Freeman
Danielle Haas
Freeman
Danielle Haas
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ArticleSunlight-driven dissolution is a major fate of oil at sea(American Association for the Advancement of Science, 2022-02-16) Freeman, Danielle Haas ; Ward, Collin P.Oxygenation reactions initiated by sunlight can transform insoluble components of crude oil at sea into water-soluble products, a process called photo-dissolution. First reported a half century ago, photo-dissolution has never been included in spill models because key parameters required for rate modeling were unknown, including the wavelength and photon dose dependence. Here, we experimentally quantified photo-dissolution as a function of wavelength and photon dose, making possible a sensitivity analysis of environmental variables in hypothetical spill scenarios and a mass balance assessment for the 2010 Deepwater Horizon (DwH) spill. The sensitivity analysis revealed that rates were most sensitive to oil slick thickness, season/latitude, and wavelength and less sensitive to photon dose. We estimate that 3 to 17% (best estimate 8%) of DwH surface oil was subject to photo-dissolution, comparable in magnitude to other widely recognized fate processes. Our findings invite a critical reevaluation of surface oil budgets for both DwH and future spills at sea.
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ThesisQuantifying the effects of sunlight on the fate of oil spilled at sea(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2024-09) Freeman, Danielle Haas ; Ward, Collin P.Oil spilled at sea is transformed by sunlight-driven photochemical reactions. The transformed oil has different properties and behavior in the environment compared to the fresh oil, resulting in different fates and effects. My work in this thesis was to put numbers on these changes, with the goal of better predicting where oil goes and how it behaves in diverse spill scenarios. First, I focused on how sunlight generates water-soluble compounds from oil, which can lead to the dissolution of oil-derived compounds in seawater (photo-dissolution; Chapter 2). To find out whether photo-dissolution could be an important fate process during an oil spill, I used a combination of experiments and photochemical rate modeling to calculate photo-dissolution rates for the 2010 Deepwater Horizon spill (DwH) in the Gulf of Mexico (GoM). I found that photodissolution likely converted ~8% of the floating surface oil to dissolved organic carbon during DwH, a fraction similar in magnitude to other well-recognized fate processes. Moving beyond DwH, I evaluated the sensitivity of oil photo-dissolution and photochemically-altered oil physical properties to temperature. I found that if a spill like DwH had occurred in 5°C water rather than the exceptionally warm 30°C water of the GoM, 7x less oil could have dissolved via photodissolution and the viscosity of the remaining insoluble oil could have been 16x higher, resulting in lower entrainment of oil into the water column as small droplets (Chapter 3). The net result is that more oil would stay at the sea surface in a cold-water spill. Finally, I determined photodissolution rates for diverse oil products beyond the light crude that spilled during DwH (Chapter 4). I found that oil photo-reactivity could be predicted from oil chemical composition. I also found that photo-dissolution likely affects oil mass balance in spills of light oils forming thin slicks but not in spills of light or heavy oils forming thick slicks. Overall, this work advances our understanding of how oil changes in the environment upon sunlight exposure. This information can be applied to better predict, evaluate, and mitigate the effects of oil spilled at sea on marine ecosystems, including humans.