Study Finds Sunlight Reduced Dispersant’s Effectiveness During Deepwater Horizon

The study authors provided an image depicting scenarios of using dispersants on a cloudy day (the oil disperses into the water column) and on a sunny day (the oil stays mainly on the sea surface).

The study authors provided an image depicting scenarios of using dispersants on a cloudy day (the oil disperses into the water column) and on a sunny day (the oil stays mainly on the sea surface).

Scientists conducted light-exposure experiments using Macondo oil and Corexit dispersant and ran model simulations to investigate how photo-chemical weathering (oxidation) affects dispersant effectiveness in oil spill response. As light exposure increased, the oil’s oxygen content increased and dispersant effectiveness decreased. Oxygen was three times higher than its initial amount after the equivalent of 53 hours of natural sunlight, and dispersant effectiveness decreased by 30%. The decrease in dispersant effectiveness from photo-chemical oxidation was at least four times greater than decreases from evaporation, challenging the paradigm that photo-chemical weathering has a negligible effect on oil oxidation. These findings provide critical insights into a “window of opportunity” for dispersant application in sunlit waters. The researchers published their findings in Environmental Technology and Research Letters: Photochemical oxidation of oil reduced the effectiveness of aerial dispersants applied in response to the Deepwater Horizon spill.

One tactic that oil spill responders employ is to spray chemical dispersants onto surface slicks to break it up so that oil disperses into the water column, which reduces human risk, enhances biodegradation, and reduces the oil that reaches coastal ecosystems.  A dispersant’s effectiveness typically decreases as oil weathers, which is usually attributed to evaporation, emulsification, and microbial degradation.

This study’s research team has been assessing the drivers of sea-surface oil oxidation to better understand how effective dispersants were when sprayed on the Deepwater Horizon oil slick. They published an earlier study that addressed the question “Is photo-oxidation fast enough to account for the rapid oxidation that was detected on the sea surface?” Their team of 10 scientists from seven institutions found that 50% of floating oil was chemically altered by sunlight in a few days.

The team’s next research focus, which resulted in this study, was motivated by questions about the premise behind oil spill guidance documents, as study author Collin Ward explained, “When oil spills happen, one of the first things responders do is refer to their guidance documents about which response tools (e.g., dispersants, booming, skimming) to use under which conditions (e.g., the type of oil, temperature, wind speed). The documents pretty much say the same thing: Oil floating on the sea surface is going to spread fast, some will evaporate, and the rest will be eaten by microbes. Notably, none of these documents recognized the major role of sunlight in the fate of oil spilled into marine environments. This ostensibly means that all response tools should be used equally, no matter if it is a sunny day or a cloudy day.”

The authors said that oil spill response tools are evaluated on “fresh” oil that had not been altered by sunlight; yet, for the Deepwater Horizon spill, the cleanup tools were applied to sunlight-weathered oil. This discrepancy motivated the team to question the guidance documents that do not account for the impact of sunlight-weathering on oil spill response tools’ performance. The tool they tested first was chemical dispersants.

The researchers exposed artificially weathered Deepwater Horizon oil to increasing durations of simulated sunlight and quantified the oil’s physical properties (viscosity, density, and adhesion) and chemical properties (oxygen content). Then they applied Corexit EC9500A at a 1:20 dispersant-oil ratio, consistent with recommendations for surface application, and analyzed dispersant effectiveness.

Sunlight rapidly (within hours to days) transformed oil into residues that were only partially soluble by the Corexit’s solvent system. The sunlight-driven decreases in dispersant effectiveness were likely caused by changes in oil’s initial chemical properties rather than its physical properties. Non-weathered Deepwater Horizon oil is primarily composed of hexane-soluble compounds that are amenable to gas chromatographic analysis (a chemical characteristic that is principle predictor of dispersant effectiveness), but photo-chemically oxidized oil is primarily comprised of hexane-insoluble compounds that are not amenable to gas chromatographic analysis. Further, photo-oxidized oil residues contain at least one order of magnitude more oxygen than non-weathered oil, making it only partially soluble by Corexit EC9500A.

Experts in oil spill response from the U.S. Environmental Protection Agency and oil spill modeling from industry joined the team and ran models that simulated the environmental conditions during the oil spill, including winds speeds and sunlight levels. They included the 412 flight paths of planes that sprayed dispersants to estimate how widespread this lower-than-expected dispersant performance could have been for the oil spill.

Simulations showed that dispersant effectiveness in the Deepwater Horizon scenario would be negligible after oil floated for 5 – 8 days. Ward explained, “Even under the best-case scenarios for aerial dispersant spraying—cloudy weather (which would limit photo-chemical 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.”

The authors noted that their goal was to provide information that could help responders refine decisions about effective dispersant use rather than passing judgments on its pros and cons. Their study showed that before dispersants were applied, the sun had already broken chemical bonds in oil compounds and created openings for oxygen attachment, making Corexit less effective than anticipated.

The team cautiously suggested that knowledge gained from this work could be incorporated into novel dispersant formulations designed specifically for photo-chemically weathered oil. However, they emphasized that further research is needed on photo-oxidation variations between oil types and its effects on other oil spill response tools before reformulating dispersant compositions can be considered.

“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,” said Ward. “Our line of thinking is simple: accurate oil spill response guidance documents translate into effective oil spill response and risk mitigation.”

Read more about how this study unfolded and its findings in these Oceanus Magazine articles:

Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at doi:10.7266/N7D50KHM.

The study’s authors are Collin P. Ward, Cassia J. Armstrong, Robyn N. Conmy, Deborah P. French-McCay, and Christopher M. Reddy.


This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Deepsea to Coast Connectivity in the Eastern Gulf of Mexico (Deep-C) consortium and to the Woods Hole Oceanographic Institution for their project The State-of-the-Art Unraveling of the Biotic and Abiotic Chemical Evolution of Macondo Oil: 2010-2018. Other funding sources included the National Science Foundation Grant (OCE-1333148), the Clark Family Foundation, Inc., and the United States Environmental Protection Agency Oil Spill Liability Trust Fund.

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit

© Copyright 2010-2018 Gulf of Mexico Research Initiative (GoMRI) – All Rights Reserved. Redistribution is encouraged with acknowledgement to the Gulf of Mexico Research Initiative (GoMRI). Please credit images and/or videos as done in each article. Questions? Contact web-content editor Nilde “Maggie” Dannreuther, Northern Gulf Institute, Mississippi State University (