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Lecture 2
Ozone layer
CHAPTER 01: Figure 1.8
The Chapman mechanism of ozone formation
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© 2012 W. H. Freeman & Co.
Dobson units (DU) for overhead ozone
One Dobson unit is equivalent to
10 μm (0.001 cm) thickness of
pure ozone at the density it would
possess if it was brought to ground
level (1 atm) pressure and 0 oC
temperature
In atmosphere about 350 DU of ozone
tropics - 250 DU, subpolar -450DU
For above the stratosphere, the air is very thin and the
concentration of molecules is so low that most oxygen
exists in atomic form, having been dissociated from O2
molecules by UV-C photons from sunlight. The main
mechanism at this altitude is
O 2 + UV − C → 2O
O + O → O2
In the stratosphere itself, the intensity of the UV-C light is
much less since much of it is filtered by the diatomic
oxygen that lies above. In addition, since the air is denser
than it is higher up, the molecular oxygen concentration is
much higher in the stratosphere. For this combination of
reasons, most stratospheric oxygen exists as O2 rather
than an atomic oxygen
Oxygen atoms are more likely to collide with molecular
oxygen than with atomic oxygen:
O + O 2 → O 3 + heat
The release of heat by this reaction results in the
temperature of the stratosphere as a whole being higher
than the air that lies below or above it.
At bottom of the stratosphere, the abundance of O2 is
much greater than that at the top. However, relatively little
of the oxygen is dissociated and thus little ozone is formed,
because almost all the high energy UV has been filtered
from sunlight before it descends to this altitude. For this
reason the ozone layer does not extend below the
stratosphere.
The Sun heats the ground, and hence the air in contact
with it. It is this reason that the air temperature falls with
increasing the altitude in the troposphere. In the
stratosphere, after the tropopause (boundary between
troposphere and stratosphere) the temperature grows
with increasing the altitude, because of the reaction
between oxygen atoms and molecular oxygen with
releasing the heat.
Ozone absorption in absolute units
Ozon absorption in visual range
Structure of ozone layer
Image of a man's face with sunscreen on the left, in visible
(left) and UV light, demonstrating how sunscreen protects
against UV exposure. The side of the face with sunscreen is
darker, showing that the sunscreen absorbs the UV light.
Damage DNA by UV-B
CHAPTER 01: Figure 1.6
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© 2012 W. H. Freeman & Co.
CHAPTER 01: Figure 1.7
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© 2012 W. H. Freeman & Co.
CHAPTER 01: Unnumbered Figure 1.2
Catalytic destruction of ozone
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© 2012 W. H. Freeman & Co.
CHAPTER 01: Figure 1.9
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© 2012 W. H. Freeman & Co.
Atomic chlorine is an effective X catalyst for ozone
destruction
At any given time, however, the great majority of
stratospheric chlorine normally exists not as Cl or as
ClO radical, but as a forms that are inactive as catalyst,
HCl or ClONO2
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© 2012 W. H. Freeman & Co.
http://ozonewatch.gsfc.nasa.gov/
Activation of Cl in ozone holes
During the dark winter time month, molecular chlorine, Cl2 accumulates within the vortex in the
lower atmosphere, and eventually becomes the predominant chlorine-containing gas.
Once a little sunlight reappears the chlorine molecules are decompose by the light into atomic
chlorine, Cl
Cl2 + sunlight --> 2 Cl
CHAPTER 02: Figure 2.6
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© 2012 W. H. Freeman & Co.
CHAPTER 02: Figure 2.7
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© 2012 W. H. Freeman & Co.