We have used X' to symbolize the catalyst in the second equation to indicate that it need not be chemically identical to the X in the first equation. Either X or X' must be a chlorine atom.

In the steps that follow the first, the two molecules XO and X'O that have added an oxygen atom react with each other. As a consequence, the catalysts X and X' are ultimately regenerated, usually after the combined but unstable molecule XOOX' has formed and been decomposed by either heat or light:

(By convention in chemistry, a species shown in square brackets is one with a transient existence.) When we sum these steps, the overall reaction is seen to be

We shall see several examples of catalytic Mechanism II in operation in the ozone holes (Chapter 2) and in the mid-latitude lower stratosphere. Indeed, most ozone loss in the lower stratosphere occurs according to this net reaction. Mechanisms I and II are summarized in Figure 1-10.

Finally, we note that while the rate of production of ozone from oxygen depends only upon the concentrations of 02 and 03 and of UV light at a given altitude, what determines the rate of ozone destruction is somewhat more complex. The rate of ozone decomposition by UV-B or by catalysts depends upon ozone's concentration multiplied by either the sunlight intensity or the catalyst concentration, respectively. In general, the concentration of ozone will rise until the net rate of destruction just meets the rate of production, and then it will remain constant at this steady-state level as long as the intensity of sunlight remains the same. If, however, the rate of destruction is temporarily increased by the introduction of additional molecules of a

Mechanism II

Mechanism I

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