The Ozone Layer and Its Depletion

As CFCs diffuse into the stratosphere, they are exposed to high-energy radiation, which can cause photodissociation. Because C—Cl bonds are considerably weaker than C—F bonds, free chlorine atoms are formed readily in the presence of light with wavelengths in the range from 190 to 225 nm, as shown in this typical reaction  

Calculations suggest that chlorine atom formation occurs at the greatest rate at an altitude of about 30 km, the altitude at which ozone is at its highest concentration. 

Atomic chlorine reacts rapidly with ozone to form chlorine monoxide and molecular oxygen  

This reaction foUows a second-order rate law with a very large rate constant  

Under certain conditions, the CIO generated in Equation 18.7 can react to regenerate free Cl atoms. One way that this can happen is by photodissociation of CIO  

The ozone layer protects Earth s surface from damaging ultraviolet (UV) radiation. Therefore, if the concentration of ozone in the stratosphere decreases substantially, more UV radiation wiU reach Earth s surface, causing unwanted photochemical reactions, including reactions correlated with skin cancer. Satellite monitoring of ozone, which began in 1978, has revealed a depletion of ozone in the stratosphere that is particularly severe over Antarctica, a phenomenon known as the ozone hole ( FIGURE 18.6). The first scientific paper on this phenomenon appeared in 1985, and the National Aeronautics and Space Administration (NASA) maintains an Ozone Hole Watch website with daily updates and data from 1999 to the present. 

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