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  Geotimes - July 2008 - Hazards of a sea breeze

Hazards of a sea breeze

Sunset over the Gulf of Mexico
Daniel Welsh-Bon
As the sun sets over the Gulf of Mexico, a chemical reaction between salts in sea spray and pollutants in the air forms a compound that releases ozone-producing chlorine atoms.

If you want to escape the dirty air of large port cities, a seaside stroll may not help. Mixtures of sea salt spray and reactive nitrogen oxides — spewed into the air by cars, ships and industry — can form a compound that leads to the production of ozone, making coastal air pollution bad. A new study published in the May issue of Nature Geoscience reports that the air along the coastline of the southeastern United States contains large amounts of nitryl chloride, a compound that releases an ozone-producing chlorine atom when hit by sunlight. A regulated pollutant, ozone irritates the lungs of humans and animals, and damages crops and other vegetation.

Hans Osthoff, an atmospheric chemist at the University of Calgary in Canada and formerly with NOAA in Boulder, Colo., and his colleagues collected air samples while aboard a research vessel sailing from Charleston, S.C., around Florida and through the Gulf of Mexico to Houston. The team found large concentrations of nitryl chloride along the coastline near major cities, where urban and industrial activities release nitrogen oxides into the air, and across the Gulf of Mexico, where the exhaust plumes of ship engines are the main polluters. They determined that a reaction between dinitrogen pentoxide and chloride salts in sea spray forms nitryl chloride at night. After sunrise, ultraviolet light cracks the molecule and produces a chlorine atom that oxidizes hydrocarbons, ultimately producing ozone. “This is the first time that nitryl chloride has been [observed] in the ambient air anywhere in the world,” Osthoff says.

The world’s oceans contain enormous amounts of chloride salts but they are harmless unless they are converted into reactive chlorine atoms. Laboratory studies show that dinitrogen pentoxide is capable of that conversion, but the consensus had been that the process does not play a big role in the atmosphere because air blowing across the open ocean is usually very clean, Osthoff says. “But things change in areas along the coast where millions of people live and where lots of traffic and industry pollute the air,” he says. “The wind blows all this pollution out to the sea and that leads to the formation of a zone along the coast where you suddenly get marine air and continental air mixed.”

The study is adding a piece to the long-standing atmospheric puzzle of how chlorine atoms are produced from sea salt, says Joel Thornton, an atmospheric scientist at the University of Washington in Seattle. Previous studies in various regions, including the Houston area, showed that chlorine atoms were bouncing around in the air, Thornton says. “We have an estimate of how much chlorine atom chemistry is out there, globally averaged, because you can look at markers in the air that essentially illustrate that chlorine atoms had been attacking them.” But until now, how these chlorine atoms formed had been largely a mystery, he says.

This is the first study of its kind and measurements in other areas are needed, Thornton says, but it’s likely that the process is important on a global scale. “You have plenty of nitrogen oxide emissions in coastal regions because they are tied to human activities and the population density in coastal regions is quite high,” he says. Osthoff and his colleagues estimate that about one-tenth of the global chlorine atom production could be due to reactions with nitrogen oxides. “To have that much driven by human activity is really quite significant,” Thornton says.

Nicole Branan
Geotimes contributing writer

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