In response to the unprecedented anthropogenic disaster following the deep ocean BP oil spill, which was initiated by an explosion at the Deep Water Horizon oil rig on April 20, 2010 the University of California, Irvine rapidly initiated whole air canister sampling on several platforms of opportunity. They employed their well established and flexible technique of whole air sampling followed by high sensitivity gas chromatographic analysis including straight-chain saturated hydrocarbons from C-1 (methane) to C-12 (dodecane) and aromatic hydrocarbons such as benzene, toluene, as well as and higher molecular weight species, with special attention to the presence of toxic contributors. Where possible, atmospheric chemical species associated with dispersal agents such as Corexit were determined as well. A variety of "platforms of opportunity" were identified. These were the 65 ft vessel "R/V Eugenie" from the deck of which 96 whole air samples were collected between May 20 and May 23. The initial results were so informative that 48 sampling canisters were sent to the R/V Pelican and 96 to the NOAA ship Thomas Jefferson. In addition 120 samples were collected from a Twin Otter aircraft.
This RAPID study is one of three that makes up the bulk of a coordinated response to examine trace gas emissions from the oil leak. The second used methane data (AGS-1048798, RAPID: Spatially Enhanced Broadband Array Spectrograph System (SEBASS) Survey Over the Gulf Oil Spill) collected from the same Twin Otter aircraft to map methane plumes and the third RAPID (AGS-1042894, RAPID: Fossil-Fuel Extraction Industry Methane Emission Ground Reference Measurements during the AVIRIS Response to the Gulf Oil Spill) used the remote sensing platform AVIRIS on the NASA ER-2 aircraft to measure methane remotely and on a larger spatial scale. The ER-2 flight plan was coordinated to overlap with the Twin Otter flight plan. From whole air samples and remote sensing, a rough estimate of methane plume size from the oil spill will be realized.
After the BP Deepwater Horizon rig explosion occurred on April 20, 2010 about 66 km (41 miles) off the Louisiana coast and initiated an unprecedented flow of crude oil hydrocarbons deep into the Gulf of Mexico, we rapidly deployed our well-established and flexible technique of whole air sampling followed by high sensitivity gas chromatographic analysis. Gas chromatography is one of the most powerful analytical methods for quantitative analysis of the atmosphere and the fate of the leaked hydrocarbons in the marine environment. The spill continued for nearly 3 months. Our samples of the near-surface atmosphere were collected aboard ship one month into the oil spill (May 20-23) and again during a follow-up period at about 2 months (June 15-27). Crude oil is a complex mixture of chemical constituents, including (but not limited to) various alkanes (e.g., butane, pentane, and hexane) and aromatic hydrocarbons (benzene, ethyl benzene, toluene, and xylenes). Some of these constituents of crude oil are toxic, especially benzene, which is considered to be human carcinogen and known to cause a host of health problems, including nausea and bone marrow damage. The only gaseous hydrocarbon normally present at a concentration of one part per million by volume in the remote atmosphere is methane with about 1800 ppbv, as seen in the "Background" samples that were collected away from the influence of the spill (Table 1). Immediately downwind of the former platform location "near site" (Table 1) we measured extremely high levels of a large number of volatile alkane type hydrocarbons such as n-hexane (see Table 1 and Figures 1 and 2), which comprised nearly 90% of the measured hydrocarbons. These hydrocarbons are typical of crude oil and indicate that they originated as part of the spill. The total average atmospheric NMHC burden in this area was more than 7 ppmC (parts per million Carbon). For comparison, these levels were more than 10 times higher than typically measured downwind of major cities. BP reported that the material coming out of the leaking pipe was about 40% by weight of natural gas (mostly methane), the remaining component being oil. However, methane levels where the other oil-derived hydrocarbons were at such high concentrations were almost unaffected. Methane and the other light hydrocarbon components of natural gas are relatively soluble, with methane being the most soluble. So they will dissolve relatively quickly at the1500m depth that the oil was coming out of the well riser. The aromatic compound benzene, which is a component of crude oil was also largely absent from the near-surface atmosphere. Benzene rings are very stable, and therefore persistent in the environment, and can have toxic effects on organisms. How much of any particular hydrocarbon in the mixture of gas and oil that was released remains in the water, and how much is released into the air is initially determined by the solubility and volatility of the individual hydrocarbon species. Quantifying this partitioning is necessary to understand gas and oil transport, to predict marine bioavailability of different fractions of the gas-oil mixture, and to develop a comprehensive picture of the fate of leaked hydrocarbons in the marine environment We conclude that during May and June the spill rapidly released huge amounts of relatively insoluble alkane hydrocarbons into the atmosphere of the Gulf of Mexico, where they were transported downwind. However, large fractions of the methane and benzene from the spill appear to have dissolved into the water column prior to the oil surfacing, remaining below the surface, possibly for some time, where they awaited consumption by aquatic microbes and/or delayed release to the atmosphere. Publications: Ryerson, T. B., et al. (2011), Atmospheric emissions from the Deepwater Horizon spill constrain air-water partitioning, hydrocarbon fate, and leak rate, Geophys. Res. Lett., 38, L07803, doi:10.1029/2011GL046726. de Gouw, J., et al., Organic Aerosol Formation Downwind from the Deepwater Horizon Oil Spill, (2011), Science, Vol. 331 no. 6022 pp. 1295-1299 DOI: 10.1126/science.1200320
