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:

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

These reactions are known as Chapman’s reactions. Reaction 2 becomes slower with increasing altitude while reaction 1 becomes faster. The concentration of ozone is a balance between these competing reactions. In the upper atmosphere, atomic oxygen dominates where ultraviolet levels are high. Moving down through the stratosphere, the air becomes denser, ultraviolet radiation absorption increases and ozone levels increase. The highest concentration of ozone is at approximately 20 km above ground level. As we move closer to the ground, ultraviolet radiation levels decrease and ozone levels decrease. The layer of ozone formed in the stratosphere by these reactions is sometimes called the ¡§Chapman layer”.

Emissions of CFCs have accounted for roughly 80% of total stratospheric ozone depletion. Another chemical that contributes to ozone depletion is methyl bromide. Methyl bromide is a colorless, odorless chemical that is used by farmers as pesticide. It is 50 times more destructive than chloroflurocarbons and causes serious health effects. Compounds containing chlorine and bromine from man-made compounds are known as industrial halocarbons. Most countries now have phased out the use of CFCs and methyl bromide in response to international agreements to protect the ozone

Some topics in this essay:
CFC CFCs, , DU Usually, Emissions CFCs, Clouds PSCs, ultraviolet radiation, Antarctica Ozone, O2 ClO, ozone layer, methyl bromide, + o2, chlorine atom, oxygen molecules, chlorine bromine, ozone molecule, atomic oxygen, ultraviolet radiation sun, destroying ozone,