To a conversation with study lead author and CIRES researcher Joost de Gouw. 6:30
When a team of researchers from NOAA and the Cooperative Institute for Research in Environmental Sciences (CIRES) raced to the scene of the BP Deepwater Horizon oil spill to assess the disaster’s impact on air quality, they found more than they expected. A significant fraction of the oil that surfaced had evaporated. Also, measurements taken onboard the NOAA WP-3D aircraft revealed that organic aerosols – a form of air pollution – formed from the oil vapors. Aside from the common culprits that create organic aerosols, the researchers discovered a new set of chemicals that contribute to diminished air quality – chemicals that also exist in urban environments.
“It was very clear that the aerosols were formed from compounds not currently measured,” said CIRES Fellow Joost de Gouw, lead author of a new paper published in the journal Science March 10. Discovering these previously unknown sources of aerosols could improve scientists’ understanding of air pollution and how to regulate it in the future, said de Gouw. “This really shows that we need to start paying more attention to these compounds,” he said.
Tiny particles, dire consequences
Aerosols are microscopic particles suspended in the air – in polluted U.S. cities about half of the air pollution particles consists of organic material. Organic aerosols are linked to asthma, cardiovascular disease, and even premature death. But scientists only know the origin of a small fraction of the organic aerosols. “The problem has been that we know there are more organic aerosols than we can account for,” de Gouw said. “So there is a lot of discussion in the literature on where this organic material comes from.”
The team’s research on the air quality impacts of the oil spill shed new light on this mystery. In early June, a team of scientists from NOAA and CIRES arrived at the scene of the spill to assess how much of the oil was evaporating into the atmosphere, and whether this oil was a concern for air quality. Using a Lockheed WP-3D Orion aircraft, the team flew for about 14 hours directly over and downwind of the oil spill. Instruments on board the aircraft measured many types of air pollution particles, including organic aerosols, and the chemicals from which they are formed in the marine boundary layer – the layer trapping most pollutants.
Based on the current understanding that the most volatile components of oil form air pollution particles, de Gouw and his colleagues knew where they expected to see the aerosols: exactly where they saw the most volatile components of the oil evaporate. “Our instrument showed a very narrow plume of oil compounds downwind from the spill site,” said de Gouw – so the scientists expected to see the organic aerosols in this same region.
Explaining the unexpected
But this is not what the scientists observed.
“We detected particles being formed, but over a much wider area,” said de Gouw. “So that was a big surprise.”
The scientists realized that other compounds, aside from the highly volatile components of the oil, had to be contributing to the air pollution. Because they recorded organic aerosols over a broad area, they concluded the heavier, less volatile compounds that are slower to evaporate were also forming aerosols. The lighter, highly volatile compounds evaporate quickly from a small area of the ocean and form air particles in that region only, de Gouw said. But heavier compounds have a chance to spread out before they evaporate, giving rise to the much wider band of organic aerosols that the team detected.
In 2007, other atmospheric scientists had proposed that heavier or “less volatile” components could—in theory—help to create organic aerosols. But it had proven to be near impossible to study this process in the real world, de Gouw said. “The problem is that volatile and less volatile species are emitted at the same time from the same combustion sources, so we could not study them separately in the atmosphere,” de Gouw said. “Until Deepwater Horizon.”
When de Gouw and his colleagues ran a series of models showing how spilled oil spread across the water, and how long it should take for various heavy, medium, and light fractions to evaporate, the conclusion was clear: Heavier compounds from the oil that are slower to evaporate were the culprit.
The finding is not one that is specific to catastrophic oil spills, de Gouw said. The oil was not a thick sludge but more similar to the highly refined oil that is used in cars or factories, he said. That means the same heavier compounds that contributed to air pollution over the Gulf Oil Spill, also contribute to air pollution in urban environments.
But these compounds are not measured in most air-quality monitoring programs designed to capture the conventional contributors to poor air quality. “This chemistry could be a very important source of aerosols in the urban United States and elsewhere,” de Gouw said. “What we learned from this study will help us to improve air quality understanding and prediction.”
Co-authors of the new paper, “Organic Aerosol Formation Downwind from the Deepwater Horizon Oil Spill,” include Ann Middlebrook1, Carsten Warneke1,2 Ravan Ahmadov1,2, Elliot Atlas3, Roya Bahreini1,2, Donald Blake4, Charles Brock1, Jerome Brioude1,2, David Fahey1, Fred Fehsenfeld1,2, John Holloway1,2, Matthiew Le Henaff3, Richard Lueb5, Stuart McKeen1,2, James Meagher1, Daniel Murphy1, Claire Paris3, David Parrish1, Anne Perring1,2, Ilana Pollack1,2, A.R. Ravishankara1, Allen Robinson6, Thomas Ryerson1, Joshua Schwarz1,2, J. Ryan Spackman1,2, Ashwanth Srinivasan3, and Laurel Watts.1,2
1 NOAA Earth System Research Laboratory, Chemical Sciences Division, Boulder, CO
2 Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO
3 University of Miami, Miami, FL
4 University of California, Irvine, CA
5 National Center for Atmospheric Research, Boulder, CO
6 Carnegie Mellon University, Pittsburgh, PA
Kathleen Human, CIRES, 303-735-0196, firstname.lastname@example.org