Refrigerants for the 21st Century
7. The Beneficial Ozone Layer

The ozone (O3) layer that exists in the stratosphere is essential to life as we know it. In contrast, O3 in the lower atmosphere [= troposphere] , which is mostly man-made, causes undesirable smog formation. The atmospheric chemical reactions leading to O3 are equilibrium reactions involving oxygen (O2):

O2 + sunlight <--> 2O

O2 + O --> O3

The second reaction releases heat which is why the stratosphere is warmer than the upper troposphere. Reverse of the second reaction is promoted by ultraviolet (uv) absorption by O3:

O3 + uv --> O2 + O

This dynamic system occurs continuously in sunlight. The position of the equilibria shown depends on temperature and the intensity of incoming solar radiation which, of course, vary globally and seasonally. As a result the stratospheric O3 concentration is not constant, varying a few percent throughout the entire atmosphere (the absolute concentration is quite small - up to about 0.1 micromol/l). In addition to this expected variability, natural phenomena, such as sun spot activity and volcanic aerosol particles, can cause periodic fluctuations in the O3 concentration.

There are natural processes occurring continuously which deplete the stratospheric O3. These involve its reaction with naturally occurring compounds of nitrogen (from the soil and oceans), hydrogen (mainly water vapor) and chlorine (from the oceans). Fortunately, O2 produced by the earth's plant life continuously replenishes its supply for equilibrium conversion to O3 in the stratosphere. Thus, a delicate balance has developed over the aeons which allows life on earth to exist as we know it.

If the stratospheric O3 shield did not exist, the earth's surface would be bombarded with intense solar radiation which would be intolarable to the earth's life forms. More intense uv-type B radiation would cause drastic increases in skin cancer and cataracts, and disrupt photosysthesis, harming food crops and other plants. Other forms of animal life would be adversely affected.

Fortunately, complete destruction of stratospheric O3 is unlikely. However, recent human activities, such as stratospheric aircraft, and wide spread and growing emissions of CFC's, have the potential for major perturbations in the O3 level.