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Up high in the stratosphere is world quite different from our own. A special particle, not welcomed in our world because it is a major constituent of photochemical smog, lives up there. This special particle is the O3. By the billions, O3 particles form a shield around the Earth called the ozone. The ozone layer reflects most of the ultraviolet radiation from the sun back into space. If all the ultraviolet radiation reaches the Earth, then the ultraviolet radiation from the sun will cause skin cancer and damage vegetation. Although the ozone can save us from ultraviolet radiation, it is very fragile and needs our care. We humans have been continuously destroying it. Ozone occurs naturally in the atmosphere. O3 particles can appears at any height in the atmosphere, but the greatest concentration is in the stratosphere. It is created when ultraviolet radiation (sunlight) strikes the stratosphere, dissociating oxygen molecules (O2) to atomic oxygen (O). The atomic oxygen quickly combines with further oxygen molecules to form ozone: O2 + hv „_ O + O O + O2 „_ O3 The ozone layer is the densest near the poles and thinnest near the tropics. The amount of ozone above a point on the earth's surface is measured in Dobson unit