Ozone has a Dr. Jeckyl and Mr. Hyde personality.
Tropospheric ozone (Mr. Hyde) is an air pollutant and a key ingredient
in photochemical smog. Stratospheric ozone (the ozone layer)
is beneficial because it absorbs dangerous high-energy ultraviolet
light. Here's a review of how ozone is produced and destroyed in
the stratosphere, a list of some of the harmful effects of ultraviolet
(UV) light, and information about anthropogenic (man-caused)
destruction of stratospheric ozone
and the ozone hole. You'll
find most of the figures below on pps. 17-22 in the photocopied
how ozone is produced in the stratosphere.
Ultraviolet (UV) light splits an O2
molecule into two O atoms
Each of the O atoms can react with unsplit O2 to make O3 (ozone).
Ozone is destroyed when it absorbs UV light and is split into O
(the two pieces move away from each other and don't recombine and
remake ozone). This is how the ozone protects
us. O3 is also
destroyed when it reacts with an oxygen
atom (thereby removing one of the "raw ingredients" used to make
ozone). Two molecules of ozone can also react with each other to
make 3 molecules
(probably the least likely of the three possibilities just because
there aren't many ozone molecules around to react with each other).
concentration in the
stratosphere will shift up and down until the natural rates of
balance each other (analogous to your bank account not changing once
the amounts of money being deposited and withdrawn are equal).
The black box represents the O3 layer concentration once
equilibrium is achieved. If an
additional man-caused destruction process is added (dotted red arrow)
the ozone layer concentration will decrease (sort of like someone else
coming along and starting to spend some of the money in your bank
account, your balance will decrease).
Knowing that you need O2
and UV light to make ozone,
to understand why the ozone layer is found in the middle of the
Skin cancer and cataracts are probably the best known hazards
associated with UV light. There is plenty of UV light high in
the atmosphere but not much
oxygen (air gets thinner at higher and higher altitude). Near the
ground there is plenty of oxygen but not as much UV light (it is
absorbed by gases above the ground). You find the optimal amounts
of UV light and oxygen somewhere in between, near 25 km altitude.
This next figure lists some of the problems associated
exposure to UV light. Thinning of the ozone layer will result in
increased amounts of UV light reaching the ground.
You may have heard of UVA and UVB; there is also UVC.
UVA is a relatively long wavelength (0.315 to 0.4
micrometers), low energy form of UV light; it is the light emitted by a
wavelength (0.28 to 0.315 micrometers). UVC has
even shorter wavelength (0.1 to 0.28 micrometers) and is the most
dangerous of the three types of UV light. Germicidal
bulbs emit UVC light and are used to sterilize and purify air,
food, and water. Fortunately All of the UVC in sunlight is
absorbed by the
There is some question
tanning booths, which emit mostly UVA, are safe. You can find
information online about
question. Here is an
example from the US Food and Drug Administration.
It is worth
mentioning that thinning of the ozone layer and increased amounts of UV
light reaching the ground is not
the cause of global warming. The worry is that increasing
concentrations of greenhouse gases and strengthening of the greenhouse
effect might cause global warming.
Human activities add substances to the atmosphere that can
reduce ozone concentration in the ozone layer (which would result in
increased exposure to UV light at the ground).
The first set of reactions above involve nitric oxide, NO. First,
NO reacts with O3
to form NO2
Then notice the NO2
reacts with an oxygen atom (one of the raw ingredients needed
to make O3)
form NO again and O2.
another ozone molecule.
At one time many countries were considering building fleets of
supersonic aircraft that would fly in the stratosphere. The
plans were scrapped partly due to concern that the NO emissions from
jet engines would damage the ozone layer.
The main threat now comes from chlorofluorocarbons (CFCs). CFCs
and had a variety of uses.
CFCs are unreactive, non toxic, and stable. Once they get into
the atmosphere they remain there a long time, as much as 100 years.
19 in the ClassNotes).
CFCs released at ground level [lower left corner in the figure
remain in the atmosphere long enough that they can eventually make
their way up into the stratophere. UV light can then break
atoms off the CFC molecule Point (a)). The resulting
"free chlorine" can react with and destroy ozone. This is shown
in (b) above. Note how the chlorine atom reappears at the end of
the two step reaction. A single chlorine atom can destroy 100,000
ozone molecules before undergoing a different reaction and being
removed from the atmosphere.
There are ways of keeping chlorine from reacting with ozone.
of these so called "interference reactions" are shown in (c)
above. The reaction products, reservoir molecules
(because they store chlorine), might serve as
condensation nuclei for cloud droplets (the small water drops that
clouds are composed of) or might dissolve in the
water in clouds. In either event the chlorine containing chemical
is removed from the atmosphere by falling precipitation. Clouds
are probably the most effective way of cleaning the atmosphere.
The ozone hole that forms above the
S. Pole every year in late
was one of the first real indications that CFCs could react with
and destroy stratospheric ozone. The hole is not really a hole in
the ozone layer, just a temporary thinning of the ozone layer above the
S. Pole and the continent of Antarctica. The ozone concentration
decreases to perhaps 30% of its normal value.
It is unusual to find clouds in the
stratosphere, most of the water vapor stays in the troposphere.
However, because it
gets very cold above the S. Pole in the winter, polar stratospheric
clouds do sometimes form (they are made from water and other
materials). This together with an
unusual wind pattern above the S. Pole in the winter (the polar vortex)
are thought to
create the ozone hole when the sun returns in the spring.
The ozone destruction reactions are circled in red above.
reacts with O3 to make ClO.
This reacts with O to
produce Cl and
O2. The Cl is now available
to react again with other
In blue is one of the "interference" reactions. ClO reacts
make ClNO3. The Cl in this "reservoir"
molecule is stored in a form that can't
any more ozone.
Now what happens above the S. Pole in the winter is that the
molecules react on the surfaces of the polar stratospheric cloud
particles to make a new and different kind of compound. This
shown in green above. The Cl that would be stored away in ClNO3
reacts and produces a new compound, HOCl, that accumulates in the
air during the winter. When the sun reappears in the spring, the
UV light splits off all the Cl molecules which react with ozone.
A lot of chlorine suddenly becomes available and the ozone
concentration takes a nosedive.