Atmospheric
Chemistry: Formation of the Ozone Layer
One billion years ago, early aquatic organisms called
blue-green algae began using energy from the Sun to split molecules of
H2O and CO2 and recombine them into organic compounds
and molecular oxygen (O2) (photosynthesis)
High in the atmosphere, some
oxygen (O2) molecules absorbed energy from the Sun's ultraviolet
(UV) rays and split to form single oxygen atoms. These atoms combined
with
remaining oxygen (O2) to form ozone (O3) molecules,
which are very effective at absorbing UV rays. The thin layer of ozone
that surrounds Earth acts as a shield, protecting the planet from irradiation
by UV light.
[The amount of ozone required to shield Earth from biologically
lethal UV radiation, wavelengths from 200 to 300 nanometers (nm), is believed
to have been in existence 600 million years ago. At this time, the oxygen
level was approximately 10% of its present atmospheric concentration. Prior
to this period, life was restricted to the ocean. The presence of ozone
enabled organisms to develop and live on the land. Ozone played a significant
role in the evolution of life on Earth, and allows life as we presently
know it to exist.]
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