Ozone has the chemical composition of a triatomic molecule that is made up of three oxygen atoms. The gas is an allotrope of oxygen and is highly unstable as compared to its diatomic counterpart. Ozone found close to the earth’s surface is an air pollutant and harmful to the respiratory systems of living creatures. Yet ozone found in the reaches of the upper atmosphere helps living organisms by shielding them from harmful ultraviolet rays. In its undiluted form ozone is a pale blue gas at standard temperature and pressure. At -112 °C or below it forms a dark blue liquid and at -193 °C or below it takes the form a violet black solid. Within the atmosphere it is colorless.
 Ozone in the Earth’s Atmosphere
The standard unit (the volume of ozone within a vertical column) within the atmosphere is by expressing them in Dobson units.
 Ozone layer
The greatest density of ozone is present in the stratosphere within a layer known as the ozone layer which is located between 10 and 50 kilometers above sea level. This layer acts as a filter, partially not allowing shorter wavelengths of ultraviolet light rays from the sun to reach the earth’s surface. A large dosage of ultraviolet rays can be harmful to living beings. Not all the ultraviolet rays are filtered though as these very rays are also responsible for the production of vitamin D, which is necessary for human bodies. Most of the ozone within the stratosphere is a byproduct of the reaction that undergoes between the UV rays and the oxygen.
The above mentioned reaction catalyzes when in the presence of specific free radicals such as hydroxyl (OH), nitric oxide (NO) and atomic chlorine (Cl) and bromine (Br). Over the past few decades the quantities of ozone in the earth’s stratosphere has been decreasing because of CFC and similar chlorinated and brominated emissions. This causes large scale ozone depletion within the atmosphere.
 Low level oxygen
Also known as tropospheric ozone, this form of ozone is considered a pollutant. It is a byproduct of sunlight coming together hydro carbons and nitrogen oxides within the air. The resulting chemicals are integral components of smog. Smog can cause bother to human eyes and respiratory systems. Ground level ozone has is also known to reduce agricultural yields as it muddles with photosynthesis and thus stunts the metabolism of plants. Ooxygen acounts for 21% of earth's atmosphere.
 Ozone as a Greenhouse
Ozone also acts as a greenhouse gas above the earth’s surface. It was always present at ground level but post industrial revolutions the levels have gone beyond the optimum level and thus these high concentrations of ozone in the upper troposphere are being seen as contributors to global warming.
 Ozone depletion
Ozone depletion takes place in the form of two very distinct phenomena, the first being a rather steady decrease in the quantity of ozone in the Earth’s atmosphere in the past three decades. The other phenomena being a seasonal, but more drastic reduction in the stratospheric ozone above the planet’s poles during the same time frame, this phenomena is better known as the ozone hole.
The mechanisms of depletion for both forms are different but the integral process behind both forms is catalytic breakdown of ozone through reactions with atomic chlorine and bromine. The above halogen atoms are emitted through the photodissociation of chlorofluorocarbon (CFC) compounds. Since the ozone layer acts as a protective layer to the earth, its depletion is extremely harmful. There are have been numerous biological repercussions due to ozone layer depletion, ranging from skin cancer, to a loss of plant life to a reduction of plankton population in the oceans.
-Environment Words- A Dictionary in Plain English, published By Images Asia