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).
This solar energy conversion
process is known as photosynthesis.
Some of the photosynthetically created
oxygen combined with organic carbon to recreate CO2 molecules. The remaining
oxygen accumulated in the atmosphere, touching off a massive ecological disaster
with respect to early existing anaerobic organisms. As oxygen in the atmosphere
increased, CO2 decreased.
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.
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
is produced naturally in the stratosphere when highly energetic solar radiation
strikes molecules of oxygen, O2, and cause the two oxygen atoms to split apart
in a process called photolysis.
NASA Goddard DAAC
a freed atom collides with another O2, it joins up, forming ozone O3. Most of
the ozone in the stratosphere is formed over the equatorial belt, where the
level of solar radiation is greatest. The circulation in the atmosphere then
transports it towards the pole . So, the amount of stratospheric ozone above a
location on the Earth varies naturally with latitude, season, and from
University Of Alaska
This animation illustrates the
formation of ozone. An oxygen molecule (O2) in the
stratosphere is broken into 2 oxygen atoms (O + O) by absorbing ultraviolet
light energy from the sun. The oxygen atom (O) is now free to react with an
oxygen molecule (O2) to create an ozone molecule (O3).
+ UV => O + O
O + O2 => O3
circumstances highest ozone values are found over places such as Canada
and Siberia, whilst the lowest values are found around the equator. The
ozone layer varies naturally with season. Over Canada is normally about
25% thicker in winter than summer. Weather conditions can also cause
considerable daily variations.
Ozone is also
naturally broken down in the stratosphere. In an unpolluted atmosphere
there is a balance between the amount of ozone being produced and
destroyed and so the total concentration remains relatively constant. At
different temperatures and pressures (i.e. varying altitudes), there are
different production and destruction reaction rates leading to a variation
in concentration. The highest ozone concentrations are in the lower
stratosphere, between about 18 and 26 km.
Ozone also occurs
in very small amounts in the troposphere. It is produced at ground level
through a reaction between sunlight and, e.g., gases emitted from cars. As
a pollutant it should not be confused with the separate problem of
stratospheric ozone depletion.
Ozone creation and depletion process text-
Shuckburgh British Antarctic Survey