Oil Spill’s Air Effects Probed

The oil spill from the April 20, 2010, explosion of BP’s Deepwater Horizon rig didn’t just foul the waters of the Gulf of Mexico; released gas and evaporating oil also polluted the air above. Now, scientists have used data collected during research flights to estimate how much oil flowed into the Gulf and what was emitted into the air, as well as to solve a vexing atmospheric chemistry puzzle.

The research flights were conducted by the National Oceanic & Atmospheric Administration’s (NOAA) WP-3D aircraft. The plane carried instrumentation to measure CH₄ and other hydrocarbons, CO₂, O₃, nitrogen oxides, and other species. The aircraft flew two flights, on June 8 and June 10, after BP loosely capped the well and started funneling some oil to a recovery vessel.

One study, led by NOAA research chemist Thomas Ryerson, compared air composition upwind and downwind from the wellhead to estimate that, on June 10, the spill emitted 2.6 × 10⁵ kg of volatile organic species, including C₂–C₁₁ alkanes and C₆–C₁₁ aromatics, into the Gulf’s atmosphere (Geophys. Res. Lett., DOI: 10.1029/2011GL046726).

The amount by weight is similar to what can be emitted daily in an urban area such as Houston, notes Kelley Barsanti, managing director of the Center for Aerosol & Climate Change Research at Portland State University. “There are potentially significant impacts on air quality,” she says.

Ryerson and colleagues also used emissions to estimate how much oil leaked from the wellhead and was collected by recovery ships. Those figures are consistent with official estimates, Ryerson says, but he cautions that further work needs to be done before making a quantitative comparison.

Researchers aboard the plane also looked at the formation of secondary organic aerosols—particles formed in the air from gaseous chemicals and play a role in air quality, human health, and climate (C&EN, July 12, 2010, page 32). Scientists historically believed that such aerosols were formed from volatile compounds, but recent research indicates that less volatile species likely play a role.

Aerosol origin is difficult to study in urban settings, because all of the chemicals in question are typically emitted together. During the Deepwater Horizon spill, however, volatile compounds evaporated quickly to form a narrow plume downwind from the wellhead, whereas less volatile species evaporated over a broader swath as the oil dispersed on the surface. During their June flights, Cooperative Institute for Research in Environmental Sciences’ fellow Joost de Gouw and colleagues found secondary organic aerosols distributed across an area consistent with formation from less volatile hydrocarbons, likely C₁₄–C₁₆ compounds (Science, DOI: 10.1126/science.1200320).

Aerosols play a role in climate by absorbing or reflecting light and seeding clouds. Understanding where they come from will allow climate scientists to better model their effects, Barsanti says.

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