The Future: Our Planet in 2050 | Ozone Depletion
What Depletes the Ozone Layer?
Kathryn Lee
The depletion of ozone is caused by the release of chlorofluorocarbons (CFCs) and other ozone-depleting substances (ODS). Some common ODS are refrigerants, insulating foams, and solvents. The following focuses mostly on CFCs, but is relevant to all ODS. Although CFCs are heavier than air, they are eventually carried into the stratosphere in a process that can take as long as 2 to 5 years. Measurements of CFCs in the stratosphere are made with the help of balloons, aircraft, and satellites.
When CFCs reach the stratosphere, the ultraviolet radiation from the sun causes them to break apart and release chlorine atoms which react with ozone, starting chemical cycles of ozone destruction that deplete the ozone layer. One chlorine atom can break apart more than 100,000 ozone molecules.
Other chemicals that damage the ozone layer include methyl bromide (a pesticide), halons (used in fire extinguishers), and methyl chloroform (a solvent used in industrial processes). As methyl bromide and halons are broken apart, they release bromine atoms, which are 40 times more destructive to ozone molecules than chlorine atoms.
CFCs and other ODS are heavier than air. In a still room, they would pool on the floor, but the atmosphere certainly not still. Numerous measurements have proven that these molecules are mixed nearly uniformly throughout all the trophosphere over the entire earth. In the same way that vinegar and oil normally separate when still, but mix when shaken, ozone depleting substances and air are thoroughly stirred together by winds in the troposphere.
Winds are also why the location of CFC and other ODS emissions is essentially irrelevant. CFCs released from a car in the U.S. are as likely to find their way to the stratosphere over India as are molecules released from much closer countries like China. Once they mix through the troposphere, CFC molecules eventually move into the stratosphere. Thousands of measurements over several decades have firmly proven the existence of these heavier-than-air molecules in the ozone layer.
As the diagram above shows, the concentration of CFC-11 is essentially constant at altitudes below 10 km. The UV radiation needed to break CFC-11 apart is shielded by the ozone layer. Because no natural processes destroy CFCs, it survives to be uniformly distributed, both vertically and horizontally. Concentrations drop off rapidly, however, in the stratosphere. As the molecules rise into and above the ozone layer, they are exposed to strong UV, break down, and release chlorine. These measurements are one link between CFCs, increased levels of chlorine in the stratosphere, and ozone depletion.
