Ozone is a colorless gas with a characteristic odor. It is a powerful oxidizer. The molecule is slightly curved and symmetrical:
The ozone accumulates mainly in the stratosphere ..
.. and here's why:
1) UV rays from the sun hit molecules of oxygen in the stratosphere. 2) Oxygen dissociates into oxygen atoms 3) The oxygen atoms are associated with oxygen molecules to form ozone. These reactions are reversible and lead to an equilibrium which maintains the ozone concentration constant in principle at a given temperature. A significant portion of UV radiation is thus absorbed, which protects the earth from its harmful influence on living beings.
Recent measurements have shown that the ozone concentration decreases abnormally in winter, especially at the poles ...
.. and here's one reason: Waste CFCs (chlorofluorocarbons) produced by man:
1) UV rays from the sun hitting molecules of CFCs and dissociate forming free chlorine atoms $Cl\cdot $ . 2) They hit ozone molecules forming oxygen molecules and $ClO\cdot $ radicals.. 3) .. which, in turn, encountering oxygen free atoms $O\cdot $ (see formation of ozone above) produce again chlorine atoms $Cl\cdot $ and oxygen. 4) ..and the cycle can thus start again and cause depletion of the ozone layer ..