Overview of Stratospheric Chemistry

About 90% of the atmosphere s ozone is found in the stratosphere, residing in what is commonly referred to as the ozone layer. At the peak of the ozone layer the 03 mixing ratio is about 12 ppm. The total amount of O3 in the Earth s atmosphere is not great if all the 03 molecules in the troposphere and stratosphere were brought down to the Earth s 

Atmospheric Chemistry and Physics From Air Pollution to Climate Change, Second Edition, by John H. Seinfeld and Spyros N. Pandis. Copyright 2006 John Wiley Sons, Inc. 

Stratospheric ozone is produced naturally as a result of the photolytic decomposition of O2. The two oxygen atoms that result each react with another 02 molecule to produce two molecules of ozone. The overall process, therefore, converts three 02 molecules to two 03 molecules. The 03 molecules produced themselves react with other stratospheric molecules, both natural and anthropogenic the balance achieved between 03 produced and destroyed leads to a steady-state abundance of 03. The concentration of stratospheric 03 varies with altitude and latitude, depending on sunlight intensity, temperature, and stratospheric air movement. 

Stratospheric 03 absorbs ultraviolet UV-B solar radiation, in the wavelength region 280-315 nm (see Chapter 4). UV-B radiation is harmful to humans, leading to skin cancer and a suppressed immune system. Excessive exposure to UV-B radiation also can damage plant life and aquatic ecosystems. If stratospheric 03 is decreased, the amount of UV-B radiation that reaches the Earth s surface increases. 

Because most of these halogen-containing gases have no significant removal processes in the troposphere, their atmospheric lifetimes can be quite long, for example  

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