Stratosphere chemistry involves hundreds of different gases, and the interaction of those gases with one another. Since ozone screens biologically harmful ultraviolet (UV) light from the Sun, ozone is a gas that's particularly important to understand.

The Sun is the primary energy source for stratospheric chemistry. Both ultraviolet and visible light (radiation) are necessary for initiating many of the reactions in the atmosphere. In fact, the absorption of ultraviolet rays by oxygen molecules, O2, high in the stratosphere of the tropics forms ozone. After formation, the ozone molecule is extraordinarily efficient at absorbing additional UV radiation, particularly the more harmful UV-b and UV-c types discussed in Chapter 1. Ozone in the stratosphere absorbs the solar UV radiation and reemits it as thermal longwave radiation (heat), which keeps the stratosphere warmer than it would otherwise be. Indeed, this ozone is responsible for the very existence of thermal layer known as the stratosphere, where temperatures rise with altitude.

Ozone is destroyed by reactions with chlorine, bromine, nitrogen, hydrogen, and oxygen gases. Reactions with these gases typically occurs through catalytic processes. A catalytic reaction cycle is a set of chemical reactions which result in the destruction of many ozone molecules while the molecule that started the reaction is reformed to continue the process. Because of catalytic reactions, an individual chlorine atom can on average destroy nearly a thousand ozone molecules before it is converted into a form harmless to ozone.

In this chapter, we will show how ozone is formed, how it screens Earth's surface from UV, and how it is destroyed. We will discuss the various cycles that lead to ozone destruction, and the sources of the gases that lead to these catalytic loss processes. Finally, we will discuss how particles in the stratosphere can change the photochemical balance of the stratosphere.

1.1 Discussion Objectives