Abstract
During the Deepwater Horizon (DWH) blowout, photooxidation of surface oil led to the formation of persistent photooxidized compounds, still found in shoreline sediments a decade later. Studies demonstrated that photooxidation modified both biodegradation rates of the surface oil and the effectiveness of aerial dispersant applications. Despite the significant consequences of this weathering pathway, the lack of measurements prevented photooxidation to be accounted for in the DWH oil budget calculations and in most predictive models. Here we develop a Lagrangian photooxidation module that estimates the dose of solar radiation individual oil droplets receive while moving in the ocean, quantifies the likelihood of photooxidative changes, and continues to track the transport of these persistent photooxidized compounds. We estimate and track the likelihood of photooxidation of Lagrangian oil droplets in the upper layers of the water column for the DWH case by coupling the net shortwave radiation from NOGAPS to the oil application of the Connectivity Modeling System (oil-CMS). The dose of solar radiation upon a droplet is computed with the intensity of the incoming irradiance at the ocean’s surface, the light attenuation coefficient, and the depth of the oil droplets. Considering a range of DWH empirical irradiance thresholds, we find that photooxidation can happen at short time scales of hours to days, in agreement with the new paradigm of oil photooxidation. Furthermore, the oxidized compounds are likely to form in a 110 km radius around the response site, suggesting that the oil reaching the coastline was already photooxidized. This new dynamic coupling provides a powerful tool to test oil weathering hypotheses, refine the oil budget during the DWH, and ultimately inform rapid response in future oil spills.
Highlights
The Deepwater Horizon (DWH) was an unprecedented spill: the deepest and the largest spill in history, it released over 130 million gallons of a mixture of oil and natural gas at 5000 feet under the sea surface for over 87 days (McNutt et al, 2012; Paris et al, 2018)
The solar irradiance median ranges from 177 Wm−2 to 716 Wm−2 over this period, with substantial daily and weekly changes, for example between June 1st and June 4th when the median solar irradiance varies from 695.9 Wm−2 on June 1st to 356 Wm−2 on June 2nd, back to 677.7 Wm−2 on June 4th
To explore how this varying solar irradiance can affect the probability of photooxidized compounds (POCs) formation, we investigate the locations where photooxidation likely took place along the spill duration, considering the higher, and most conservative, threshold for photooxidative changes (Ward et al, 2018a)
Summary
The Deepwater Horizon (DWH) was an unprecedented spill: the deepest and the largest spill in history, it released over 130 million gallons of a mixture of oil and natural gas at 5000 feet under the sea surface for over 87 days (McNutt et al, 2012; Paris et al, 2018). The critical weathering pathway of near-surface photooxidation still lacks from oil spill numerical models, the well-defined effects of varying sunlight on photooxidation (Ward et al, 2018a,b; Ward and Overton, 2020). We fill this lacuna by translating recent laboratory and field studies into a photooxidation module for the oil fate and transport application of the Connectivity Modeling System (oil-CMS, Paris et al, 2012, 2013). The novel photooxidation Lagrangian algorithm integrates spatio-temporally explicit solar irradiance from Earth Systems models to estimate the cumulative irradiance received by each droplet as they are transported, along with estimating the likelihood of photo-induced changes on individual Lagrangian droplets
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