Ocean's surface could have big impact on air quality, study says

03/03/04

COLUMBUS, Ohio Certain ions bouncing around on the ocean's surface and in droplets formed by waves may play a role in increasing ozone levels in the air we breathe, new research suggests.

These ions cover the surface of the sea in an ultra-thin blanket about one-millionth the thickness of a sheet of paper. Researchers call this region the "interface."

Using a technique that employs highly accurate laser beams, chemists for the first time saw the actual structures formed by these halogen ions, or halides. They could see just how molecules of water surround these ions and also determine the halides' whereabouts within the interfacial area.

This kind of information can help researchers predict which halides are more likely to react with other chemicals and ultimately form ozone, a naturally occurring gas which enhances the upper atmosphere's defense against harmful ultraviolet rays.

"Interfacial halides have a significant effect on atmospheric chemistry which, in turn, could pose serious implications for respiratory health," said Heather Allen, the study's lead author and an assistant professor of chemistry at Ohio State University. The study appears in the current issue of the Journal of Physical Chemistry B.

Scientists have noted increased ozone levels in urban areas near seawater, and suspect that halides may play a key role.

"In marine areas, halides can react with other molecules that form ozone and ultimately increase ozone production in nearby urban areas," Allen said.

While the ozone layer in the upper atmosphere is essential for shielding the earth from some solar radiation, high amounts of ozone in the lower atmosphere can cause serious respiratory problems.

In a series of laboratory experiments, Allen and her colleagues studied water structures created by three halides commonly found in the marine interfacial zone chloride, bromide and iodide.

The researchers mixed each halide with water to create experimental interfacial zones. They then projected two beams of laser light onto each solution in an attempt to see the structure and location of each halide in the interface.

Allen said that while these kinds of pristine interfaces wouldn't be found on the ocean's surface, where many more chemicals are at play, knowing the concentration and structure of interfacial halides could help scientists better understand atmospheric chemistry.

"Studying liquid surfaces is difficult," Allen said. "They may look flat, but they're nowhere near flat on a molecular level. The addition of halides and other chemicals alters water's surface structure."

When mixed with water, halogen salts become halides charged particles that, by nature, are unstable and are looking to combine with other elements in order to regain their stability. Two of these halides iodide and bromide like to combine with ozone-forming chemicals.

"Even though the halides are only one part of the chemical mix in the interface, we didn't really understand how important they were to atmospheric chemistry until we were able to separate out their individual characteristics," Allen said.

The researchers found that the concentrations of halides changed deeper into the interfacial layer. Iodide ions favored the surface of the interface, followed by bromide ions. Chloride ions were in abundance in the lower portion of the interface and did not affect the water's surface structure. By virtue of their position in the interface, the iodide and bromide may have a greater impact on the air we breathe.

"Iodide turned out to be the most important halide when it came to surface reactions, because it had the highest concentration at the interfacial surface," said Allen, adding that just a little iodide or bromide can influence ozone creation. Chloride appears to be less likely to do so.

"Halogens compete with other radicals that are normally used to create ozone," Allen said. "But when enough halogen radicals are available, they actually react faster than do other radicals.

She said the next step is to examine the actual reactions between the halides and non-halogen molecules near the sea surface to see if they can actually determine how much ozone is formed and where it's created in greatest quantities.

Allen conducted the study with fellow Ohio State researchers Dingfang Liu, Gang Ma and Lori Levering. The team received funding for this work from the National Science Foundation-funded Ohio State Environmental Molecular Science Institute and in part by Research Corporation, based in Tucson, Ariz.

Source: Eurekalert & others

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