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 Deepwater Horizon 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 (25), 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.