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The Sun's Overlooked Impact on Oil Spills On cloudy days, the solvents are effective, enabling surfactants to create small droplets that can more easily sink, or be diluted in the ocean, eaten by microbes, or evaporated. On sunny days, sunlight oxidizes oil into compounds that are only partially soluble in the solvent. This limits surfactants’ ability to break oil into droplets. (Natalie Renier, WHOI Creative)

The Sun’s Overlooked Impact on Oil Spills

Research sheds new light on use of aerial dispersants


The 2010 Deepwater Horizon disaster released more oil into the Gulf of Mexico than any other spill in United States history. When it occurred, responders did what they usually do when oil spills happen: They referred to their guidance documents to select the best strategies to minimize risks to humans and ecosystems, especially sensitive areas such as coastlines. 

These guidance documents provide state-of-the-art knowledge on which tools—dispersants, booming, skimming, for example—might be most effective in different circumstances, such as the type of oil spilled and temperature and wind conditions in the area.

No matter who wrote these guidance documents—officials from industry, government agencies, or the National Academies of Sciences, Engineering, and Medicine—they all pretty much say the same thing: Oil floating on the sea surface is going to spread fast, some will evaporate, and microbes will eat the rest. 

None of these documents recognized that sunlight plays a major role in what happens to oil spilled into marine environments. This ostensibly suggests that all response tools should be used evenhandedly, no matter if it is a sunny or cloudy day. The oil spill community may have been underestimating the role of sunlight all along.

In February 2018, I published a study in the science journal Environmental Science & Technology showing that 50 percent of the oil floating on the Gulf of Mexico was chemically altered (or weathered) by sunlight within a few days. Importantly, sunlight does not remove oil from the sea surface, as evaporation does, so the sunlight-weathered oil still has to be cleaned up.

A second study, published in April 2018 in Environmental Science & Technology Letters showed that the chemical alteration of oil by sunlight lowers the performance of chemical dispersants—a commonly used response tool that helps break up oil into droplets and reduces the amount of oil that reaches coastal ecosystems. 

The question people often ask is: “How did the guidance documents get it wrong?” The answer really boils down to not having an opportunity to study how oil reacts with sunlight in the field.

Early pioneers 

In the early 1970s, several scientists, well before their time, reported that oil spilled into marine environments could be transformed by sunlight. The solar energy photo-
oxidizes the compounds, producing compounds that have more oxygen in them.

But making the leap from the lab to the field is extremely challenging. It demands analyzing hundreds of field samples collected across a wide range of space and time; nuanced and intricate photochemistry experiments; and interdisciplinary modeling efforts.

The first step in making this leap is showing that the longer oil floats on the sea surface, the more it becomes oxidized—a relationship that had never been reported for any oil spill anywhere in the world.

Why? Because large, offshore spills such as Deepwater Horizon are extremely rare. The spill released nearly 500 million barrels of oil and gas into the Gulf of Mexico. The oil floated on the sea surface for 102 days.

This provided responders an opportunity to skim oil off the ocean at different time points, which we then analyzed for its oxygen content. In contrast, most spills take place close to the shore and abruptly release only a small amount of oil. By the time the responders get to the spill, most of it has stuck to sorbent booms or washed up on beaches.

Those samples, skimmed off the sea surface by first responders, were a gold mine for research. For the first time, we were able to report a strong link between the length of time that oil remained on the sunlit sea surface and the oxygen content of that oil. The fact that the oil was oxidized was not too surprising (think back to the studies from the 1970s), but the amount of oil that was oxidized and how fast the oxidation happened certainly caught us, and the oil spill community, by surprise.

At least 50 percent of the oil floating on the Gulf of Mexico was chemically altered in less than five days.

Sunlight, not microbes 

The only hitch was that I didn’t know how it was oxidized. The two possibilities were sunlight and marine microbes. The guidance documents said that microbes were the likely culprit, so we checked that out first.

I analyzed hundreds of samples for dozens of chemical markers of biological degradation. I found very little evidence for biological degradation while the oil was floating on the sea surface, at exactly the time when the oil was oxidized. Despite what the guidance documents told us, microbes weren’t the primary driver of oil oxidation on the sea surface.

Logically, the next question was: Is photo-oxidation fast enough to account for the rapid oxidation detected on the sea surface?

This question is incredibly complicated and called for a diverse group of collaborators, including aquatic photochemists, analytical chemists, modelers, and remote sensing experts. In total, the team included ten scientists from seven institutions, including the National High Magnetic Field Laboratory in Florida and Bigelow Laboratory for Ocean Sciences in Maine.

To figure out how fast oil is oxidized on the sunlit sea surface, you need to know:

  • How sunny it is (less light = slower photo-oxidation)
  • How efficient sunlight is at oxidizing the oil film (lower efficiency = slower photo-oxidation)
  • How thick the oil film is (thinner film = less light absorption = slower photo-oxidation)
  • How widespread the oil film is (smaller surface area = slower photo-oxidation)

If you figure these four things out for all 102 days that the oil floated on the Gulf of Mexico (which is not a trivial exercise), you can calculate the potential for sunlight to add oxygen to floating oil.

It turns out that the potential is enormous. In fact, all lines of evidence from the field, lab, and modeling efforts point toward sunlight, rather than microbes, as the primary driver of oil oxidation on the sea surface.

Testing dispersants’ effectiveness

Having established how, and how fast, the oil was oxidized, I started building a case for why the oil spill community should care about oil photo-oxidation. Oil spill response tools generally are evaluated on “fresh” oil that has not been altered by sunlight, yet for the Deepwater Horizon spill, the cleanup tools were applied to sunlight-weathered oil. This discrepancy motivated us to question the accuracy of the guidance documents that tell us that sunlight-weathering doesn’t affect the performance of oil spill response tools.

The tool I tested first was chemical dispersants. I assembled another diverse team of scientists, this time including experts in oil spill response from the U.S. Environmental Protection Agency (Robyn Conmy) and oil spill modeling from industry (Deborah French-McCay, RPS Ocean Science). Together we found that the equivalent of a few days of sunlight exposure decreased the effectiveness of chemical dispersants by at least 30 percent. The results from the lab were similar to those from the field, which led us to the conclusion that sunlight was a primary factor driving aerial dispersant performance.

Next, I embarked on a modeling effort to estimate how widespread this lower-than-expected dispersant performance could have been for the Deepwater Horizon spill. The results showed that under average wind and sunlight conditions, the majority of dispersant applications—hundreds of them—would not have achieved even the minimum effectiveness levels designated by the EPA, because they targeted photochemically weathered oil. Even under the best-case scenarios for aerial dispersant spraying—cloudy weather (which would limit photochemical weathering) and high-wind conditions (which would transport oil farther from the spill area before sunlight transformed it)—dozens of aerial dispersant applications still would not have achieved EPA-designated effectiveness levels.

Toward better guidelines

I expect that this research may lead to some negative statements about dispersant use. This would be unfortunate. The goal of our research is not to pass judgment on the pros and cons of dispersant use. Instead, I am simply providing more knowledge to help officials refine decisions on using dispersants in ways that are most effective.

Notably, these studies provide a solution for the lower-than-expected dispersion of photo-oxidized oil. The research suggests that the dispersant-solvent system is not compatible with photo-oxidized oil and doesn’t effectively break it into droplets that can more easily disperse in the ocean. I offer this solution with a great deal of caution: It’s conceivable that the knowledge gained from this work could be incorporated into novel dispersant formulations designed specifically for photochemically weathered oil.

Before we jump the gun and start to reformulate dispersants, a lot more research needs to be done. First, the collective research community needs to figure out how photo-oxidation varies for different types of oil—from heavy, sour bitumen oil from the Canadian tar sands to light, sweet oil from the Bakken oil field in North Dakota.

Next, we need to figure out how oil photo-oxidation affects a wider range of oil spill response tools—from chemical agents, such as herders and surface washing agents, to tools that physically remove oil, such as sorption and skimming. 

Lastly, we need to put all this information together and write algorithms to calculate how fast various types of oil will oxidize in different waters on Earth across all seasons. That will allow us to tailor more effective response plans for future spills.

The times are exciting—but nerve-wracking, too! This is not a game or just another means to publish more papers or secure more research funds. The stakes are high. Oil spills pose major risks to humans and ecosystems. Oil spill cleanups are extremely expensive, and they will continue to occur, at least in our lifetimes. My line of thinking is simple: Accurate oil spill response guidance documents translate into effective oil spill response and risk mitigation. 

This research was funded by the Gulf of Mexico Research Initiative.