The Ozone Layer
Importance
• Ozone, O3, shields the earth’s surface
from biologically harmful UV-B radiation,
which damages the genetic information in
the DNA of all living cells.
• In humans, UV-B radiation causes
sunburn, can eventually lead to skin
cancer, and may suppress the normal
functioning of the immune system.
• Only a trace of the gas is sufficient.
Location
• 90% of the world’s O3 is high above the
surface in the stratosphere, where its
concentration is 4 parts per million.
• Here nearly all of the sun’s UV-B radiation
is absorbed by the ozone molecules,
effectively filtering out most of the harmful
rays so they do not reach the lower
troposphere and earth surface.
Ultraviolet (UV) Radiation
• The sun emits light energy mostly in a broad band of
wavelengths from 400 to 700 nm, which is the visible
spectrum our eyes detect.
• A small portion of sunlight energy is carried by photons
of shorter wavelengths (<400 nm), called Ultraviolet light.
UV-A radiation ranges from 400 to 320 nm, UV-B ranges
from 320 to 280 nm, and UV-C is <280 nm.
• Photons of UV light have more energy than those of
visible light, thus causing more damage to cells. UV-A is
not filtered out as it passes through the atmosphere to
the surface, but DNA is not sensitive to UV-A.
• Harmful UV-C is totally absorbed by O2 molecules, which
make up 21% of the air in the atmosphere.
Ozone Stability
• Ozone is kept at a steady state
concentration in the stratosphere by equal
rates of reactions for its formation and
destruction.
• But now air pollutants, mainly CFCs, or
chlorofluorocarbons (produced for roughly
60 years), have accelerated ozone
destruction, effectively depleting
stratospheric ozone concentrations.
Ozone Formation
• Ozone formation involves gaseous oxygen
(O2) and ultraviolet radiation from the sun.
• First: O2 + UV-C → O + O
• Then: O + O2 → O3
Natural Ozone Destruction
• Ozone destruction occurs naturally a few
different ways:
• O3 + UV-B → O2 + O
• O3 + O → 2O2
• O3 + NO → NO2 + O2
Destruction of Ozone by Pollutants
• Pollutant X + O3 → XO + O2
• Then X is regenerated back into the
atmosphere by the following reaction,
perpetuating the cycle until X is removed
by a side reaction.
• XO + O → X + O2
• Cl (from CFCl3) and NO (from car exhaust)
are some examples of pollutants that are
causing ozone depletion.
Antarctic Ozone Hole
• In September and October, during the first
few weeks of spring following the six
month darkness of the Antarctic night, an
enormous gap in the stratospheric ozone
layer appears above the Antarctic
continent and the Southern Ocean,
including at least one city – Ushuaia,
Argentina (in Patagonia).
• Scientists blame this effect on CFCs
NASA’s Goddard Space Flight Center used
a mapping spectrometer to visualize the
hole in the ozone layer.
Why Over Antarctica?
• During the sunless winter, nitric acid vapor
(HNO3), which helps to lessen the amount of
ozone destroying Cl in the air when there is
sunlight, is frozen in polar stratospheric clouds
and catalyzes the formation of chlorine gas.
ClONO2 + HCl → Cl2(gas) + HNO3(ice)
• Because the Antarctic air is colder than
anywhere on earth, the clouds form at much
higher altitudes. Strong winds form a vortex,
isolating the clouds from air of lower altitudes
until spring.
Spring Sunrise
• The UV-A radiation breaks the Cl2 into individual
Cl atoms that, because there is little or no HNO3
vapor to react with, begin the cycle of ozone
destroying reactions until virtually all of the
ozone inside the vortex has been used up (the
hole seen in the 3-D image).
• Gradually the vortex looses its integrity and in
mid-November the hole breaks up and mixes
with the air of the rest of the world, further
diluting the world’s ozone concentration as the
Antarctic ozone void is filled.
Overall Effects
• In closing, studies have shown that the
stratospheric ozone layer thinned by 10% from
the early 1950’s to the early 1990’s.
• For every 1% decrease in ozone, there is a
corresponding 2% increase in the amount of
ultraviolet radiation that reaches the surface of
the earth and a 4% increase in skin cancer.
• So it is evident that this is one of the more
pertinent issues that we will face in the near
future.