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Clean Air Initiative – Smog: A Primer

11/01/03 by Halley Brantley

Photochemical smog. Ozone. PAN. Ter- penes.

Where did these terms come from, what causes them and what effect do they have on the air we breathe?


The term “smog” was coined in 1905 by Dr. H.A. Des Voeux, a member of the Coal Smoke Abatement Society in England. A contraction of the words “smoke” and “fog,” the air pollution we typically see today has nothing to do with either. Auto exhaust is the major culprit in the production of most smog today, says Dr. James Beard, author of Chemistry, Energy and the Environment and chair of the Chemistry Department at Catawba College, but it takes more than automobiles to produce this toxic substance.

Two other conditions have to be present with auto exhaust before photochemical smog emerges:

1) It has to be warm — at least 68 degrees Fahrenheit. “You don’t have smog of that type in Chicago in the wintertime,” Beard says. “It’s not warm enough.”
2) Sunlight must also be present.

“Another thing that can make it really nasty is when there is a method of concentrating the smog,” Beard says. “The reason Los Angeles has such a problem is that it’s hemmed in by mountains. The city gets these constant sea breezes off the ocean in the summertime, so the photochemical smog builds up in the morning and just sits in the valley.”

Scientists first determined the basis of photochemical smog in the late 1950s. At first, they caught auto exhaust in boxes and tried to duplicate the effects they witnessed in areas that suffered from smog. “Nothing happened until somebody got the idea of putting a bright light in the box,” Beard says. “That was the beginning of understanding how this
stuff gets generated.”

Basically, photochemical smog involves the action of sunlight on hydrocarbons in the air. Hydrocarbons are chemicals found in petroleum products like gasoline, turpentine or kerosene. Nitric oxide is produced when fuels are burned. “Any time you get anything really hot, you set up a situation where the oxygen and nitrogen in the air can react with each other,” Beard says.

It can come from any number of sources — a lightening strike, a hot boiler, an auto engine.
While nitric oxide is toxic, it is not a long-lived molecule, according to Beard. However, ni- tric oxide reacts with the air and produces nitrogen dioxide which, in turn, can react with sunlight to produce ozone.


Ozone is a form of oxygen. While molecules of ordinary oxygen are comprised of two oxygen atoms, ozone molecules have three. “Ozone is a respiratory irritant and has toxic effects at low levels,” Beard says. “A lot of air pollutants are measured in parts per million. We generally measure ozone in parts per billion because it is that sensitive and that nasty.”



Ozone reacts with hydrocarbons to create droplets, says Beard. “That’s what gives you the white haze associated with photochemical smog.” The haze then reacts with more ozone to create PAN (peroxyacyl nitrate), a class of substances that cause the eyes to tear. “If you have a smog alert, your eyes will water,” he says. “It’s the PAN in the air.”

While humans suffer from ground level ozone and PAN, plants are even more sensitive to the pho- tochemical soup. “One of the first things people noticed in L.A. was yellow spots on leaves,” Beard says.



The haze that often hangs over the Great Smokies and the Blue Ridge Mountains is related to photochemical smog. However, the source is different. In this case, the hydrocarbons come naturally from the trees. Called terpenes, these hydrocarbons cause the haze. “The nitrogen dioxide levels and ozone levels are low enough that they are pretty much harmless,” Beard says. “But there’s a relation between what goes on in the Blue Ridge and what goes on in L.A. in a smog alert. It’s just a matter of degree.”

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