Illustration for the mechanism for ozone destruction by CFCs

Illustrate the mechanism for ozone destruction by CFCs.

The sun emits different types of wavelength, among those, we have UV light (Ultra violet light). The UV light would be very damaging if it were to come in contact with us including damaging the DNA and giving sunburns.

Figure 1 types of ultra-violet radiation penetrating into the earth’s atmosphere and ozone layer.

As shown in figure 1, we are protected by a layer of Ozone which accumulates in the upper atmosphere of the earth and absorbs the UV radiation (UVB and UVC) before it reaches us. Stratospheric ozone (O_3) is produced by the combination of an oxygen atom (O) with an oxygen molecule (O_2).

 O_3+ UV => O + O,UV light strikes an oxygen O_2 molecule, splitting of the O atoms

O+O_3↔O_2,O_2  reacts with O to form O_3, since its reversible, UV light also splits O_3to O_2+O

In the upper atmosphere, allotropes of oxygen exist. They include O_3, O_2and o. They are all in equilibrium with each other hence they interconvert between O_3  and o with O_2. In equilibrium there is always an O_3. 

Chlorofluorocarbons (CFCs) are produced for use in refrigeration, air conditioning, solvents, aerosol spray cans, and Styrofoam™ puffing agents. They are very stable in the troposphere with lifetimes of approximately 100 years. This long lifetime allows CFCs that are emitted near the surface to be carried by the winds upward. In the stratosphere, CFCs are dissociated by UV light producing chlorine atoms.

Presence of chlorine atoms in the upper atmosphere catalyzes the breakdown of ozone into O_2. Cl atoms accelerate the decomposition of O_3   relative to the reformation of ozone, hence the ozone concentration goes down. E.g. Freon 12 reaching the upper atmosphere ,it reacts with O_3  to form Chlorine monoxide and diatomic oxygen. The atomic oxygen reacts with chlorine monoxide forming another molecule of O_2and regenerating the Cl atom. And this reaction takes place repeatedly. Hence Cl catalyzes the breakdown of ozone into O_2.

 

O_3+Cl→ClO+O_2

ClO+O→O_2+Cl

O_3+O→O_2+O_2  

A single chlorine atom can do this to hundreds of thousands of molecules of O_3, hence destruction of the ozone layer. Destruction of O_3 exceeds creation of O_3  .If the use of CFCs and other ozone-destroying chemicals are banned, the ozone layer O_3  should establish itself in the upper atmosphere.

References:

ChemSurvival. (June 7,2014). How CFC’s Deplete the Ozone Layer [video file]. Retrieved From  https://www.youtube.com/watch?v=5BM4wXCP3Vc&t=1s  (September 27,2021)

The university of Wisconsin. Ozone Formation and Destruction [article]. Retrieved from https://cimss.ssec.wisc.edu/wxwise/ozone/OZONE2.html (September 27,2021)

ResearchGate. Diagram-showing-how-different-types-of-UV-radiation-penetrate-or-interact-with-the-ozone [picture]. Retrieved from https://www.researchgate.net/figure/Diagram-showing-how-different-types-of-UV-radiation-penetrate-or-interact-with-the-ozone_fig1_285056396/download  (September 28,2021)