simulations of fresh oil reaching the sea surface and aged oil spreading in a wider area downwind are key to understanding the evaporation processes of more or less volatile hydrocarbon compounds,” said Paris, a biophysical modeler. “The model predictions that included oil behavior, advection, and wind drift helped link the measured organic aerosols to their source and mechanism of emission.”

Paris, the research team, and Meteorology and Physical Oceanography Research Associate Professor Villy Kourafalou were awarded a National Science Foundation RAPID grant in July 2010 to model the three-dimensional dynamics of the oil spill and assess its fate and extent.

The study provides researchers with a better understanding of the effects of air pollutants, and their secondary chemical counterparts on the environment, human health and climate change.

“The study also shows the benefit of having the right scientific capabilities available for rapid hazard response,” said Atlas. “It was fortuitous that we were able to get out there quickly with the necessary instruments and expertise, which turned out to be very useful.”

Organic aerosols (OA) make up nearly 50 percent of the air pollution particles in polluted US cities. Air pollution particles can damage lung and heart function, and also affect the climate, with some aerosol, including OA, partially offsetting the warming from greenhouse gases by reflecting incoming sunlight or changing cloud properties, and other aerosol amplifying warming by increasing the amount of sunlight absorbed in the atmosphere.

De Gouw said his team knew where to expect OA particles downwind from the oil spill based on conventional understanding. OA forms when the most lightweight (volatile) components of surface oil evaporate, undergo chemical reactions, and condense onto existing airborne particles.

Roughly 30 percent of the surface oil fell into this volatile category, evaporating into the atmosphere within hours, the new analysis showed. That gave it little time to spread out, so emissions came from the area immediately surrounding the spill. A steady wind drew those emissions into a thin, linear streak of pollution in which organic aerosol was expected to form.

“But that’s not what we saw,” said De Gouw. “We saw this very broad plume of organic aerosol instead.” OA levels in that plume were similar to levels found in U.S. urban air.

So he and his colleagues set out to figure out what else might have contributed to the pollution particles. Atmospheric scientists, in 2007, had proposed that heavier (less-volatile) components could theoretically help to create OA, but it had proven to be near impossible to study this process in the real world.

“The problem is that the heavier and lighter species are emitted at the same time from the same sources, so we could not study them separately in the atmosphere until Deepwater Horizon,” said De Gouw.

Since the heavier components of oil take longer to evaporate, they have more time to spread on the surface farther from the spill source than the lightweight particles. When De Gouw and his colleagues ran a series of models showing how the oil spread across the Gulf, and how long it should take for various particles to evaporate, the results were clear.

The less-volatile compounds measured by instruments on the aircraft that took the data were the culprit. These heavier compounds are not measured in most air quality monitoring systems, which were designed to capture the conventional contributors to poor air quality. The findings could help scientists understand why there is more organic aerosols in the polluted atmosphere than they can explain.

“This chemistry could be a very important source of aerosol in the United States and elsewhere,” De Gouw told Reuters. “What we learned from this study will actually help us to improve air quality understanding and prediction.”

The study, titled “Organic Aerosol Formation Downwind from the Deepwater Horizon Oil Spill” was published in the March 11 issue of the journal Science.

BP’s Deepwater Horizon oil rig exploded on 20 April 2010, killing 11 people and spilling millions of gallons of oil into the Gulf of Mexico, disrupting the regional economy and ecosystem for more than 6 months, as well as causing air pollution.

Image Caption: The oil slick, seen from the window of the Lockheed WP-3D Orion aircraft. Best known as one of NOAA’s “hurricane hunters,” the plane was outfitted with chemistry instruments for a mission in California during the spring of 2010. NOAA diverted it to the Gulf for several days in June, as part of a multi-agency effort to assess the atmospheric consequences of the spill. Credit: Daniel Lack, CIRES and NOAA

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