Imagine a typical Hollywood "gotcha" film. The plot involves the discovery that the apparent "bad guy" is in fact the "good guy," and that the seemingly nice guy is in fact a fiendishly evil genius. This is the case with ozone and chlorofluorocarbons.

Ozone is a molecule consisting of three oxygen atoms. Its chemical notation is O3. Ozone is a "bad guy" in that breathing it is lethal at dosage levels of a few molecules per million air molecules. This is why ozone at the surface is referred to as a pollutant. Yet ozone high in the atmosphere screens out biologically harmful solar ultraviolet radiation, keeping it from reaching the surface. Such ultraviolet radiation is destructive of genetic cellular material in plants and animals, as well as human beings. Without the "ozone layer" high up in the atmosphere, life on the surface of the Earth would not be possible as we know it.

Chlorofluorocarbons (CFCs) are a family of chemical compounds developed back in the 1920's as safe, non-toxic, non-flammable alternative to dangerous substances like ammonia for purposes of refrigeration and spray can propellants. Hence their "good guy" image. Their usage grew enormously over the years. One of the elements that make up CFCs is chlorine. Very little chlorine exists naturally in the atmosphere. But it turns out that CFCs are an excellent way of introducing chlorine into the ozone layer. The ultraviolet radiation at this altitude breaks down CFCs, freeing the chlorine. Under the proper conditions, this chlorine has the potential to destroy large amounts of ozone. This has indeed been observed, especially over Antarctica. As a consequence, levels of genetically harmful ultraviolet radiation have increased.

This "online textbook" on stratospheric ozone will help clarify the "good guy-bad guy" issues by providing a broad overview of the fundamental science related to the study of stratospheric ozone. They are structured to give a basic description of: our atmosphere and ozone (Chapters 1, 2, and 3); the basic physical and chemical processes that control our atmosphere and ozone levels (Chapters 4, 5, and 6); ozone measurement (7); ozone variability (8) and trends (9); the pollution of the ozone layer (10); the Antarctic ozone hole (11); and modeling and the future of ozone (12). This first chapter provides a background on stratospheric ozone and addresses current concerns about the ozone layer and the role CFCs play.

1.1 Chapter 1 Objectives

This purpose of this first chapter is to provide background information that will enable the reader to