The Earth's atmosphere is composed of several layers: (a) the troposphere (the layer closest to the ground) where most of the weather occurs (such as rain, snow, and clouds), (b) the layer above the troposphere (the stratosphere), an important region in which effects such as the ozone hole and global warming originate. Supersonic jet airliners fly in the lower stratosphere (a historical example was the French Concorde), whereas subsonic commercial airliners are usually in the troposphere. The narrow region between these two parts of the atmosphere is called the tropopause.

Ozone forms a layer in the stratosphere, thinner in the tropics (around the equator) and denser toward the poles. The amount of ozone above a given point on the Earth's surface is measured in Dobson units (DU)—and is typically about 260 DU near the tropics and higher elsewhere, although there are large seasonal fluctuations. Ozone is produced when ultraviolet radiation—generated in the Sun— strikes the stratosphere, dissociating (or separating) dioxygen molecules (02) into atomic oxygen (O). Atomic oxygen quickly combines with more dioxy gen molecules to form ozone:

Up in the stratosphere, ozone absorbs some of the potentially harmful ultraviolet (UV) radiation from the Sun (i.e., at wavelengths between 240 and 320 nm) that can cause skin cancer and damage vegetation, among other effects.

Although the UV radiation dissociates the ozone molecule, ozone can reform through the following reactions, resulting in no net loss of ozone:

Ozone is also destroyed by the following reaction: O + O3 ^ 02 + 02 (4)

The Chapman Reactions

The reactions 1 to 4 are known as the Chapman reactions. Reaction 2 becomes slower with increasing altitude, while reaction 3 becomes faster. The concentration of ozone is a balance between these competing reactions. In the upper atmosphere, atomic oxygen dominates where UV levels are high. Moving down through the stratosphere, UV absorption increases and ozone levels peak at roughly 20 km. As we move closer to the ground, UV levels decrease and ozone levels fluctuate (with a general decreasing trend). The layer of ozone formed in the stratosphere by these reactions is sometimes called the Chapman layer.

Molecular chlorine is easily photodissociated (i.e., split by sunlight):

This is the key to the timing of the ozone hole. During the polar winter, the cold temperatures that form in the "vortex" lead to the formation of polar stratospheric clouds. Heterogeneous reactions convert the reservoir forms of the ozone-destroying species (like chlorine and bromine) to their molecular forms. When the sunlight returns to the polar region during the spring in the southern hemisphere (corresponding to the northern hemisphere autumn), the Cl2 is rapidly split into chlorine atoms, which lead to the sudden loss of ozone. This sequence of events has been confirmed by measurements before, during, and after the ozone hole.

There is still one more ingredient to consider in the broad picture of the ozone destruction. We still have most of the ozone, but we have not explained the chemical reactions in which atomic chlorine actually participates to destroy ozone. We discuss this next.

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