How is formed

    How is formed the ozone lawyer? 


   Versión en español                                                                            English version


The primitive atmosphere of the Earth there was not  oxygen (O2). Therefore is not possible to suppose the existence of ozone (O3), an allotropic form of oxygen, on the original time.


About three thousands millions years ago the first photosynthetic cells sprang up and they began to modify the composition of air. The archaic reductive atmosphere was becoming to an oxidant atmosphere that made possible the appearance and development, around 570 million years behind, of life marine ways capable to obtain energy by means of the breathing.


More than 170 million years it should still happen so that it was formed a stratospheric ozone that could absorbed the ultraviolet radiation hard of the solar rays. Thanks to this protective mantel and to the establishment in the planet of favorable climatic conditions, the first spiders and acari appeared in mainland and then, some dozens of millions of years later, the amphibians would invade the earth. 


If we compare the age of the planet, estimated in about 4 600 million years, with the 400 millions of years of existence of the layer of ozone, through the parable of the cosmic day, the period of existence of the ozone layer represents the last two hours of the planetary day. 


The formation of ozone is a photochemical process that takes place in the stratosphere.

At an altitude between 15 and 50 Km, the solar ultraviolet radiation produces the dissociation of the oxygen molecules in oxygen atoms in its fundamental state (3P) that quickly recombined with not dissociated oxygen molecules, in presence of a third body, M, (that can be another oxygen molecule or of nitrogen) able to capture the energy surplus, 

O2 + hn ® 2 O(3P) (1)

O(3P) + O2 + M ® O3 + M

 O2 + hn ® 2 O(3P) (1)

O(3P) + O2 + M ® O3 + M

This mechanism is the responsible for the ozone forming in the stratosphere in relatively high concentrations, approximately 300 to 500 Dobsons (1 Dobson = 1 molecule of ozone for each 109 molecules, or what is same 0,3  to 0,5 ppm), defining a layer that in the polar latitudes achieve a height among the 16 –18 Km and on the equator to approximately 25 Km. In terms of mass a 3 thousand million tons of ozone have accumulated in the stratosphere but the volume that occupies is so great than its concentration is really low (it doesn't overcome the 5 ppm)


In the sequence of images it is represented the ozone production through the   molecular oxygen photolysis. A molecular dissociation of oxygen (O2) yield simple atoms. This process is produced by the absorption of the energy associated to the ultraviolet radiation of  the solar spectrum. Then, these simple atoms (O) when combining with an oxygen molecule (O2) form ozone (O3).




The Green LaneTM, Environment Canada's World Wide Web site (2005):

Stratospheric Ozone: Understanding Stratospheric Ozone.