Ozone and the Atmosphere

The Atmosphere and UV Radiation

Ozone is a key element of the atmosphere. The atmosphere doesn't have much ozone gas, O3, and most of it occurs in a layer of the atmosphere called the stratosphere. Ozone is important because it prevents some ultraviolet (UV) radiation from reaching Earth's surface.

Solar radiation includes three forms of ultraviolet radiation: UV-A, UV-B, and UV-C. UV-C would be very harmful to life, but it is absorbed by oxygen gas (O2) in the atmosphere and does not reach Earth's surface. UV-C plays a role in the formation of ozone.

UV-B is also harmful to life, because it can damage the DNA of living things. Luckily, ozone in the atmosphere absorbs most of the UV-B radiation so that only ~10% normally reaches Earth's surface.

UV-A can cause sunburns and skin damage. About 50% of UV-A reaches Earth's surface.

Normal Ozone Formation and Breakdown

In the atmosphere, ultraviolet light causes the normal formation and breakdown of ozone.

Consider oxygen molecules in the atmosphere. High-energy UV-C rays can split oxygen molecules. When an oxygen molecule absorbs UV-C, it splits into two single oxygen atoms, each of which is highly reactive.

If such an atom strikes another oxygen molecule, the two may form ozone.

If an ozone molecule absorbs a slightly less energetic UV-B ray, it splits back into an oxygen atom and an oxygen molecule.

Another reaction also occurs in the atmosphere, when free oxygen atoms strike ozone molecules to produce two oxygen molecules.

The presence of ultraviolet light results in the continual formation and breakdown of ozone. Under natural conditions, the rate of formation and destruction of ozone is about the same, resulting in a stable ozone layer in the atmosphere.

Activity 1

In the atmosphere, oxygen and ozone molecules are continually being broken down and re-formed. Complete these reactions by dragging the items to their correct locations. Some items are used more than once.

CFC-Driven Ozone Breakdown

Although ozone is naturally formed and broken down in the atmosphere, human-made substances, such as CFCs, can catalyze the additional breakdown of ozone, leading to ozone depletion.

With ozone depletion, more damaging UV-B rays are able to reach Earth's surface.

CFCs, which are now banned from use, can react with ozone in the atmosphere. The reaction begins when UV-B radiation breaks the bond between a chlorine atom and the carbon atom in the CFC. This is the most important reaction, because it releases a free and highly reactive chlorine atom.

The chlorine atom reacts with ozone, resulting in the formation of O2 (oxygen gas) and ClO (chlorine monoxide). Another reaction in the atmosphere liberates the chlorine atom again, allowing the free chlorine atom to attack yet another molecule of ozone.

A chlorine atom can eventually convert to an inactive form by combining with different molecules in the atmosphere, but it typically destroys thousands of ozone molecules before inactivating.

Activity 2

CFCs can react with ozone in the atmosphere. Complete these reactions by dragging the items to their correct locations.