Study: Oil spill’s impact weighs heavy on atmosphere

Platform supply vessels battle the blazing remnants of the off shore oil rig Deepwater Horizon April 20, 2010

The black smoke that rose from the water's surface during controlled burns of surface oil from the Deepwater Horizon oil spill last year pumped more than 1 million pounds of soot pollution into the atmosphere, according to a new study by the National Oceanic and Atmospheric Administration and the University of Colorado Boulder.

The study, published online this week, showed the amount of black carbon, or soot, is roughly equal to the total black carbon emissions normally released by all ships that travel the entire Gulf of Mexico during a nine-week period, said lead author Anne Perring. Perring is a scientist with the Cooperative Institute for Research in Environmental Sciences -- a joint research center between CU-Boulder and NOAA --and works at the NOAA Earth System Research Laboratory in Boulder.

Black carbon is known to degrade air quality and contribute to warming of the Earth's atmosphere. The new study, published online in Geophysical Research Letters, provides some of the most detailed observations made of black carbon sent airborne by burning surface oil.

"Scientists have wanted to know more about how much black carbon pollution comes from controlled burning and the physical and chemical properties of that pollution. Now we know a lot more," said Perring.

During the 2010 Gulf oil spill, an estimated one of every 20 barrels of spilled oil was deliberately burned off to reduce the size of surface oil slicks and minimize impacts of oil on sensitive shoreline ecosystems and marine life. In response to the spill, NOAA quickly redirected its WP-3D research aircraft to survey the atmosphere above the spill site in June.

During a flight through one of the black plumes, scientists used sophisticated instrumentation on board to characterize individual black carbon particles. NOAA's single particle soot photometer was key to making the black carbon measurements.

Black carbon is the most light-absorbing airborne particle in the atmosphere and the reason for the black color in the smoky plumes that rise from the surface oil fires. Black carbon can also cause warming of the atmosphere by absorbing light. Prolonged exposure to breathing black carbon particles from human and natural burning sources is known to cause harmful human health effects.

During the nine weeks of active surface oil burning, a total of 1.4 million to 4.6 million pounds, or 0.63 million to 2.07 million kilograms, of black carbon was released into the atmosphere of the Gulf of Mexico, the study estimated.

The study also found that the hot soot plumes from the controlled burns reached much higher into the atmosphere than ship emissions normally rise, potentially prolonging the amount of time the black carbon can remain in the atmosphere which would affect where the black carbon ends up.

The researchers also found that the average size of the black carbon particles was much larger than that emitted from other sources in the Gulf region, and that the emitted particles produced were almost all black carbon, unlike other sources such as forest fires that tend to produce other particles along with black carbon.

"The size and makeup of the black carbon particles determine how fast the particles are removed from the atmosphere by various processes, which ultimately affects their impact on climate," said Perring. Larger particles are removed from the atmosphere more quickly and thus have smaller climate impacts. And the same properties of black carbon are important for assessing human health impacts.

Finally, Perring and her colleagues found that of the oil that was burned, 4 percent of the mass was released as black carbon, an important metric rarely observed during cleanup of an oceanic oil spill, which could help guide future decision-making.

The new paper, "Characteristics of Black Carbon Aerosol from a Surface Oil Burn During the Deepwater Horizon Oil Spill," has 15 co-authors from the NOAA Earth System Research Laboratory and from CIRES and can be found at europa.agu.org/?view=article&uri=/journals/gl/gl1117/2011GL048356/2011GL048356.xml&t=gl,perring.