How and why has the situation regarding the ozone layer changed over the past 30 years?

Stratospheric ozone has decreased over the globe since the 1980s. Averaged over the globe, ozone in the period 1996-2009 is about 4% lower than before 1980. Much larger depletion, up to 40%, occurs over the high latitudes of the Southern Hemisphere in October.
The increase in reactive halogen gases in the stratosphere is considered to be the primary cause of the average ozone depletion. The lowest ozone values in recent years occurred after the eruption of Mt. Pinatubo volcano in 1991, which increased the number of sulphur-containing particles in the stratosphere. These particles remained in the stratosphere for several years and increased the effectiveness of reactive halogen gases in destroying ozone. Observed ozone depletion varies significantly with latitude on the globe. The largest losses occur at the highest southern latitudes as a result of the severe ozone loss over Antarctica that occurs every year during winter and early spring. The next largest ozone losses are observed in the high latitudes of the Northern Hemisphere, caused in part by late winter/early spring losses over the Arctic. Ozone-depleted air over both Polar regions is dispersed away from the poles during and after each winter/spring period. Ozone depletion also occurs directly at latitudes between the Equator and Polar regions but is much smaller. Figure 3 shows the average measured and predicted amounts of equivalent effective stratospheric chlorine (EESC) concentration based on the findings of the latest Scientific Assessment Report. EESC is used as an indicator of the amount of ozone depleting substances in the atmosphere. This figure shows that the amounts of these substances in the stratosphere have already reached a maximum and now are decreasing. The concentration of the ozone is inversely proportional to the concentration of the EESC.