An Assessment of the Sea-Surface Interactions of Offshore Ozone with the Neuston Layer in the Gulf of Mexico. P. F. Roscigno and A. Lugo-Fernández, Minerals Management Service, Gulf of Mexico OCS Region, 1201 Elmwood Park Blvd., New Orleans, LA 70458
The Gulf of Mexico’s Federal Outer Continental Shelf (OCS) is a region where the large number of exploration and production operations for oil and natural gas require high energy consumption. This activity releases to the marine atmosphere substantial amounts of NOx (~146,000 t/y) and total hydrocarbons (~294,000 t/y), which, under appropriated meteorological conditions, react to form ozone (0-54 Tg/m3 over water) that can potentially affect the marine environment or be transported elsewhere. Naturally occurring ozone in the troposphere has an ambient concentration of about 35 Tg/m3. Elevated (395 Tg/m3) ozone concentrations are indicative of pollution and result from photochemical reactions between hydrocarbons and NOx, aided by stagnant air conditions and adequate sunlight. For summer conditions in the Gulf of Mexico, winds blow generally from the southeast and stagnant conditions occur 5 to 8 percent of the time. High ozone concentrations occur over water for up to 6-7 days and respond to wind pattern changes at similar time scales. The offshore ozone plume is the result of an integration of mixing, dispersion, advection, and emissions over time and space and represents the cumulative effects of these factors over time. The fate and effects of ozone in aquatic media depend on its concentration which is controlled by dry deposition, solubility, and chemical reactions on the water surface. The physical, chemical, and biological conditions of the sea-surface microlayer (SSM) are characterized by large gradients of temperature and salinity and by high irradiance that are coupled with the SSM’s ability to concentrate pollutants and result in a dynamic and stressful habitat. This work examines the amount of OCS-derived ozone deposited in the sea surface of the Gulf of Mexico and assesses its impact on the neuston of the sea-surface microlayer. Estimated ozone deposited over the sea surface ranged from 400 to 1800 kilograms during the episode’s duration which resulted in sea-surface concentrations of ~15 Tg/m3. Based on this information and published toxicity values for ozone, impacts to the neuston range from acute to chronic during the episode’s duration. Other indirect impacts can occur because of changes to carbon pathways and bromate production; however, present work is directed toward quantifying these uncertainties.