­The stratosphere -- the layer of our atm­osphere just above the one we breathe -- includes a thin layer of ozone. This layer is thicker over the poles than the equator. It might seem insignificant compared to the depth of the rest of the atmosphere, but it does a very important job. It prevents much of the sun's ultraviolet (UV) light from reaching the Earth. UV light can cause skin cancer, cataracts and other disorders.

Ozone protects us from the sun by interacting with light. When ultraviolet light hits oxygen molecules (O2) in the stratosphere, it splits the molecules into two atoms of oxygen (O). When this atom encounters another oxygen molecule, the two combine to make ozone (O3). Ultraviolet light also breaks ozone back down into an oxygen molecule and an oxygen atom. Check out this animation from NASA to see how this works.

Oxygen molecule + light = two atoms of oxygen. Oxygen atom + oxygen molecule = ozone molecule.

Images courtesy NASA

 

This process is called the ozone-oxygen cycle, and it converts UV radiation into heat, protecting the Earth. Other substances in the stratosphere, like chlorine, break the ozone back down into oxygen molecules and atoms. Usually, the building up and breaking down is a balanced process, but it can change according to seasons and due to natural events like volcanic eruptions.

But most scientists agree that human activity has caused an imbalance in the oxygen-ozone cycle. More ozone breaks down than the sun can rebuild. This imbalance comes from the "hole," or thinning, in the ozone layer over Antarctica. Human-produced ozone depleting compounds, like chlorofluorocarbons (CFCs), are doing most of the damage.

Complex chemical reactions, which can only take place in Antarctica during the winter and spring, destroy ozone. In the winter, the sun doesn't reach the South Pole. A polar vortex, or air current around the pole, forms, which isolates the air. The CFCs that arrive can't get out, so they become concentrated. Polar stratospheric clouds, made of water ice and nitric acid, also form in the vortex.

When sunlight returns to Antarctica in the spring, chemical reactions on the surface of these clouds break CFCs down into atoms of chlorine and bromine. These atoms are deadly to ozone. One chlorine atom can break apart 100,000 ozone molecules, and bromine is 40 times more destructive. This happens naturally in the stratosphere, but the chlorine and bromine there isn't as concentrated as it becomes during the Antarctic spring. These atoms destroy much of the ozone over Antarctica, throwing off the balance of the rest of the ozone layer.

Chlorine + ozone = chlorine monoxide + oxygen molecule. Chlorine monoxide + oxygen atom = chlorine + oxygen molecule.

Images courtesy NASA

The polar vortex and polar stratospheric clouds exist only over Antarctica, which is why the ozone "hole" exists only there. But the bigger the hole gets, the thinner the ozone layer will become over the rest of the earth. This means more dangerous UV radiation will reach the Earth's surface.

 

Unfortunately, we can't make more ozone to patch the hole. It takes a lot of energy to make ozone - in the atmosphere, the intense energy of the sun drives most of the work. Ozone is also a dangerous pollutant at ground level.

At ground level, ozone is a component of smog.

Image courtesy U.S. EPA

To repair the ozone layer, we must stop releasing ozone-depleting compounds into the atmosphere. Scientists hope that the ozone layer will return to normal within fifty years of completely stopping the release of these compounds. The EPA, the University of Cambridge and NOAA have lots more information on the ozone layer.

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