4 June 2004

An ozone refresher course

Forrest M. Mims III

Why is there so much concern about too little ozone high in the stratosphere and too much in the troposphere down below?

Because the spring ozone season is now underway across the northern hemisphere, this is a good time to review this topic.

Ozone is a very pale blue, pungent gas. No matter how it is formed or where it is, ozone is a special kind of oxygen molecule with 3 oxygen atoms instead of the usual two.

High in the stratosphere above where jet planes fly, sunlight splits oxygen molecules into individual oxygen atoms. Ozone is formed when these free atoms combine with oxygen molecules.

The result is the blanket of ozone around the entire Earth known as the ozone layer. The ozone layer efficiently absorbs dangerous rays of ultraviolet sunlight. Without the ozone layer, most life as we know it could not exist.

Around 90 percent of the ozone in Earth's atmosphere is in the ozone layer. The remaining 10 percent is in the troposphere between the surface and the stratosphere.

The same process that forms the ozone layer can also form some of the ozone near the ground. Ozone is also formed near the ground by many different chemical reactions.

For example, nitrogen oxide in car exhaust efficiently destroys ozone. This is why ozone concentrations in and downwind from big cities can plummet to zero overnight. However, during daylight hours, nitrogen oxide eventually becomes nitrogen dioxide. Ozone is formed when nitrogen dioxide and organic vapors are illuminated by ultraviolet sunlight.

The organic vapors are often called volatile organic compounds or VOCs. They can come from gasoline and other petroleum products and vegetation, especially oak trees.

The ozone layer is considered good ozone, because it shields us from excessive ultraviolet. Ozone near the ground can also be good, for it kills viruses and helps purify the air.

However, excessively high concentrations of ozone can suppress the growth of plants and cause problems for people with asthma. This is why the Federal Clean Air Act regulates ozone in the air we breathe.

The Clean Air Act is responsible for major improvements in air quality. Unfortunately, however, some of the science in the act is out of date. For example, the act was passed before scientists discovered that smoke from fires can increase ozone.

Nor did the writers of the act know that pollen, spores and indoor air pollution cause far more problems for asthmatics than outdoor ozone.

Because the Clean Air Act also penalizes counties and cities for dirty air episodes over which they have no control, the Congress is considering revisions based on the latest scientific findings. Reformers claim this will place the emphasis on cleaning up the air at the source of the problem. While many environmentalists agree that regional air pollution is a major problem, they argue that loosening current controls is a step backward that will overlook local source of pollution.

Forrest M. Mims III has measured the ozone layer since 1989. His science is featured online at http://www.forrestmims.org/.

This feature was originally published in Forrest Mims's weekly science column in the Seguin Gazette-Enterprise, Seguin, Texas. The column is written for a general audience.