Study Sheds New Light on Formation of L.A.’s Smog
Sunshine and salty air are among the sought-after pleasures of coastal living. But an intense chemical interaction of sunlight, sea salt and air may be contributing to Southern California air pollution, according to a UC Irvine study to be published today.
“If you want to understand the climate, truly you have to understand the dynamics of sea salt,” said Donald Dabdub, an assistant professor of mechanical and aerospace engineering who wrote the article in the journal Science with chemistry professor Barbara Finlayson-Pitts.
Their findings may have significant implications for addressing greenhouse gases and pollution in coastal urban areas, as well as for understanding global climate change, since salt water covers two-thirds of Earth’s surface, Dabdub said.
It remains unclear how much the new findings might change the current understanding of air pollution in southern California. Sam Atwood, spokesman for the South Coast Air Quality Management District, declined to comment on the specifics because he had not yet seen the study.
“We’re definitely interested to take a look at the full study. Whether it will demonstrate a deficiency in our air pollution modeling remains to be seen,” he said.
The process the researchers studied starts with particles of sodium chloride, or sea salt, and water in the air over the ocean and at the coast. Offshore breezes push that ocean air inland.
Researchers found that when sea salt is exposed to daylight, it undergoes a previously unrecognized reaction on the surface of the salt particle that eventually results in the release of chlorine gas.
The chlorine gas, with emissions from the burning of oil, coal and gasoline, increases the amount of ozone--the key constituent of smog--in the lower atmosphere.
Scientists had thought the reaction occurred inside the particle, and therefore did not release chlorine gas.
“The chemistry that we proposed was basically a reaction on the [surface] of the particle,” Dabdub said. “We were able to show that this interface has a key role that orchestrates this reaction. In that way, we were able to explain all sorts of things that were misunderstood about chlorine chemistry.”
“The plot thickens when all these chlorine gases are in the presence of products of combustion,” he added.
Though ozone in the stratosphere is beneficial because it blocks the sun’s ultraviolet rays, ozone in the lower atmosphere is an irritant to lungs and eyes. In the Southern California basin, ozone levels often exceed federal Clean Air Act standards.
Smog--a brew including chemicals, solvents, emissions and exhaust--is a hallmark of Southern California, but has been steadily declining because of increased regulation of vehicle and industrial emissions. However, the region’s mountains and inland winds trap what smog remains in the Los Angeles Basin.
Last summer, there were no full-scale smog alerts for the first time in half a century. Last year also was the first in which the region’s air quality was not the worst in the nation--it was second, trailing Houston by one smoggy day.
“Geography plays a very significant role in our pollution problem,” said Atwood. “It, of course, is not the only factor.”
The study was funded by the U.S. Department of Energy, the North Atlantic Treaty Organization, the National Science Foundation and the UC Irvine Council on Research, Computing and Library Resources.
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Suspicious Sea Salt
Scientists at UC Irvine have discovered a chemical reaction between the sun, air and sea salt that may be contributing to Southern California’s pollution problem:
1. Sunlight breaks apart ozone molecule (O3)
2. One of the resulting oxygen molecules (O) attracts a hydrogen molecule (H) from water in sea salt (H2O and NaCl)
3. The resulting hydroxyl radical (OH) adheres to and reacts with chloride (Cl) on surface of sea salt, eventually producing chlorine (Cl2) gas, which, when combined with emissions from burned fuel, produces more ozone.
Source: UC Irvine
