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Oceans Remove Nitrogen Oxides from Air at Night, Researchers Find

Mar 07, 2014 10:57 AM EST

Oceans act as nitrogen oxide sink and clear up photochemical smog during night, according to a new study.

The study, conducted by researchers at University of California, San Diego, found that an ocean removes as much as 15 percent of nitrogen oxides at night along the coast.

Photochemical smog is acondition that develops when pollutants react in sunlight to form a mixture of compounds that are hazardous to health.

Nitrogen oxides are a family of highly reactive gases. Cars, trucks and power plants are few sources of the gas. Nitrogen oxides react with volatile organic compounds to produce ozone (smog) on hot summer days, according to the U.S. Environmental Protection Agency.

The latest finding could help researchers understand the fate of nitrogen oxide molecules.

"One often neglected path is reaction at the surface of the sea," said Tim Bertram, an assistant professor of chemistry at the University of California, San Diego, who led the research. "The sea has a salty, rich, organic surface with the potential for a variety of chemical reactions."

In the present study, researchers decided to follow dinitrogen pentoxide (N2O5), which is a molecule formed after oxidation of nitrogen oxides. Dinitrogen pentoxide can react with chlorides in water to form nitryl chloride. In the presence of sunlight, nitryl chloride regenerates nitrogen oxides and frees a chloride radical that reacts with other molecules in the reaction to form ozone.

To follow the flow of the molecules, Michelle Kim, a graduate student at UC San Diego's Scripps Institution of Oceanography placed instruments at the end of a pier in La Jolla, Calif.

The night of February 20, 2013, provided researchers with an opportunity to study the fate of nitrogen oxide molecules as the usual offshore breeze reversed. The atmospheric conditions helped researchers look at the gas exchange between air and sea.

Kim found that dinitrogen pentoxide was taken by the ocean, but didn't find nitryl chloride being released in air.

"We knew from previous work that nitrogen oxides are lost to various surfaces - sea spray and other aerosols, even snowpack," she said in a news release. "This study shows - for the first time - that the ocean is a terminal sink for nocturnal nitrogen oxides, and not a source for nitryl chloride under these sampling conditions."

The study is published in the journal Proceedings of the National Academy of Sciences.


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