New organic aerosol science on the horizon

An estimated 68,000 barrels of oil were released to the sea surface every day during the five month long leak – in addition to large amounts of methane. The environmental disaster that was the Deepwater Horizon (DWH) oil spill in 2010 has however provided scientists with an unexpected opportunity to learn about the formation processes of atmospheric particles, known as aerosols.

Much organic aerosol in the atmosphere is “secondary”, that is to say generated from reactions of gases, rather than emitted directly as a particle. These gases come from natural sources like forests or the ocean, and from anthropogenic activities such as fuel use. As far as we know, secondary organic aerosol (SOA) generation requires particularly volatile organic precursor gases; the volatility of a compound describes how keen its molecules are to be in the gas phase. However, the formation of SOA has been observed to be much more efficient in polluted air than can be explained by the presence of these volatile gases alone; at the moment scientists don’t know whether this discrepancy could be attributed to the reactions of less volatile compounds that are also emitted.

The DWH oil leakage provided an opportunity to investigate this since compounds of different volatilities evaporated from different regions of the slick. Scientists from a number of US institutions, including the National Oceanic and Atmospheric Administration (NOAA) and the University of Colorado, collaborated to collect and analyse data from two flights over and downwind of the oil slick in June 2010. As expected, they were able to detect a narrow plume of volatile gases in the atmosphere that had evaporated from the surface of the water.

Interestingly, a much wider plume of organic aerosol was observed downwind of the spillage; suggesting that since the less volatile compounds take longer to evaporate, the spilled oil had time to travel further and generate aerosol particles further downwind.

For the first time, scientists were able to make observations in a location where compounds of different volatilities had been spatially separated naturally and these results suggest that less volatile compounds could be much more important to aerosol formation than originally thought.

Whilst we can measure the emission of precursor gases, and indeed the concentration of the resulting aerosol, if we want to understand how the atmosphere might change in the future, we need to really understand the formation processes involved – and perhaps we are one step closer.

© Cat Scott |

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