Sea surface acts as sink for nitrogen oxides at night

A new research on the coast of southern California has shown that the surface of the sea takes up nitrogen oxides that build up in polluted air at night.

The ocean removes about 15 percent of these chemicals overnight along the coast, a team of atmospheric chemists said.

Nitrogen oxides, formed by the burning of fossil fuels, generate photochemical smog. Atmospheric chemists would like to account for the fates of these molecules in a kind of budget that indentifies their sources and sinks - ways in which they are removed from the air.

"One often neglected path is reaction at the surface of the sea," Tim Bertram, an assistant professor of chemistry at the University of California, San Diego, who led the research, said.

"The sea has a salty, rich, organic surface with the potential for a variety of chemical reactions," he said.

To track the cycle of nitrogen in the atmosphere, they studied dinitrogen pentoxide, a molecule that results from the oxidation of nitrogen oxides. It can react with chloride from sea salt, for example, to form nitryl chloride.

When sunlight hits nitryl chloride the next morning, it regenerates nitrogen oxides and frees a chlorine radical that attacks other molecules in reactions that can lead to the formation of ozone.

Michelle Kim, a graduate student at UC San Diego's Scripps Institution of Oceanography working with Bertram, deployed intruments at the end of the institute's pier in La Jolla, Calif., to measure the flux of these molecules.

"We knew from previous work that nitrogen oxides are lost to various surfaces - sea spray and other aerosols, even snowpack," she said.

"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," she added.

The findings are published online in the Proceedings of the National Academy of Sciences.