Kengfoo - Actually, it's really easy to make ozone.
Ozone is produced naturally when sunlight shines on air.
It is produced by a chemical reaction in which several molecules of oxygen react with each other. But this reaction only takes place when there's sunlight. So as long as it's sunny, ozone will constantly be produced. So we have one reaction that makes ozone and one that destroys it. In general, the sunlight is more than enough to make up for all of the ozone lost to pollution.