Answer:  

Dr. Hiroshi Hukunishi, who is actually studying atmosphere, kindly answered the question above.
 
 
The composition of the earth’s atmosphere is constant from the ground to about 100 kilometers above it. The main components are nitrogen molecules, oxygen molecules, and argon, and the mixture ratios are 78%, 21% and 0.9%, respectively. So, this region is called “the homogeneous sphere”. The atmosphere in this region is homogeneous because the atmosphere is mixed well by diffusion which is caused by various sizes of vortex (this type of diffusion is called “vortex diffusion” or “turbulent diffusion”).
 
On the other hand, where the altitude is above 100 kilometers, the composition of atmosphere changes with the altitude. The lighter the gas, the more it is distributed at higher altitude. So this region is called “heterogeneous sphere”. The region is heterogeneous because the density of atmosphere in this region is 1 million times less than that at the ground, so the diffusion by collision of molecules and atoms (called “molecular diffusion”) becomes faster than vortex diffusion in this region. The lighter the molecules and atoms, the faster the diffusion velocity. Therefore, the lighter the molecules, the higher the altitudes they are distributed into. And since molecules at high altitudes are dissociated to atoms by ultraviolet light from the sun, the composition of the atmosphere changes in the following order (from low to high altitude): nitrogen molecules (N₂), oxygen molecules (O₂), nitrogen atoms (N), oxygen atoms (O), helium atoms (He), and hydrogen atoms (H).
  

Layers of the atmosphere    From EPA

  
However, the above-mentioned fact that the composition of atmosphere becomes heterogeneous at high altitudes is not the reason why the ozone layer is formed at around 10 - 50 kilometers above the ground. The actual reason is that ultraviolet light from the sun, which is necessary for producing ozone, only reaches this altitude. Sunlight consists of light which has a broad range of wavelengths such as ultraviolet light, visible light, and infrared light. Visible light can reach the ground, but almost all of the ultraviolet light is absorbed by the atmosphere. Then only the nearest ultraviolet light, which is the light closest to visible light, can reach the ground. Ozone (O₃) is generated when an oxygen molecule (O₂) is dissociated to an oxygen atom (O) by ultraviolet light and the oxygen atom combines with an oxygen molecule (O₂). 
 
The dense ozone layer is formed at 20 - 30 kilometers above the ground. The number of oxygen molecules, which are an ingredient of ozone, decreases with increasing altitude, but the intensity of ultraviolet light, which breaks down the oxygen molecules, increases as altitude increases. According to these two factors, ozone generation becomes highest at around 20 - 30 kilometers.
 
 
Acknowledgement:
Dr. Hiroshi Hukunishi
He is studying the atmospheric phenomena on several planets (Venus, Earth, Mars, Jupiter, etc). Please take a look at his website for details. 
 
 

This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp.  Please let us know if you find any errors.