IV.

Degradation of the Ozone Layer

The first explanation for the presence of a layer of ozone in the atmosphere was given by the scientist Chapman in 1930, who noted that ozone is destroyed by ultraviolet, and created by a combination of one oxygen molecule with one oxygen atom. In the 1960s, however, it was realized this simple explanation could not account for the observed levels of ozone in the atmosphere. It is now known that catalytic cycles, involving other chemical species, influence the concentration of ozone. A catalytic cycle is essentially a chain of reactions:

• 1. X + O₃ → XO + O₂
• 2. XO + O → X + O₂
• Net: O + O₃ → O₂ + O₂

The first two reactions produce no overall change in the reactive chemical, X, but result in the loss of an ozone molecule. Such catalytic reactions are very important in understanding how the ozone hole is produced chemically; since X is neither created nor destroyed, one molecule of X can go through many cycles. This means that some chemical species in the atmosphere with much lower concentrations (1,000 times less) can significantly affect the amount of ozone; for example, chlorine, bromine, nitrogen, and hydrogen species. Ozone is therefore produced and destroyed by a balance between different reactions. If this balance is disturbed, large changes can be expected in ozone, and this is what occurs in the stratosphere over Antarctica to produce the ozone hole.