A3.8 Depletion of the Ozone Layer


Ozone depletion occurs when the natural balance between the production and destruction of stratospheric ozone is tipped in favour of destruction.

In the 1970s, scientists discovered that chlorofluorocarbons (CFCs) could destroy ozone in the stratosphere [Drs. M. Molina and S. Rowland in 1974]. These were commonly used as refrigerants, coolants, and propellants for aerosol cans. However, this idea was not taken seriously until the discovery of the ozone hole over Antarctica in 1985.

CFCs do not break down in the lower atmosphere and they can remain in the atmosphere for over 100 years. They are instead transported into the stratosphere where they are eventually broken down by UV radiation, releasing free chlorine. The chlorine becomes actively involved in the process of destruction of ozone. Ozone is converted to oxygen, leaving the chlorine atom free to repeat the process up to 100,000 times, resulting in a reduced level of ozone.

Based on data collected since the 1950s, scientists have determined that ozone levels were relatively stable until the late 1970s. Severe depletion over the Antarctic has been occurring since 1979 and a general downturn in global ozone levels has been observed since the early 1980s. Global ozone levels have declined an average of about 3% between 1979 and 1991. This is demonstrated in the graph below:

Annual Decline in October Stratospheric Ozone over Antarctica

 Source: http://www.ozone-hole.org.uk

A3.8 Min_ozone