Forecasting Photo-Dissolution for Future Oil Spills at Sea: Effects of Oil Properties and Composition

Photo-dissolution, the photochemical production of water-soluble species from oil, can transfer oil-derived dissolved organic carbon (DOC) from floating surface slicks to the underlying seawater. Photo-dissolution was likely a quantitatively relevant fate process for the Macondo crude oil spilled during the 2010 <i>Deepwater Horizon</i> spill, but the importance of photo-dissolution for other oils is poorly constrained. This study evaluated the photo-dissolution reactivities (apparent quantum yields) and modeled rates for oils with diverse physical properties and chemical compositions, including an ultra low sulfur fuel oil (ULSFO). Photo-dissolution from UV (310 nm) light was strongly positively correlated with the fraction of small, gas-oil range compounds (<C₂₅), resulting in faster rates for lighter crudes. However, photo-dissolution rates and importance to oil mass balance varied as a function of both reactivity and properties that govern slick thickness and light absorbance. Thicker slicks (∼1 mm) of light and heavy crudes produced more DOC by photo-dissolution compared to thin slicks due to higher rates of light absorbance. However, the mass lost from thin slicks (∼1 μm) was quantitatively relevant for calculations of oil mass balance, with a modeled ∼5% loss for a simplified, hypothetical spill after 1 day of sunlight exposure. The ULSFO was unusual in its exceptionally low photo-reactivity, suggesting distinct fates for this high-spill-risk product. The results show that photo-dissolution is a relevant fate process for a wide range of oil products and that it is controlled by oil properties and composition, making possible predictions of oil fate and effects for future spills at sea.