The “window-of-opportunity” to respond to oil spills is impacted by time-sensitive weathering processes (e.g., evaporation or emulsification) that alter the efficacy of response tools, such as in-situ burning or skimming. One response tool used to protect sensitive coastal ecosystems from oiling is chemical dispersants. Our understanding of the relationship between dispersant efficacy and oil weathering processes may be incomplete. Current response plans consider the effects of photochemical weathering to be minimal. For example, the ExxonMobil oil spill response field manual suggests that photochemical weathering is “negligible relative to other weathering mechanisms.” Contrasting this paradigm, photochemical oxidation was reported to be a dominant weathering process for the fate of <i>Deepwater Horizon</i> surface oil, causing substantial alterations to the chemical and physical properties of surface oil on timescales of hours to days. Here, we link these rapid changes in oil chemical and physical properties (i.e., oxygen content and viscosity) to the efficacy of chemical dispersants used in response to oil spills. We show that photo-oxidation of oil substantially reduces dispersant efficacy, and this reduction greatly outweighs even the impacts of evaporation, a weathering process regularly factored into oil spill response plans. Evaporation of up to 30% of the initial oil mass reduced dispersant efficacy by less than 10%, whereas 24 hours of simulated sunlight exposure (the equivalent of 52 hours of natural sunlight on the Gulf of Mexico) reduced efficacy by 30%. Overall, our results show that the "window of opportunity" to apply chemical dispersants to oiled sunlit waters is impacted by photo-oxidation. Therefore, we propose a modification to oil spill response manuals, textbooks, and models to accurately reflect the impact of photochemical weathering on the efficacy of chemical dispersants applied in response to oil spills in sunlit waters.<br/>

Photochemical oxidation reduces the efficacy of aerial dispersants applied in response to oil spills

255th National Meeting (2018)

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