The Deepwater Horizon spill gushed crude oil into the Gulf of Mexico from April 20 to July 15 last year. It was a devastating disaster that had an overwhelmingly negative effect on the environment. However, in a bittersweet way, the oil spill also provided scientists with a good opportunity. It let them study the profile and chemical origin of atmospheric aerosols, which affect climate, air quality, cloud formation, and ozone.

Atmospheric aerosols (particles with radii of 1µm or less) can reflect sunlight and provide a surface area for chemical reactions to occur (e.g., they enhance chlorine’s ability to destroy ozone), and water condenses on their surface to modify cloud particles. An understanding of aerosol composition and formation is absolutely essential for predicting the impact of pollution and formulating sound environmental regulations. Unfortunately, there are still gaps in our knowledge regarding atmospheric aerosols. In particular, we don't have a thorough understanding of what are called secondary organic aerosols.

Organic aerosols makeup a large fraction of atmospheric aerosols, and secondary organic aerosols are the major component of the aerosols in polluted air. Secondary organic aerosols aren’t directly released from a source of pollution, like the oil in the spill; instead, they form in the atmosphere through the chemical modification of the primary aerosols that are directly released from the source.

The problem is that there are different types of primary aerosols, and scientists aren’t sure which types are responsible for creating secondary aerosols. Primary organic aerosols are separated into three categories that are ranked in order of decreasing volatility: volatile organic compounds, intermediate volatile organic compounds, and semi-volatile organic compounds.

Before the oil spill, scientists had a tough time studying the three primary organic aerosols separately, as all three are usually released together. However, the Deepwater Horizon oil spill provided a unique field study opportunity, as the time-dependent spread of oil across the ocean surface created a natural separation.

A NOAA (National Oceanic and Atmospheric Administration) research aircraft did flights on June 8 and 10, 2010 to study the atmospheric impacts of the oil spill. Joost de Gouw and his coworkers looked at data collected by the NOAA aircraft to find the source of secondary organic aerosols. Their observations appear in a recent issue of Science.

At the time of the flights, the oil coating the ocean surface directly above the wellhead was fresh, and it released mostly volatile organic compounds. De Gouw found that 23 percent of the oil spill mass evaporated within two hours. The NOAA aircraft picked up a narrow atmospheric plume corresponding to the volatile organic compounds a short distance downwind from the spill site.

The rest of the oil flowed and spread across the ocean. The farthest reaching oil, being the oldest, released mostly semi-volatile organic compounds, as those are the least volatile. Oil that was in middle range from the wellhead mostly released intermediate volatile organic compounds.

The NOAA aircraft picked up a wider plume of aerosols farther downstream from the wellhead that mainly consisted of secondary organic aerosols.

To determine the source of these secondary organic aerosols, De Gouw and his team examined the volatility distribution of the oil, the hydrocarbon composition of the oil, and the evaporation rates of the hydrocarbons. They then performed simulations of surface oil trajectories. Their calculations suggest that the precursors to the secondary organic aerosols evaporated 10 to 100 hours after being spilled.

Furthermore, they estimate that it would have taken three hours for the primary aerosols to be chemically converted into their secondary products. Overall, this meant that the intermediate and semi-volatile organic compounds are the precursors to the secondary organic aerosols.

These results indicate that intermediate and semi-volatile organic compounds should be carefully measured and studied, as they might be the main contributors to secondary organic aerosols. They also validate recent theoretical models of atmospheric aerosols.

While it's ideal to have more than one source of data to make conclusions, we might have to settle for this single set of field observations until better techniques become available to parse mixtures of primary organic aerosols. No one wants another oil spill just so grad students can get a bit more data.

Science, 2011. DOI: 10.1126/science.1200320 (About DOIs)

Listing image by Department of Ecology State of Washington