Study Finds Dispersants Reduce VOC Emissions and Increase Airborne Particles with PAHs

Ratios of nano-scale particle concentrations for seawater surface with oil-dispersant mixture compared to that of clean seawater at different times after initial wave impact. From Figure 3 of the study, provided by Afshar-Mohajer.

Ratios of nano-scale particle concentrations for seawater surface with oil-dispersant mixture compared to that of clean seawater at different times after initial wave impact. From Figure 3 of the study, provided by Afshar-Mohajer.

Researchers conducted laboratory wave tank experiments to investigate how plunging breaking waves affect the concentration of particulate and gaseous emissions from oil slicks. Large waves generated total volatile organic compound (VOCs) emissions for oil-only trials that were 1.5-2 times higher than small waves, and the addition of dispersant reduced total VOC concentration up to 60% for small waves and 76% for large waves, suggesting that dispersant may suppress organic compound diffusion or increase dissolution rates. Crude oil and oil-dispersant trials generated particle-bound polycyclic aromatic hydrocarbon (PAH) concentrations 3-5 times higher than dispersant-only trials, with oil-dispersant trials generating very small particle (1 – 100 nanometers) concentrations 10-100 times higher than oil-only trials but did not greatly affect particulate matter larger than 1 micrometer. The increased nanoparticle aerosolization could represent a health concern as nanoparticles are able to travel large distances and affect human and animal respiratory systems. The researchers published their findings in Atmospheric Environment: A laboratory study of particulate and gaseous emissions from crude oil and crude oil-dispersant contaminated seawater due to breaking waves.

Oil spill incidents occur frequently and can cause a variety of occupational, ecological, and environmental problems. The use of chemical dispersants reduce the amount of surface oil and helps prevent shoreline oiling, but it may also alter the emissions of particulate matter and VOC gases (like benzene and toluene). Particulate matter and VOC gases can enter the atmosphere through breaking waves and bursting bubbles, and exposure to these substances has been associated with negative health effects including respiratory and cardiovascular disease.

To gain insight into how dispersants may affect airborne particulate matter and VOC emissions, this study’s researchers conducted wave tank experiments with breaking waves of different strengths, properties, and energy dissipation rates of the same order as oceanic conditions. The experiments used oil only (Louisiana Light Sweet crude oil), an oil-dispersant mix (1:25 ratio Corexit 9500A, within the range of previously tested ratios in wave-tank experiments by Li, 2017), and dispersant alone. To measure airborne particles, the team used an aerodynamic particle sizer, a scanning mobility particle sizer, digital holographic microscopy, and a photoelectric aerosol sensor. The team used a handheld photoionization detector to monitor VOC concentrations.

The authors posited that reduced interfacial tension generated by dispersant application likely caused the observed increase in aerosolized nanoparticle in oil-dispersant treatments. Study author Kirsten Koehler explained the significance of increased aerosolized nanoparticles, “It is likely that these nano-sized particles contain toxic crude oil compounds, which may deposit in the alveolar region of the human respiratory system where gas exchange occurs. Companies should consider wind direction when using dispersants to minimize the potential exposures of workers to nano-sized particles.”

The researchers suggest that future studies include longer experiments, specification of the compounds involved, and consideration of other natural contributors that might affect VOC concentration, such as wind and sunlight.

Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at doi:10.7266/N75M63RJ.

The study’s authors are Nima Afshar-Mohajer, Cheng Li, Ana M. Rule, Joseph Katz, and Kirsten Koehler.

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This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Dispersion Research on Oil: Physics and Plankton Studies II (DROPPS II) consortium.

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.

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