SUMMARY OF CONDITIONS NECESSARY FOR AND STEPS INVOLVED IN OZONE LOSS

We now review those conditions necessary for and the steps involved ozone loss inside the polar vortex. The necessary conditions for the formation of the ozone hole are: (1) temperatures cold enough to form polar stratospheric clouds (PSCs); (2) isolation of the polar vortex; (3) persistence of cold temperatures late into spring, or complete denitrification; and (4) sufficient levels of chlorine. Figure 11.60 summarizes the main elements of the theory.

The main chemical and physical processes are listed below as follows:

Step 1 - HCl and ClONO2 react on PSCs to form Cl2.

Step 2 - The Cl2 is degassed while HNO3 remains on the surfaces of the PSCs.

Step 3 - Sunlight returns in spring and the Cl2 is immediately photolyzed into free chlorine, Cl.

Step 4 - The free Cl then destroys ozone primarily via the ClO-ClO and ClO-BrO reactions.

Without the existence of PSCs, the first of the necessary conditions, the conversion of reservoir chlorine species to reactive forms cannot occur. Because midlatitude air contains high concentrations of NO and NO2, the second condition (isolation of the polar vortex) is necessary to prevent these reactive nitrogen compounds from reacting with the ClO, hence interrupting the ClO-ClO and ClO-BrO catalytic loss processes. The third necessary condition is that there must be a small amount of sunlight to drive the catalytic loss process by splitting the Cl2 (molecular chlorine) into highly reactive free chlorine. The fourth necessary condition is that there be sufficient chlorine, otherwise ozone loss is too small.