Oil is more likely to stick around in a cold, sunny ocean

Much of what scientists know about how crude oil breaks down when exposed to sunlight comes from studying the aftermath of the Deepwater Horizon oil spill, which began in the spring of 2010 in the warm waters of the Gulf of Mexico. But new research suggests that oil spills in colder locations with lots of sun exposure could play out much differently.

On Sunday at the ACS Spring 2023 meeting in the Division of Environmental Chemistry, Danielle Haas Freeman, a PhD candidate at Woods Hole Oceanographic Institution, detailed how sunlight-driven photo-oxidation could leave a larger-than-expected fraction of spilled oil suspended on the surface of cold water instead of breaking up and moving below the surface.

“What that means is that you would have more oil to clean up, and then you’d also end up with potentially more oil reaching sensitive coastal ecosystems,” said Collin P. Ward, a geochemist at Woods Hole Oceanographic Institution who led the study.

Freeman and Ward looked at the physical properties governing two different pathways for oil to be naturally removed from the water’s surface: entrainment, a process in which oil gets broken up and drawn down into the water column, and dissolution.

For the study, the researchers used oil from the same deposit as was spilled when the Deepwater Horizon oil rig exploded. They plotted the oil’s viscosity, density, interfacial tension, solubility, and predicted entrainment rate at different temperatures over the equivalent of 12 days of simulated sunlight exposure.

They found that the photo-oxidized oil was both more viscous and more soluble than crude oil at room temperature­—and also that its physical properties were much more sensitive to temperature. Compared to a spill in 30 °C water under the same conditions, their model predicted that photo-oxidized oil in 5 °C water would be 7 times less soluble and have an entrainment rate 9 times lower.

Freeman said that her work in Ward’s lab aims to contribute to more accurate models that make it easier to effectively respond to oil spills. “The predictions that we’re making right now for where oil goes and how it behaves during a spill are based on these mathematical relationships that really don’t work for oil after [it’s] been exposed to sunlight. But the real world has sunlight in it,” she said.

Seeing how light exposure changed the oil’s temperature sensitivity underscores the need for models that better incorporate photochemical weathering, said Garrett McKay, an environmental photochemist at Texas A&M University, who was not involved with the study. “People are going to need to pay attention to this.”

Freeman and Ward expect to publish a paper later this year detailing their findings. They next plan to look at how photo-oxidation affects other types of oil.