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Myth: CFCs Are Heavier Than Air, So They Can't Reach the Ozone Layer

[Graph showing CFC-11 concentrations constant in the troposphere and rapidly declining in the stratosphere]

Source: World Meteorological Organization, Scientific Assessment of Ozone Depletion: 1998, WMO Global Ozone Research and Monitoring Project - Report No. 44, Geneva, 1998.

CFCs and other ozone depleting substances ( ODS) are heavier than air. In a still room, they will pool on the floor. However, the atmosphere is anything but still. Numerous measurements have confirmed that these molecules are mixed nearly uniformly worldwide. 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 graph above shows, the concentration of CFC-11 is essentially constant at altitudes up to 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.

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