Stratospheric ozone is formed
naturally by chemical reactions involving solar ultraviolet
radiation (sunlight) and oxygen molecules, which make up
21% of the atmosphere. In the first step, solar ultraviolet
radiation breaks apart one oxygen molecule (O2) to produce
two oxygen atoms (2 O). In the second step,
each of these highly reactive atoms combines with an oxygen
molecule to produce an ozone molecule (O3). These reactions
occur continually whenever solar ultraviolet radiation is present
in the stratosphere. As a result, the largest ozone production
occurs in the tropical stratosphere.
