Research Highlight: The Ocean as Air Filter

UC San Diego researchers have demonstrated that the ocean acts as an air filter at nighttime, absorbing pollutants that mingle with the sea surface after the sun goes down.

Nitrogen oxides are among the pollutants created by the burning of fossil fuels. Scripps Institution of Oceanography graduate student Michelle Kim and UC San Diego Department of Chemistry and Biochemistry researcher Tim Bertram followed the path of nitrogen oxides from the smoggy skies over Los Angeles to the surface of the ocean off San Diego, where that air pollution often drifts.

They determined that the ocean is what scientists call a “sink” for these chemicals that react with sunlight, removing about 15 percent of these compounds from the atmosphere. What doesn’t get absorbed stays in the atmosphere. When the sun shines again and the photochemicals break down, they are available to mix with other chemicals in the atmosphere to create the global warming agent known as ozone.

“We knew from previous work that nitrogen oxides are lost to various surfaces – sea spray and other aerosols, even snowpack,” said Kim, who is co-advised by Bertram and Scripps oceanographer Grant Deane. “This study shows – for the first time – that the ocean is a sink for nitrogen oxides at night.”

The National Science Foundation-funded research is part of a general quest among climate scientists to understand where air pollution goes once it is created and how it changes natural processes. Bertram and Kim said that researchers had wondered if the ocean could be the final resting place for some nitrogen oxides but lacked the technology to follow its path.

Instruments developed by Bertram and private companies including Boston-based Aerodyne and the unique measurement facilities housed at the end of the Scripps Pier led to a way to answer the question.

The instruments the researchers developed made  simultaneous measurement of updrafts and downdrafts over the ocean surface and of the chemical composition of those winds, a condition known as eddy covariance. The Scripps Pier, said Bertram, is one of the few places in the world where atmospheric chemists can “test drive” micrometeorology methods such as this.

 “The facility is fantastic. We can put these instruments essentially out in the atmosphere while still controlling the setting,” said Bertram. “I can’t think of any other place in the world to do these hard measurements.” 

Kim selected nights with favorable onshore flows (winds that blow in from the ocean) that would allow her to observe overnight changes in quantities of compounds created by the importing of nitrogen oxides from Los Angeles to the La Jolla coast. She found a net reduction of certain forms of the pollutant but no net gain of other forms often created when nitrogen oxides mix with sea salt particles.

The study appears in the journal Proceedings of the National Academy of Sciences.   

Though Bertram and Kim did not consider what effects the absorption of the nitrogen oxides have on the ocean, Bertram suggested they were likely to be very slight. Though a little of it goes a long way in forming ozone in the atmosphere, the parts-per-billion concentration of the compound in seawater was probably negligible.

“It would be like putting a very light dusting of flour on top of a really, really large cake,” he said.

– Robert Monroe

Related Image Gallery: The Ocean as Air Filter