How Does Chlorine Rise to the Stratosphere?

The ozone layer helps protect life on Earth from ultraviolet radiation.

Photons of ultraviolet light have more energy than visible light; consequently, they can damage DNA in humans and other organisms. The ozone layer helps protect us from UV rays, but is vulnerable to chlorine atoms from compounds called chlorofluorocarbons (CFCs).

Features

Earth's surface is unevenly heated by solar radiation. This fact, together with the earth's rotation, explains the formation of large-scale wind circulation patterns. Over time, wind mixes gases in the atmosphere. While it takes a long time for gases in the lowest layer of the atmosphere (the troposphere) to mix with the stratosphere, some long-lived compounds like CFCs can eventually find their way into the stratosphere.

Function

Once CFC molecules are in the stratosphere, they are exposed to higher levels of ultraviolet light. UV radiation can break up CFCs to release chlorine. Chlorine reacts with ozone to break up ozone molecules.

Considerations

Even though CFCs are considerably heavier than many other molecules like nitrogen, oxygen, and CO2, over time, air currents can ensure that some of them reach the stratosphere. Molecules in the atmosphere are not sorted solely by weight, since other processes are involved. It takes months or years for air in the troposphere to mix with air in the stratosphere, but CFCs are relatively long-lived compounds.

References

• Max Planck Institute for Chemistry in Mainz: Upper Atmosphere
• NASA Earth Observatory: Ozone