Ocean can clean air at night, study suggests

LOS ANGELES — The ocean doesn’t just moderate temperatures and influence weather in some of the world’s biggest cities; it also has the power to cleanse the air, new research suggests.

At night, the sea surface can absorb and remove up to 15 percent of smog-forming nitrogen oxides that build up in polluted air in coastal cities like Los Angeles, according to a study published this week in the Proceedings of the National Academy of Sciences.

Researchers at UC San Diego, came to that conclusion after deploying scientific instruments at the Scripps Institution of Oceanography pier last year to measure the exchange of trace gases between the air and the sea.

The conditions were just right one night in February when onshore winds blew a polluted air mass from the Los Angeles Basin along the coast and toward the sea, allowing the researchers to track what happened to the nitrogen oxide gases as they swept across the surface of the sea.

Tim Bertram, an atmospheric chemist at UCSD who conducted the research with graduate student Michelle Kim, said the measurements taken that night provided one of the first real-world answers to a long-standing question: To what extent does the ocean surface remove the ingredients of smog?

All day long our vehicle tailpipes, factories, trains and ships emit nitrogen oxides, gases that react in sunlight to form ozone, or smog, Bertram explained. After dark, the nitrogen oxide emissions continue, but the chemical reactions they go through get a bit more mysterious.

Bertram said he was expecting the pollutants to react at the ocean surface to form other compounds. To his surprise, the analysis showed that ocean water is a “terminal sink” for nitrogen oxides, meaning it permanently removes them from the air.

“As soon as it’s lost to the ocean surface it’s gone,” Bertram said.

The interaction of the ocean, atmosphere and the pollution we generate is so complex that it is too soon to say whether the study’s findings mean that seawater has an overall benefit for air quality in coastal cities, Bertram said.

“It certainly is important, but it’s yet to be quantified exactly how important that process is to smog formation,” he said.

But it’s a question worth further research, the article says, because nearly half of the world’s population lives within about 125 miles of the coast, releasing much of the world’s nitrogen oxide pollution close to saltwater.

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