Photochemical weathering is a major fate process for oil spilled at sea. For example, sunlight transformed over half of the floating surface oil spilled during the 2010 <i>Deepwater Horizon </i>disaster (DwH) into oxygenated hydrocarbons in less than a week. The diverse pool of oil photo-products have distinct physical properties from crude oil, changing the relative importance of oil fate processes such as entrainment or dissolution into seawater. A key variable that may alter the physical properties of oil photo-products in spill scenarios beyond DwH is temperature. Photochemical weathering of oil is expected to have low temperature sensitivity, but we don’t know how the physical properties of oil photo-products change with temperature. Addressing this knowledge gap is particularly timely as the Arctic, a cold, high-light environment in the summer, is currently considered a high-risk location for future oil spills, while much of our contemporary understanding of oil photochemistry is sourced from DwH, which occurred during the spring and summer in the Gulf of Mexico. In this study, we quantified the impact of photochemical weathering on oil physical properties as a function of temperature for a single, well-studied crude oil. We found that photochemical weathering substantially increases oil viscosity and dissolution, decreases oil-seawater interfacial tension, and causes a minor increase in density. Further, the changes in oil properties are temperature-dependent, altering the relative importance of key oil fate processes such as entrainment and dissolution in warm versus cold environments. Specifically, photochemical weathering can decrease entrainment rates by one to two orders of magnitude compared to evaporative weathering at room temperature, and this difference is even more pronounced at low temperature. Conversely, photochemical weathering increases the production of water-soluble species by a factor of 5 at room temperature, and this difference is more pronounced at high temperature. Our results show that the ecosystem impacts of photochemically weathered oil in a cold, high-latitude spill may be substantially different from the impacts in a low-latitude spill such as DwH due to the temperature dependence of photochemically-altered oil physical properties.

Temperature mediates the fate of oil at high versus low latitude

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