Chlorofluorocarbons in the atmosphere are broken down by UV radiation, releasing chlorine atoms. These wayward atoms destroy ozone molecules, by pulling away an oxygen atom.

How Ozone Is Destroyed

Stratospheric ozone is destroyed by the chlorine contained in certain chemicals (other substances also "eat" ozone, but chlorine is the most important). Many of these chemicals, called chlorofluorocarbons (CFCs), were widely used until 1980 in spray cans, plastic foams, refrigerants, and firefighting equipment. CFCs remain intact in the lower atmosphere and eventually drift into the stratosphere (the concentration of chlorine in the stratosphere is now about four times the natural level). There, they are broken down by UV radiation. Millions of tons of chlorine atoms have been released, each one able to destroy thousands of ozone molecules before it sinks down to the troposphere. Governments have banned the use of CFCs, but it will take several decades to halt damage to the ozone layer.

 

Levels of ozone in the atmosphere are also influenced by three other natural factors:

El Nino

Every three to seven years, the eastern equatorial Pacific Ocean warms up. This periodic warming event is called an El Nino. Along with it comes a change in air pressure in the upper troposphere over the Southern Hemisphere. This pressure change affects the normal movement of ozone and can cause changes in ozone levels. For example, the strong El Nino of 1997–98 deepened a large low-pressure area over the northeastern Pacific. That strengthening may have made the Arctic vortex less stable and more vulnerable to sudden stratospheric warmings during the winter of 1998. During the following spring, there was less ozone depletion in the Arctic.

Solar Cycle

Every eleven years or so, there is a peak in sunspot activity on the surface of the sun, which changes the amount of energy that reaches the earth. This solar cycle alters the amount of UV radiation available to break oxygen molecules apart to make ozone. More ozone is made when sunspot activity is at its maximum. Scientists have linked this solar cycle maximum to a slight warming of the winter stratosphere and more ozone moving toward the poles.
The occurrence of solar prominences, like those depicted in this NASA photo, fluctuate on a periodic cycle of about 11 years. The year 2000 represented a peak in this solar cycle.  

Volcanoes

Chemical reactions, such as those in cold polar clouds (see The Polar Connection), can also happen on fine droplets or particles of sulphate that form in the stratosphere after volcanic eruptions. Two years after Mt. Pinatubo erupted in the Philippines in 1991, spring ozone amounts over Canada and the High Arctic fell below normal.

 
The eruption of Mt. Pinatubo.