Comparison of Experimenal Photooxidation Rates and Patterns in Glass- and Water-Based Oil Slicks with Daily Weathering Observed in the Gulf of Mexico

Charles Sharpless1, Christoph Aeppli2, Christopher M Reddy3, Bob Swarthout4, Orlando C Stewart1, Matthew Walters1 and David L Valentine5, (1)University of Mary Washington, Chemistry, Fredericksburg, VA, United States, (2)Bigelow Lab for Ocean Sciences, East Boothbay, ME, United States, (3)Woods Hole Oceanographic Institution, Woods Hole, MA, United States, (4)Appalachian State University, Environmental Science Program, Boone, NC, United States, (5)University of California Santa Barbara, Marine Science Institute, Santa Barbara, CA, United States
Abstract:
Photooxidation is a well-known degradation route for toxic components of oil (e.g., PAHs). However, recent research suggests that it may play a broader role by enhancing oil’s dark1 and photo-toxicity2 and producing persistent, oxidized hydrocarbons.3To better understand photooxidation’s importance to oil weathering in marine systems, we are combining laboratory studies with field measurements of compositional changes under controlled conditions. Lab experiments have employed a solar simulator to photooxidize slicks of Dorado Well crude oil on water (Instant Ocean) and glass surfaces. Qualitatively similar compositional changes were seen in both systems, such as rapid loss of aromatics and production of oxidized hydrocarbons as assessed by GCMS, GCxGC, TLC-FID, and FTIR. Rates were much faster on water, a finding tentatively ascribed to the film on glass (~750 um) being much thicker than on water (~140 um). Further experiments have been conducted with thinner films on glass, and the results are being analyzed to clarify the importance of film thickness versus surface substrate for photoxidation kinetics. Naturally weathered samples were also collected in the Gulf during a cruise in June, 2015. Surface slicks from natural seeps were tracked and sampled daily under very calm seas with full sun, and solar irradiance was simultaneously measured. These samples, currently undergoing GCxGC and TLC-FID analyses, provide a unique reference with which to assess in-situ transformation rates and compositional changes due to photooxidation. Comparison between results from the field samples and lab experiments should help clarify the absolute contribution of photooxidation to marine oil weathering and improve efforts to use lab results to constrain estimates of environmental transformation rates.

1. D. Rial et al. J. Haz. Mat. 2013, 260, 67

2. J.P. Incardona et al. Proc. Natl. Acad. Sci. 2012, 109, E51

3. C. Aeppli et al. Environ. Sci. Technol. 2012, 46, 8799

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