How the 2010 Deepwater Horizon spill reshaped our understanding of crude oil photochemical weathering at sea: a past, present, and future perspective

Collin P. Ward *a and Edward B. Overton *b
aDepartment of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA. E-mail:; Tel: +1 508-289-2931
bCollege of the Coast & Environment, Louisiana State University, Baton Rouge, Louisiana 70803, USA. E-mail:; Tel: +1 225-578-8634

Received 17th January 2020 , Accepted 18th March 2020

First published on 23rd March 2020

The weathering of crude oil at sea has been researched for nearly half a century. However, there have been relatively few opportunities to validate laboratory-based predictions about the rates, relative importance, and controls of oil weathering processes (e.g., evaporation, photo-oxidation, and emulsification) under natural field conditions. The 2010 Deepwater Horizon (DWH) spill in the Gulf of Mexico provided the oil spill science community with a unique opportunity to evaluate our laboratory-based predictions in nature. With a focus on photochemical weathering, we review what we knew prior to the DWH spill, what we learned from the DWH spill, and what priority gaps in knowledge remain. Three key findings from the DWH spill are discussed. First, the rate and extent of photochemical weathering was much greater for the floating surface oil than expected based on early conceptual models of oil weathering. Second, indirect photochemical processes played a major role in the partial oxidation of the floating surface oil. Third, the extensive and rapid changes to the physical and chemical properties of oil by sunlight may influence oil fate, transport, and the selection of response tools. This review also highlights findings and predictions about photochemical weathering of oil from several decades ago that appear to have escaped the broader scientific narrative and ultimately proved true for the DWH spill. By focusing on these early predictions and synthesizing the numerous findings from the DWH spill, we expect this review will better prepare the oil spill science community to respond to the next big spill in the ocean.

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Collin P. Ward

Collin P. Ward is an Assistant Scientist in the Department of Marine Chemistry & Geochemistry at the Woods Hole Oceanographic Institution. He earned a BS and MS in Environmental Sciences from The Ohio State University, and a PhD in Earth and Environmental Sciences from the University of Michigan. His research characterizes how and how fast sunlight and microbes alter the physical and chemical properties of organic carbon in aquatic ecosystems. He works on a wide range of organic carbon types, including natural organic matter, crude oil, and plastics. His study sites span fresh and saline surface waters from the Alaskan Arctic to the Gulf of Mexico.

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Edward B. Overton

Edward B. Overton is a Professor Emeritus in the Department of Environmental Sciences at Louisiana State University in Baton Rouge, Louisiana. He is an Analytical and Environmental Chemist. He started working at Louisiana State University in 1984 and held the Clairborne Chair in Environmental Toxicology and Air Quality prior to his retirement in 2009. Since 2009, he has served as Research Professor during the response to the Deepwater Horizon oil spill. His research interests include understanding the fates and distributions of hydrocarbons following oil spills, the environmental chemistry of hazardous chemicals, and the detection of environmental pollutants at the site of sample collection.

Environmental significance

Crude oil in the ocean undergoes a wide range of weathering processes (e.g., evaporation, photo-oxidation, emulsification) that alter its properties. Despite decades of studying these processes, most of our perceived knowledge has not been confirmed under natural field conditions. One silver lining of the 2010 Deepwater Horizon (DWH) oil spill was that it provided the community with an opportunity to validate our laboratory-based predictions in a natural setting. This critical review focuses on the photochemical weathering of oil and provides a diverse range of perspectives about: (i) what we knew prior to the DWH spill, (ii) what we learned from the DWH spill, and (iii) what future research priorities remain.

1. Introduction

When crude oil is released into the ocean it undergoes a series of weathering process that influence its fate and impacts (Fig. 1). These processes include, but are not limited to, dissolution, evaporation, oxidation, emulsification, and sedimentation.1,2 Some of these processes (dissolution, evaporation, and sedimentation) move the oil's hydrocarbon compounds to different locations and can change the physical properties of the liquid oil, but the chemical properties remain essentially unaltered. For example, low molecular weight alkanes in floating oil (boiling points > ∼n-C5, < ∼n-C16) are partially soluble in water and volatile, so they are prone to dissolution into the water column and evaporation into the atmosphere. These small alkanes are lost from the floating liquid oil, but not chemically transformed in the process. Their loss leaves behind the higher molecular weight, denser, and more viscous mixture of hydrocarbons (e.g., boiling points > ∼n-C16) that compose floating surface oil. Thus, dissolution and evaporation impact the composition of surface oil by enriching certain compounds and removing others because of their physical properties, but they do not cause chemical transformations that result in new compounds.
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Fig. 1 Weathering processes of oil at sea.

Emulsification occurs when water is mixed with and retained by floating oil. Similar to evaporation, no chemical transformations occur due to emulsification. However, emulsification substantially alters physical properties, including density, interfacial tension, and viscosity. Emulsified oil can interact with solids and be deposited on the sea floor or on beaches and marshes, a process collectively referred to as sedimentation (Fig. 1). Importantly, no chemical transformations of the oil occur during sedimentation; it is merely a translocation process.

Photochemical and biological oxidation are transformative processes in which the oil is oxidized by sunlight or microbes into new oxygenated compounds (Fig. 1). Photo-oxidation of oil can undergo two different pathways.3 First, light-absorbing compounds within oil, such as polycyclic aromatic hydrocarbons or asphaltenes, are directly oxidized by the absorption of sunlight. Second, the absorption of sunlight generates reactive oxygen species, which can indirectly oxidize a wide range of crude oil constituents, not just those that absorb light directly. Both light-driven pathways result in the formation of new, oxygenated compounds with vastly different physical and chemical properties than the spilled oil. For example, some of these newly oxidized oil components are water soluble and enter the waters surrounding the spilled oil (Fig. 1).

Scientists have been studying photochemical and other weathering processes over the past five decades, as summarized by numerous review articles.1–6 One central discussion point of these review articles is that there have been relatively few opportunities to validate our laboratory-based predictions about the rates, relative importance, and controls of these weathering processes under natural field conditions. The Deepwater Horizon (DWH) spill of 2010 provided the community with this rare opportunity. In the largest marine oil spill in United States history, upwards of five million barrels of oil and gas was discharged into the Gulf of Mexico over an 87 day period.7 Oil floated on the sea surface for 102 days, providing researchers a unique sample set with which to assess our understanding of oil weathering at sea.

This review disseminates results from a synthesis workshop focused on the role of weathering processes in oil spill science, with emphasis placed on photochemical weathering. Participants included representatives from the federal government, academia, and industry. Multiple generations of oil spill scientists were intentionally invited to gain diverse, long-term perspectives about what we knew prior to the DWH spill, what we learned from studying the DWH spill, and what priority gaps in knowledge remain. This past, present, and future framework was applied to three guiding questions about the relative importance of photochemical versus other weathering processes, the reactants, pathways, and products of photochemical weathering, and the role that photochemical weathering plays in oil spill modelling and response operations.

2. Is photo-oxidation an important weathering process for oil spilled into the ocean?

(a) Knowledge gained prior to DWH

Prior to the DWH oil spill, photochemical oxidation (photo-oxidation) was widely recognized as a minor, secondary weathering process that operates on timescales of weeks to months.1,2,4,8–14 For example, Oil in the Sea III2 states that “photo-oxidation is unimportant from a mass balance consideration.” Consistent with this perspective, in the most comprehensive mass balance study of an oil spill prior to DWH, photo-oxidation of oil released into Prince William Sound, AK during the Exxon Valdez spill of 1989 was assumed to only take place in the atmosphere.13 That is, photo-oxidation of the spilled oil supposedly occurred only after evaporation, making photo-oxidation a secondary process. This perception trickled down into international, industry-developed cooperatives that provide consulting services to all major oil companies in response to spills, like Oil Spill Response Limited. In the Oil Spill Response Limited guidance document,14 oil oxidation is described as follows: “oil reacts with oxygen to form new by-products, including soluble products or persistent tars. The process is promoted by sunlight (photo-oxidation). The process occurs at the surface of the oil and even in strong sunlight, is very slow.”

Numerous conceptual diagrams9,14,15 depicting the relative importance of different weathering processes also characterize photo-oxidation of floating surface oil as a minor, secondary process relative to evaporation, emulsification, and biodegradation (Fig. 2). In these conceptual models, the importance of the process is represented by the size of the bar, and time since the spill is shown on the x-axis. Evaporation is often the most important process on the diagram, starting immediately after the spill, and lasting up to a year. Emulsification is depicted as a less important process than evaporation, starting hours after the spill, and lasting up to a year. Biodegradation is typically considered to be of similar importance as emulsification, starting several hours after the spill, and lasting up to a year. Photo-oxidation is consistently considered the least important process, starting days after the spill, and lasting up to a week. Collectively, the consensus prior to the DWH spill was that sunlight-driven processes have little impact on the fate, transport, or response strategies to oil spilled into the ocean because it is a slow, secondary weathering process impacting a minor fraction of spilled oil.

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Fig. 2 The relative importance of floating surface oil weathering processes as understood prior to and after the Deepwater Horizon spill of 2010. Ref. 1, 2, 4 and 8–14 support the “Before DWH” conceptual model.

The rationale behind classifying photo-oxidation as a slow, secondary weathering process is unclear. In a seminal review article,3 it was clearly stated that under certain conditions, photo-oxidation can occur at rates comparable to evaporation and at rates greater than biodegradation. In fact, one of the studies highlighted in the review hypothesized that photo-oxidation is a primary step in the formation of stable water-in-oil emulsions.16 Nevertheless, thisevidence that photo-oxidation plays an important role in oil weathering was either overlooked or not accepted by the broader oil spill community.

There are three plausible explanations for the perception that photo-oxidation is a minor, secondary weathering process of crude oil spilled at sea. First, few studies have actually teased apart the relative role of weathering processes under natural field conditions. Well over a dozen primary studies were conducted on photo-oxidation of oil prior to DWH,15–35 informing at least three review articles.3,5,6 The overwhelming majority of these studies were conducted under controlled laboratory conditions where weathering by sunlight is often assessed in isolation of other weathering processes. Accordingly, few studies validated findings from the laboratory with samples from an actual spill. In fact, no study prior to DWH developed a relationship between the time oil resided at the sunlit sea surface versus its extent of oxidation.

Second, despite making up a small percentage of total oil mass (<10% for most crude oils), a large emphasis of study has been placed on the photochemical fate of polycyclic aromatic hydrocarbons (PAHs).22,25–27 The rationale for this focus on PAH degradation is multi-faceted. PAHs directly absorb natural sunlight at Earth's surface and are thus logical components of crude oil to probe for their susceptibility to photo-oxidation. Moreover, PAHs are high-priority pollutants as designated by the US Environmental Protection Agency, so their fate and transport are important from a damage assessment perspective. Furthermore, PAHs are amenable to analysis via gas chromatography coupled with flame ionization detection or mass spectrometry (GC-FID or GC-MS), instruments that are widely used in oil spill sciences. Lastly, PAH standards are commercially available, allowing for quantification via GC-based approaches. This focus on PAHs may have misdirected the scientific community away from the importance of indirect photo-oxidation affecting a larger pool of oil compounds, not just the components that directly absorb sunlight.

Third, technology likely limited a comprehensive assessment of the role of photo-oxidation of crude oil spilled into the ocean. For example, prior to DWH assessing the composition of photo-products at the molecular level was analytically challenging. This is largely because photo-oxidation products generally reside outside the analytical window of GC-based approaches (see Section 3(b) for further discussion).37 Moreover, the spatial extent and thickness of oil floating on the sea surface is a key parameter for estimating rates of oxidation (see Sections 2(b) and 2(c) for further discussion). However, prior to DWH no technology existed that allowed for remote sensing of oil film thicknesses across large bodies of impacted water.38 This limited the community's capacity to estimate rates of photo-oxidation and compare to the magnitude of oxidation observed in oil residues from a spill.

(b) Knowledge gained from DWH

The unique circumstances of the DWH spill, with 87 days of continual oil release and 102 days of surface oiling, provided a rare opportunity to collect hundreds of naturally weathered oil residues across space and time. This situation, combined with technological breakthroughs and a decade long commitment to fund field- and laboratory-based research by the Gulf of Mexico Research Initiative, considerably advanced our understanding of the importance of photochemical weathering of oil at the sea surface relative to other weathering processes.

Using a myriad of analytical approaches, numerous field studies reported that the insoluble, non-volatile surface oil was partially transformed into new oxygenated compounds (i.e., partial oxidation) in the aftermath of the spill.39–49 Multiple studies39,40,44,46 reported that surface and deposited oil residues contained lower GC-amenable fractions, higher elemental oxygen content (determined by elemental analysis; EA), higher oxidized hydrocarbon content (determined by thin-layer chromatography couple with flame ionization detection; TLC-FID; Fig. 2), and higher hydroxyl and carbonyl content (determined by Fourier transform infrared spectroscopy; FTIR). Looking at the asphaltene fraction of surface and deposited oil residues (i.e., operationally defined as the fraction that is insoluble in iso-octane), Lewan and others43 documented a high degree of oxygenation (>10% oxygen by mass). The molecular-level composition of oil residues deposited on Gulf of Mexico (GoM) beaches was also assessed using Fourier transform ion-cyclotron resonance mass spectrometry (FT-ICR MS).42,45,49 This high-resolution analysis revealed substantial partial oxidation, including at least two-fold higher molecular complexity in the weathered versus the spilled oil, with most of the added complexity attributed to oxygenated products. The significantly enriched stable oxygen isotope signature (δ18O) of weathered oil residues relative to the native oil also suggested substantial partial oxidation, with isotopically heavy molecular oxygen rather than lighter water found as the primary source of oxygen incorporation.41 Ramped pyrolysis-GC-MS analysis of weathered oil residues further indicated extensive oxygenation of DWH oil with increasing time in the environment.48

It is compelling that dozens of field studies concluded that DWH surface oil underwent substantial partial oxidation despite extreme differences in the analytical tools used to probe the process. The types of analytical tools ranged widely, from quantitative to qualitative, bulk to molecular-level, narrow to wide analytical windows, cheap to expensive, routine to specialized, and low to high throughput. Given these vast differences, it is actually quite remarkable that the results of these studies aligned. This alignment is a testament to technological innovations and widespread and long-term support provided by the Gulf of Mexico Research Initiative, thereby allowing researchers to expand the analytical window beyond traditional GC-based approaches.

In coordination with these studies that documented the extent of surface oil partial oxidation in the field, experimental and modelling studies were simultaneously conducted to assess the location, timing, and pathway of partial oxidation. By comparing the extent of oxidation for oil resides skimmed off the surface ocean versus those that deposited on GoM beaches, multiple studies documented that a major fraction of the oxidation occurred while at the sea surface.39,41,43,46 For example, the most comprehensive, long-term sample set available, including 34 surface oil and 306 deposited oil residues, showed that two-thirds of total partial oxidation from 2010 to 2016 occurred while the oil was floating on the sea surface.46 However, the extent of oxidation of surface oil residues alone cannot be used to assess how fast the oxidation took place at the sea surface. This is because any given “patch” of surface oil is subject to highly variable (both in magnitude and direction) surface currents and winds that impact how long the oil resided at sea prior to collection and analysis. Consequently, oil spill modelling efforts were required to link residence time to the extent of oxidation. This linkage revealed that oil oxidation at the sea surface occurred rapidly, on timescales of hours to days. Oxidation, determined by EA, TLC-FID, and FTIR, rose exponentially from hours after surfacing until plateauing approximately one week after surfacing.46 Collectively, these data demonstrated that approximately 50% of the insoluble, non-volatile surface oil hydrocarbons were oxidized in less than one week at the sea surface. This magnitude and rate of partial oxidation was unprecedented and ultimately raised the question: how was the oil oxidized?

There are three main pathways to increase the oxygen content of surface oil: preferential enrichment of native oxygenated hydrocarbons via evaporation, and partial oxidation via microbial and photochemical processes. To assess the relative importance of these pathways, a multi-tiered approach using biomarker analyses, experimental assays, and kinetic modelling was required. Preferential enrichment of native oxygen in the parent oil via evaporation was largely ruled out by normalizing changes in oxidized hydrocarbon content to hopane, an internal conservative tracer.50 This approach demonstrated that the magnitude of oxidation far exceeded that possible via preferential enrichment caused by evaporation.39 From a simple mass balance perspective, this makes geochemical sense. For example, if 50% of the surface oil were to evaporate,51 then the oxidized hydrocarbon content would double from 10% to 20%, far less than the 60% oxidized hydrocarbon content detected in floating surface oil residues (Fig. 3).39,46 Using the same logic and applying it to elemental O content, O would increase from 0.4% to 0.8%, far less than the 6% O detected in dozens of floating surface oil residues.39,46 Similarly, evaporation has only a minor impact on the stable oxygen isotope composition of the surface oil, and thus cannot explain the 8‰ shift in δ18O of the surface oil.41 Collectively, preferential enrichment of native oxygen in the parent oil via evaporation was a minor contributor to the apparent oxygenation of surface oil, indicating that microbial and/or photochemical processes governed oxygenation.

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Fig. 3 Thin layer chromatography coupled with flame ionization detection analysis of the oxidized hydrocarbon content of parent Macondo Well oil, surface oil residues skimmed off the Gulf of Mexico (S1–17), deposited oil residues collected off of Gulf of Mexico beaches (B1–94), and oil scrapped off of rocks along the Gulf of Mexico shoreline (R1–18). The data are plotted chronologically as a function of days after the start of the spill. Reprinted with permission from Aeppli et al., 2012.39 Copyright 2012, American Chemical Society.

A combination of biomarker analyses and mesocosm experiments strongly suggest that microbial oxidation did not contribute substantially to the oxygenation of DWH floating oil. Several studies used a multitude of widely vetted biomarker ratios to assess the biodegradation of surface and deposited oil residues (e.g., n-C18 to phytane, n-alkane abundance normalized to hopane).39,46,52 In all cases, the biomarker ratios were stable while the surface oil was at sea, the period of oxygenation. Consistently, mesocosm experiments designed to replicate microbial oxidation of surface oil indicated that the magnitude of oxygenation was not substantial enough to account for observations from field oil residues.49 Moreover, the molecular composition of oxygenated metabolites detected in the biodegradation mesocosms using FT-ICR MS did not match those detected in field oil residues (Fig. 4). That is, ketone-containing oxygenated compounds spanning one to five oxygen atoms were widely abundant in field oil residues but were generally not produced by microbes (Fig. 4).49 Together, the biomarker analyses and mesocosm assays indicate that microbial oxidation likely did not contribute substantially to the partial oxidation of DWH surface oil.

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Fig. 4 Chemical composition of oil residues from biodegradation (row 1) and photo-oxidation (row 2) mesocosms, and from a Gulf of Mexico beach collected 57, 587, and 1378 days after the start of the spill (rows 3–5). The x-axis represents the number of carbon atoms in each formula detected by FT-ICR MS. The y-axis represents the number of carbon–carbon double bond equivalents (bonds + rings) in each formula. In these analyses, only the derivatized ketone/aldehyde fraction of the oxygenated oil residues are considered. The columns represent the oxygen class, ranging from formulas that contain one to five oxygen atoms. The color is proportional to the abundance of each formula. Reprinted with permission from Niles et al., 2019.49 Copyright 2019, American Chemical Society.

A combination of biomarker analyses, mesocosm experiments, and kinetic rate modelling collectively led to the conclusion that sunlight exposure governed the oxygenation of DWH oil at the sea surface. The distribution of naphthobenzothiophene and benzo(a)anthracenes/chrysene homologs is an indicator of photochemical weathering, where the magnitude of depletion (relative to hopane) is expected to increase with alkylation,22 consistent with field observations from the DWH spill.52,53 Moreover, the degree of photochemical weathering was also related to molecular weight, where higher molecular weight PAHs, like chrysenes that have higher and red-shifted light extinction coefficients, were more depleted than lower molecular weight PAHs like phenanthrenes that have lower and blue-shifted light extinction coefficients.52 Similarly, the peri-condensed PAH benzo[a]anthracene was two-fold more depleted in the weathered surface oil residues compared to its cata-condensed isomer, chrysene.53 This is consistent with the light absorption properties of these isomers, where benzo[a]anthracene has a higher and red-shifted light extinction coefficient compared to chrysene.27 Collectively, extensive biomarker analyses suggest that sunlight played a major role in the oxidation of DWH oil at the sea surface.

Photochemical mesocosm experiments in the laboratory corroborated the results from biomarker analyses. The chemical composition of ketone-containing photo-products in laboratory incubations was remarkably similar to the composition of transformation products extracted from a field oil residue collected 57 days following the DWH blowout (Fig. 4).49 Interestingly, the composition of the photo-irradiated oil and a 57 day field residue was also similar to that of residues collected 587 and 1378 days after the blowout, further indicating that most of the oxygenation occurred immediately after oil surfacing.

Lastly, photochemical rate modelling estimates of partial photo-oxidation overlapped with the extent of oxidation observed in surface oils, suggesting a photochemical basis for the partial oxidation of DWH surface oil.46 The modelling efforts used photochemical rate equations that were written decades before the DWH oil spill,54,55 but could only be implemented for a spill the size of DWH due to recent technological advances in remote sensing of oil film spatial extent and thickness.38 The rate of photo-oxidation is defined by eqn (1):

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where Φλ is the apparent quantum yield spectrum for photo-oxidation of oil (mol of O2 photochemically consumed divided by mol of photons absorbed), and Rλ is the rate of light absorption by oil, defined by eqn (2):
Rλ (mol photons per m2 per d per nm) = Edλ(1 − eKdλz)(2)
where Eλ is the daily downwelling spectral photon irradiance at the sea surface (mol photons per m2 per d per nm), Kdλ is the light attenuation coefficient of the oil film (m−1), and z is the depth of the oil film (m).

Mesocosm experiments were used to assess Φλ of photochemical oxygen consumption by the spilled and field-weathered oil. Reactivity decreased with increasing wavelength from the UV throughout the visible, although, consistent with previous reports,16 photo-reactivity in the visible was appreciable.

For the first time for any oil spill, daily areal rates (i.e., mol O2 per m2 per d; integrated across the depth of the oil film) of photo-oxidation were calculated for the 102 days that oil floated on the GoM. By multiplying the daily rates by the area of surface oiling (m2), oil oxygenation for each day of surfacing was calculated (mol O2 per d). The estimates of total photo-oxidation derived from rate modelling overlapped with the extent of oxidation observed in surface oil residues.46

Collectively, biomarker analysis, mesocosm experiments, and photochemical rate modelling all point towards the same conclusion: photo-oxidation was the governing process of DWH surface oil oxidation. This consensus point challenges the community perception that photo-oxidation is a minor, secondary weathering process operating on timescales of weeks to months. Accordingly, conceptual models depicting the importance of photochemical weathering relative to other weathering processes have been changed to accurately reflect the role of sunlight (Fig. 2).56

(c) High priority knowledge gaps for future research

Was the Deepwater Horizon spill unique with respect to the rate and extent of surface oil photo-oxidation?. If a different type of oil spilled in an off-shore, deep water setting, in a different geographical location, and at a different time of year, would we expect this magnitude of partial photo-oxidation? Do the results from DWH apply to surface spills from oil tanker wrecks, for example? To assess this knowledge gap, the susceptibility to photo-oxidation (i.e., apparent quantum yields) across a wide range of sunlight types (UV to visible) for a wide range of oil types (i.e., light to heavy, sweet to sour) should be assessed. Furthermore, the influence of different environmental conditions (e.g., temperature, salinity) and weathering conditions (e.g., dissolution and evaporation) should be determined. Ultimately, by identifying the controls on oil photo-oxidation, we will improve our predictive understanding of the rate and extent of oil photo-oxidation in a different spill scenario.
Establishing why the magnitude of partial oxidation appeared to plateau after one-week at sea. Several independent lines of evidence suggest that the magnitude of surface oil oxidation plateaued after approximately one week of floating on the sea surface.41,46 These observations suggest that there may be an upper limit of surface oil photo-oxidation. Four hypotheses have been proposed to explain these observations:41 (i) self-shading,46 (ii) limited diffusion rates of O2,36 (iii) decreasing reactivity with increasing light exposure,46 and (iv) photo-oxidation to water soluble products.57 Explicitly testing the relative importance of these four hypotheses is critical to predict future rates and impacts of oil photo-oxidation at sea.
Developing higher resolution and more precise approaches to quantify oil film thickness over space and time. One of the key variables controlling oil photo-oxidation rates is the thickness of the oil film. Even for light oils, like the oil spilled in DWH, sunlight is rapidly attenuated with depth: UV light penetrates ∼10 μm and visible light penetrates ∼100 μm. Because light is attenuated exponentially by oil, the rates of oxidation increase exponentially up to 100 μm, where no more light is available to oxidize oil.46 Current remote sensing approaches to estimate oil film thickness typically bin the film into two categories: thin (∼1 μm) or thick (∼70 μm).38 Moreover, the coefficient of variation of surface area estimates approached 100% for both the thin and thick oil films.38 This low resolution and precision represents a major source of uncertainty in modelled estimates of oil photo-oxidation rates.46 Therefore, in addition to improving estimates of oil volume at the sea surface, next-generation remote sensing packages for quantifying film thickness and spatial extent will also improve estimates of oil photo-oxidation rates.

3. What are the reactants, products, and pathways of oil photo-oxidation?

(a) Knowledge gained prior to DWH

Prior to DWH, the community established two schools of thought about the reactants, products, and pathways of crude oil photo-oxidation. The first school of thought argued that only those compounds that absorb sunlight (>∼280 nm) are capable of being oxidized by sunlight (i.e., “direct photolysis”). These light-absorbing “chromophoric” compounds include PAHs and asphaltenes. In the left panel of Fig. 5, direct photolysis is depicted, where a black aromatic ring is transformed into a partially oxidized orange aromatic ring, the expected major product of direct photolysis.
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Fig. 5 A conceptual model depicting direct and indirect photo-oxidation. In direct photo-oxidation (left panel), a light absorbing molecule (depicted as a black aromatic ring) is partially oxidized into a new molecule (depicted as an orange aromatic ring). Indirect photolysis occurs when the absorption of light leads to the production of reactive oxygen species (middle panel), like singlet oxygen, peroxyl radicals, and hydroxyl radicals. These reactive intermediates can oxidize a wide range of compounds, not just the compounds that directly absorb light (right panel).

Perhaps the most widely cited study to embrace the direct photolysis school of thought is Garrett et al., 1998.22 In this study, Alaska North Slope crude oil to was exposed to UVB light and compositional changes to the oil were characterized after exposure. The authors reported that “saturated compounds are resistant, but the aromatic compounds are particularly sensitive to photo-oxidation.” For example, TLC-FID analysis of the dark-control and light-exposed oil demonstrated that the production of oxidized compounds by sunlight could be completely accounted for by the oxidation of aromatics.

At the same time the direct photolysis school of thought was being developed, many studies reported that saturated compounds were susceptible to oxidation, leading to the alternative, indirect photolysis school of thought. Indirect photo-oxidation occurs when a chromophore, like a PAH or asphaltene, photochemically generates reactive oxygen species (ROS), like singlet oxygen, peroxy radicals, and hydroxyl radical (Fig. 5 middle panel). These sunlight-generated ROS can indirectly react with a wide range of crude oil constituents (Fig. 5 right panel), not just those that absorb light directly.

There is widespread evidence in support of the indirect photolysis school of thought, although no consensus has been reached on the specific ROS-meditated pathway that controls oxidation. The earliest studies were actually motivated by the idea that addition of sensitizers in response to spills in remote locations (e.g., the Arctic) could accelerate photo-oxidation of both light-absorbing and non-light absorbing constituents of crude oil.58 For example, the addition of xanthone to crude oil can sensitize the oxidation of hexadecane (a non-light absorbing constituent of crude oil) into hydroperoxides and alcohols.59 This indirect oxidation was reported to be mediated by peroxy radicals. A laboratory study conducted in the absence of sensitizers demonstrated that alkanes (straight-chained, branched, and cyclic) were susceptible to photo-oxidation, yielding ketones and alcohols.19 This study also invoked peroxy radicals as a critical oxidative intermediate. Additional laboratory studies reported that singlet oxygen was a critical intermediate in the photo-oxidation process.16,31 For example, addition of beta-carotene, a singlet oxygen quencher, hindered photo-oxidation31 and the formation of water-in-oil emulsions16 (see Section 4(a) for further discussion). A mechanism was proposed,31 and later debated,36 where singlet oxygen leads to the formation of organic peroxides which dissociate photochemically to form peroxy radicals. These radicals subsequently oxidize a broad range of crude oil constituents. Incubations of oil spilled during the 1979 Ixtoc I oil spill in the Gulf of Mexico also revealed evidence of indirect photo-oxidation.28 Oil was collected close to the source of the spill and then incubated in natural sunlight alongside dark controls. After four days of natural sunlight exposure, the irradiated oil visually resembled that of naturally weathered oil on the sea surface, whereas the dark control oil showed no visual evidence of change. Consistently, the irradiated oil contained a suite of fatty acids, transformation products that were not detected in the dark control oil. Because singlet oxygen does not directly react with saturated alkanes, the authors proposed that alternative intermediates must have been involved. Collectively, a wide range of evidence supported the indirect photolysis school of thought, although the exact pathway of oxidation was debated.

(b) Knowledge gained from DWH

Understanding the relative importance of direct vs. indirect photolysis is critical for predicting how much oil is vulnerable to photo-oxidation during an oil spill. For example, if direct photolysis is dominant, oil photo-oxidation would be limited by the percent of oil that directly absorbs natural sunlight. In the case of DWH, the PAH content of the spilled oil was 4% by mass,51 whereas the PAH content of the insoluble, non-volatile surface oil was <1% by mass.39 Therefore, if direct photolysis is the dominant pathway, sunlight could only oxidize a minor fraction of the spilled oil. On the other hand, if indirect photolysis is dominant, a much larger fraction of the spilled oil could be oxidized by sunlight.

As discussed in Section 2(b), upward of 50% of the floating surface oil was partially oxidized by sunlight on a timescale of days to one week. Given that light absorbing PAHs comprised only 4% of the spilled oil and <1% of the surface oil, the only plausible conclusion to explain this magnitude of oxidation is that indirect photolysis was a dominant pathway. Consistently, the loss of aromatics as determined by TLC-FID (16% of initial mass, including volatile one-ring aromatics that do not absorb natural sunlight)51 simply cannot account for the nearly 50% gain in polar compounds with increasing time on the sea surface, suggesting that saturated compounds must have been oxidized by sunlight. Building on these simple mass balance assessments, multiple advanced analytical platforms independently reached this same conclusion that indirect photolysis was a dominant pathway for the photo-oxidation of DWH surface oil.

Comprehensive GC × GC-FID was used to characterize over 40 oil residues collected in the GoM following the spill.40 A chemometrics approach was used to correlate the loss of different compounds classes to the formation of oxidized hydrocarbons (determined by TLC-FID). The analysis indicated that saturated compounds were the dominant precursors to the formation of oxidized compounds. The key advantage of the approach is that the multi-dimensional separation allowed the authors to track compounds that would not be resolved using traditional one-dimensional GC-based approaches (e.g., alkylcyclopentanes, alkyl cyclohexanes, alkylated bicyclic saturated compounds, tricyclic terpenoids, and alkylbenzenes). However, because oxygenated photo-products are largely non-amenable to GC analysis,39 tracking the products of photo-oxidation required an expansion of the analytical window beyond traditional GC-based approaches.37

Several novel techniques proved successful in characterizing the composition of indirectly photo-oxidized products. FT-ICR MS analysis detected thousands of aliphatic oxygenated photo-products in oil residues that washed up on GoM beaches,42 further suggesting that non-light absorbing constituents were major precursors of oxidation. Additional evidence from field oil residues and laboratory incubations revealed abundant aliphatic ketone and aldehyde photochemical transformation products (Fig. 4).49 Lastly, derivatization of weathered oil residues prior to GC × GC-FID analysis revealed the presence of a suite of aliphatic acids and alcohols.47

Despite the many lines of evidence indicating that indirect photolysis played a critical role in the oxidation of DWH surface oil, the key ROS pathways that govern oxidation remain unknown. Singlet oxygen, hydroxyl radical, and reactive organic triplet states were all reported to be produced upon exposure of DWH oil to sunlight.60–62 However, the relative importance of these indirect oxidative pathways remains unknown. The stable isotopic composition of dissolved oxygen (δ18O) provided insights into the oxidative pathway.46 The isotope effect of dissolved O2 photochemically incorporated into hydrocarbons was independent of crude oil physical and chemical properties (i.e., = 18εoxidation of DWH oil = −9.0 ± 0.3‰, 18εoxidation of Alaskan North Slope = −9.4 ± 1.3‰), raising the possibility that a variety of crude oil types share a common photo-oxidation mechanism. The identity of this common mechanism is unknown; however, δ18O could prove to be a promising, novel approach to identify it.

(c) High priority knowledge gaps for future research

Determining the mechanism of oil photo-oxidation. The specific ROS pathway(s) driving oil photo-oxidation has escaped our understanding for nearly fifty years. Identifying the exact pathway, and how it may vary for different types of oils, is critical to further explore the idea of adding chemicals to quench or enhance photo-oxidation in response to future spills, an idea initially proposed in 1973.58
Assessing the role of asphaltenes in oil photo-oxidation. Relative to the study of PAHs as sensitizers of oil photo-oxidation (see Section 2(a)), very few studies have explored the role of asphaltenes as photochemical sensitizers. These native compounds are high molecular weight, highly conjugated,63 and absorb deep into the visible light region. Given that ten-fold greater visible light reaches Earth's surface compared to UV light, and visible light penetrates ten-fold deeper into oil films than UV light, further investigation of the photochemical reactivity of asphaltenes is warranted. Two studies16,46 have reported that visible light plays an important role in oil photo-oxidation, although the chemical structures driving this visible light-mediated oxidation are unknown.

4. What role does photo-oxidation play in oil spill response?

(a) Knowledge gained prior to DWH

It is well established that as oil weathers at the sea surface, the effectiveness of tools used in response to oil spills decreases.4 The term “window-of-opportunity” defines the time after a spill where a specific tool can be used effectively. Although the “window-of-opportunity” varies for different tools, oils, and spill locations, the response community reached a consensus that evaporation and emulsification were the two most important weathering processes impacting oil spill response decisions. The rationale was that these two processes alter the physical properties of oil, namely viscosity and density. For example, the 2005 National Academies report4 on chemical dispersant use explicitly states that “the two most important weathering factors include evaporation and the formation of stable water-in-oil emulsions, because they both affect the spilled oil's in situ viscosity on the water surface.” Moreover, “viscosity was found to be the most important physical property in determining dispersibility.” The Exxon Mobil oil spill response manual9 states that “viscosity changes affect the selection of response options,” and “emulsification and mousse formation increase the density and viscosity, which may affect the selection of clean up options.”

As discussed above in Section 2(a), prior to DWH the oil spill science community considered photo-oxidation to be a minor, secondary weathering process that operates on timescales of weeks to months. Therefore, response operations do not consider the extent of photochemical weathering when designing response plans. In fact, the Exxon Mobil oil spill response manual9 explicitly states that the effects of photo-oxidation are “negligible relative to other weathering mechanisms.” Collectively, this perspective suggests that the “window-of-opportunity” to respond effectively to an oil spill is not affected by the extent of photochemical weathering.

The absence of photochemical weathering algorithms in oil spill fate and trajectory models reinforces this perspective.12,64,65 These models are often used by the response community to predict the trajectory and thus volume of oil across space and time after a spill. Physical properties like viscosity, density, and surface tension are key parameters that drive model predictions and influence the rates of processes like spreading, emulsification, and entrainment. The exclusion of photo-oxidation suggests that it does not influence these physical properties or processes. In fact, a review paper in 1998 (ref. 64) states that photo-oxidation “is unimportant over the first few days of a spill,” whereas a review paper in 1999 (ref. 12) does not mention photo-oxidation as a relevant process impacting the model output. Photochemical degradation of PAHs is notably included in the Spill Impact Model Applications Package (SIMAP), however, changes to oil physical properties by sunlight are not accounted for by the model.65

This perspective that oil spill response operations are not impacted by photochemical weathering of oil overlooks multiple instances in the literature where photochemical weathering had substantial impacts on the physical properties of oil, like viscosity and water content. Most notably, Thingstad and Pengerud 1983 (ref. 16) reported that visible light exposure (i.e., >400 nm) and physical mixing play essential roles in the formation of stable water-in-oil emulsions, also known as “mousse” (Fig. 6). In their study, laboratory incubations with Statfjord crude oil (a light sweet crude from the North Sea) were conducted under varying environmental conditions (light, shaking speed, and temperature). The amount of water incorporated into the oil during the incubations was quantified, as was the establishment of stable water-in-oil emulsions. The treatments with high light and shaking speed retained the most water and most quickly formed stable water-in-oil emulsions. For example, the amount of water incorporated into a Statfjord crude oil after six days of visible light exposure and shaking was nearly 7-fold higher than the same oil shaking in the dark. Decreasing irradiance and shaking speed retarded emulsification but decreasing temperature did not. Moreover, in the same experiment, the addition of beta-carotene to the light-exposed treatment yielded the same amount of water uptake as the dark control. Beta-carotene is a quencher of singlet oxygen, a reactive intermediate produced by sunlight and hypothesized to play a major role in indirectly oxidizing crude oil (see Section 3(a) for more discussion about singlet oxygen). The authors even hypothesized a mechanism to explain their results: partial photo-oxidation of large hydrophobic molecules yields products that act as oil soluble, surface-active agents and cause water droplets to be retained during physical mixing. Prior to the DWH spill, this hypothesized mechanism had not been explicitly tested, in part because the technologies required to test it were not yet developed.

image file: d0em00027b-f6.tif
Fig. 6 The formation of water-in-oil emulsions as defined by water content in Statfjord crude oil following exposure to varying laboratory conditions (visible light, shaking speed, and temperature). (A) Open circle: high light, shaking, and temperature. (B) Closed circle: high light and shaking, but low temperature. (C) Open square: high light, low shaking, and high temperature. (D) Closed square: low light, high shaking and temperature. (E) Open triangle: no light, high shaking and temperature. (F) Closed triangle: no light, high shaking, low temperature. Error bars represent one standard deviation from the mean of ten replicates. This figure was originally published in Thingstad T. and Pengerud B., The Formation of “Chocolate Mousse” from Statfjord Crude Oil and Seawater, Mar. Pollut. Bull., 1982, 14(6), 214–216,16 Copyright Elsevier (1979).

This primary research was included in a 1985 review article written Payne and Phillips,3 where the authors explicitly state that “changes in viscosity, spreading or contraction rates, and water-in-oil emulsification tendencies also may occur as a function of oil photo-oxidation.” They go on to state that these light-driven changes “can occur at rates comparable to evaporation,” a weathering process that is routinely considered to affect the selection of response tools.4,9,14

(b) Knowledge gained from DWH

One key, response-relevant finding from researching the DWH spill was that sunlight exposure substantially altered the physical properties of DWH oil at the sea surface. By systematically and independently testing the change in density, viscosity, and adhesion as a function of evaporation and photo-oxidation, the relative importance of these weathering processes was assessed.66 Simulated sunlight exposure of crude oil to the equivalent of < three days of natural sunlight in the GoM resulted in a two-fold greater increase in density, a three-fold greater increase in dynamic viscosity, and a two-fold greater increase in adhesion compared to the same oil that was evaporated to 30% mass loss.66 These results corroborate previous reports3 that photo-oxidation can rapidly impart major changes to the physical properties of crude oil, comparable in magnitude to evaporation.

Another key, response-relevant finding from researching the DWH spill was that sunlight is a key process in the formation of water-in-oil emulsions. Using a novel method developed to isolate surface active material67 and subsequent characterization of this material using FT-ICR MS, it was demonstrated that photo-oxidation yields compounds that partition to the oil–water interface and effectively behave as emulsifying agents.68 This is a critical finding because it corroborates the hypothesis put forth by Thingstad and Pengerud 1983,16 a hypothesis that was not testable at the time due to analytical limitations (i.e., both the isolation method and FT-ICR MS). This finding is also critical because it validates a “new view” that, for some oils, the formation of stable water-in-oil emulsions is a secondary process and is enhanced by photo-oxidation, a primary weathering process (Fig. 2). It is important to note that not all oils need photo-oxidation to form stable water-in-oil emulsions. However, it appears that for light-sweet crude oils, like DWH and Statfjord, photo-oxidation stimulates emulsification.

Lastly, through a combination of mesocosm experiments, kinetic modeling, and geospatial analysis, it was determined that photo-oxidation of DWH surface oil had the potential to hinder the performance of chemical dispersants.66 For example, simulated sunlight exposure of crude oil to the equivalent of <three days of natural sunlight in the GoM resulted in a ∼30% reduction in dispersant effectiveness. This decrease in dispersant effectiveness due to photo-oxidation exceeded that of evaporation (30% mass loss) by four-fold. This finding challenged the prevailing assumption that, at least for oils with chemical compositions similar to those produced in the northern GoM, the effects of photo-oxidation are negligible relative to other weathering mechanisms.9 When accounting for photo-oxidation, modelling efforts suggested that a considerable fraction of aerial dispersant applications targeting DWH surface oil may have had low effectiveness (Fig. 7).

image file: d0em00027b-f7.tif
Fig. 7 Assessing the impact of photo-oxidation on chemical dispersant effectiveness. The location of the Macondo well in the Gulf of Mexico is indicated by the white star. The location of the 412 flight paths for aerial dispersant applications are presented in yellow and are outlined by the white polygon. Each concentric circle represents the distance DWH surface oil travelled before photo-oxidation decreased dispersant effectiveness to <45%, assuming high irradiance and slow transit speed (red inner circle), mean irradiance and transit speed (green intermediate circle), and low irradiance and fast transit speed (black outer circle). Under mean conditions, a substantial fraction of aerial applications targeted photo-oxidized oil that may have had low effectiveness. Reprinted with permission from Ward et al., 2018 66 Copyright 2018, American Chemical Society.

The exact mechanism behind the apparent decrease in dispersant effectiveness is unknown. One explanation could be the low solubility of photochemically oxidized oil in the carrier solvent of dispersants (i.e., food-grade kerosene).66 This incompatibility would limit interaction between the surfactant molecules in the dispersant mixture and the oil, and thus the amount of oil that can be chemically dispersed. Another plausible mechanism could be the photochemical production of surface-active molecules that act as emulsifying agents and limit dispersant–oil interactions.16,68 Independent of the mechanism, laboratory studies to date support the conclusion that sunlight plays a critical role in the alteration of crude oil properties (both physical and chemical), and this may impact the performance of chemical dispersants applied in response to oil spills at sea. This new perspective is incorporated into updated guidance documents for the weathering of oil at sea and the use of dispersants in response to marine oil spills.56

(c) High priority knowledge gaps for future research

Assessing if other oil types and dispersant mixtures are negatively impacted by photochemical weathering. The next big ocean spill will likely not be northern GoM crude oil. Moreover, the reserve of dispersants available to use in response to the next spill may also be different. Therefore, it is critical that a broader range of oils (light to heavy, sweet to sour) and dispersant mixtures are evaluated for the impact of photo-oxidation on dispersant effectiveness. Pending the results, this experimental matrix could reveal insights into the mechanism of suppressed dispersibility of photo-oxidized oil, which may inform next-generation dispersant formulations.
Establishing the impact of photochemical weathering on alternative chemical agents used in oil spill response. Despite the widespread attention that chemical dispersants garner by the scientific community, media, and general public, they represent just one type of chemical agents used in response to spills. For example, surface washing agents are used to lift oil from substrates like rocks on coastlines. Chemical herders are used to “herd” oil like cattle into thick films prior to skimming or in situ burning. Lastly, emulsion breakers are used to help decant water from oil and maximize recovered oil storage capacity in skimming vessels. The impacts of photochemical weathering on the effectiveness of these alternative chemical agents is currently unknown but should be determined to enhance response operations.
Incorporating photo-oxidation into oil spill fate and transport models. Despite calls for inclusion of photochemical weathering into oil spill models over 30 years ago,64 current models do not account for changes to oil properties by photochemical weathering. Incorporating new algorithms that account for photochemical weathering could substantially improve model predictions, and, in turn, optimize response operations.

5. Concluding remarks

The DWH spill of 2010 provided the community with a rare opportunity to study how oil behaves in the ocean. The long duration of the spill presented scientists access to field samples to assess how predictions from the laboratory actually play out in nature. Moreover, sustained resources over a ten-year period after the spill afforded fresh perspectives from scientists outside of the oil spill science discipline and new technologies to probe oil weathering processes. Ultimately, our improved understanding of how oil behaves at sea is a notable silver lining of this devastating environmental disaster.

In this review, we highlighted three key findings from the DWH spill of 2010 related to the photochemical weathering of oil at sea. First, the rate and extent of photochemical weathering was much greater for DWH surface oil than expected based on early conceptual models of oil weathering. This finding led to revised conceptual models that accurately reflect the role of sunlight in oil weathering. Second, indirect photochemical processes played a major role in the partial oxidation of the floating surface oil. This finding settled a long-standing debate about the relative importance of direct vs. indirect processes and explains why such a large fraction of DWH surface oil was oxidized by sunlight. Third, the extensive and rapid changes to the physical and chemical properties of oil by sunlight may influence oil trajectory at sea and the selection of response tools. A comprehensive characterization of this relationship is critical because it could improve the accuracy of oil trajectory model predictions and the effectiveness of response operations moving forward.

Scientists in the oil spill community openly discuss the next big spill as an inevitable event, rather than a possibility. It's not if, but when and where. In this review we highlighted several instances where predictions made about the photochemical weathering of oil at sea several decades ago proved true for DWH. These early predictions were either overlooked or not accepted by the broader oil spill community at the time of the DWH spill. Nevertheless, by highlighting these predictions from the early oil spill literature and synthesizing the numerous novel findings from studying the DWH spill, it is our belief that the community is better positioned to respond to the next big oil spill in the ocean.

Conflicts of interest

There are no conflicts to declare.


We a grateful to all participants of the Gulf of Mexico Research Initiative Photo-oxidation Synthesis Workshop, which took place 4 to 5 December 2018 in Washington, DC, USA. Many of the discussion points raised during this workshop helped frame this manuscript. Participants included: Robyn Conmy (US EPA), Deborah French-McCay (RPS ASA Associates), Greg Hall (US CGA), Zhanfei Liu (UT Austin), Jim Payne (Payne Environmental Consultants), Jagos Radovic (U. of Calgary), Christopher Reddy (WHOI), Ryan Rodgers (FSU/NHFML), Charles Sharpless (U. of Mary Washington), Matthew Tarr (UNO), and Phoebe Zito (UNO). Many thanks to the workshop participants, John Farrington (WHOI), Danielle Freeman (WHOI), and two anonymous reviewers for constructive feedback on the manuscript. We acknowledge support from the Gulf of Mexico Research Initiative to coordinate the workshop, synthesize the findings of the workshop, design the graphics, and write the manuscript.


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