Photochemical Oxidation of Oil Reduced the Effectiveness of Aerial Dispersants Applied in Response to the Deepwater Horizon Spill

Chemical dispersantsare one of many tools used to mitigate the overall environmental impact of oil spills. In principle, dispersantsbreak up floating oil into small droplets that disperseinto the water column where they are subject to multiple fate and transport processes. The effectiveness of dispersantstypically decreases as oil weathers in the environment. This decrease in effectiveness is often attributed to evaporation and emulsification, with the contribution of photochemical weathering assumed to be negligible. Here, we aim to test this assumption using Macondo well oil released during the Deepwater Horizon spill as a case study. Our results indicate that the effects of photochemical weathering on Deepwater Horizon oil properties and dispersanteffectiveness can greatly outweigh the effects of evaporative weathering. The decrease in dispersanteffectiveness after light exposure was principally driven by the decreased solubility of photo-oxidized crude oil residues in the solvent system that com...