Stratospheric Ozone

The Earth is covered with a thin layer of gas we call the atmosphere. Atmospheric pressure decreases rapidly as we move away from the surface. In the region known as the stratosphere, far above the troposphere, in which all our weather occurs, the atmosphere is very thin. Here short wavelength high-energy radiation from the sun causes dissociation of oxygen molecules. When short wavelength ultraviolet light strikes and O2 molecule, it dissociates into oxygen atoms. When an oxygen atom collides with an oxygen molecule an ozone molecule, O3, may be formed. The O3 molecule that is formed has excess energy; the energy released when the oxygen atom combines with the oxygen molecule. If it does not get rid of that excess energy, it rapidly dissociates back into O2 and O. However, if the O3 molecule collides with any other species present, most likely an N2 molecule, it transfers the excess energy to this molecule and becomes stabilized. When solar radiation from the ultraviolet region of the spectrum strikes an ozone molecule, it may be absorbed and the O3 molecule dissociates. The ozone cycle in the stratosphere involves: - Formation of oxygen atoms and reaction of these with O2 to form O3 - Stabilization of the newly formed O3 by energy transfer through collision - And dissociation of O3 by solar radiation The ozone layer that is established in the stratosphere by these equilibria shields earth’s surface from the ultraviolet radiation that is harmful to living systems.