Oil spills quickly photooxidise and produce tar in the ocean

Researchers studying the 2010 Deepwater Horizon (DWH) oil spill have found that the oil photooxidised within hours, forming tar in the nearby coastal areas.

A new study lead by scientists at the University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science suggests that, under realistic environmental conditions, oil drifting in the ocean after the DWH oil spill photooxidised into persistent compounds within hours, instead of over long periods of time, as was thought during the 2010 spill. This is the first model result to support the new paradigm of photooxidation that emerged from laboratory research.

After an oil spill, oil droplets on the ocean surface can be transformed by a weathering process called photooxidation, which results in the degradation of crude oil from exposure to light and oxygen into new by-products over time. Tar, a by-product of this weathering process, can remain in coastal areas for decades after a spill. Despite the significant consequences of this weathering pathway, photooxidation was not considered in oil spill models or the oil budget calculations during the spill.

The UM Rosenstiel School research team developed the first oil-spill model algorithm that tracks the dose of solar radiation that oil droplets receive as they rise from the deep sea and are transported at the ocean surface. The authors found that the weathering of oil droplets by solar light occurred within hours, and that roughly 75% of the photooxidation during the spill occurred in the same areas where chemical dispersants were sprayed from aircraft. Photooxidised oil is known to reduce the effectiveness of aerial dispersants.

Claire Paris, a UM Rosenstiel School faculty and senior author of the study, said: “Understanding the timing and location of this weathering process is highly consequential. It helps directing efforts and resources on fresh oil while avoiding stressing the environment with chemical dispersants on oil that cannot be dispersed.”

Ana Carolina Vaz, assistant scientist at UM’s Cooperative Institute for Marine and Atmospheric Studies and lead author of the study, said: “Photooxidised compounds like tar persist longer in the environment, so modelling the likelihood of photooxidation is critically important not only for guiding first response decisions during an oil spill and restoration efforts afterwards, but it also needs to be taken into account on risk assessments before exploration activities.”

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