The document discusses the ozone layer and the Antarctic ozone hole. It explains that the ozone layer formed over time as oxygen accumulated in the atmosphere from photosynthesis. The ozone layer peaks between 20-30km above Earth's surface and protects life from UV radiation. In the 1980s, scientists discovered an ozone hole forming over Antarctica each spring caused by chlorofluorocarbons reacting with ozone. The Montreal Protocol was enacted to phase out CFC production and allow the ozone layer to recover over the coming decades. However, the ozone hole will persist for some time due to CFCs' long lifetime in the atmosphere.

1. ES 1111
Lecture 2 - The Ozone Layer
& Ozone Hole
(Page 222 of first text, but mostly from
book “Earth Under Siege”)

2. The Beginning of Ozone
 It is the ozone layer that allowed life to
evolve onto land
 Prior to the ozone layer, organisms lived in
the murky waters or in the deep ocean
 The ozone layer wasn’t always there
because oxygen itself was rare at the start
 Photosynthesis by aquatic plants enriched
the atmosphere with oxygen

3. The Ozone Layer
 The ozone layer is at its maximum between 20
and 30 kilometers above the surface, and is
located in the stratosphere
 The ozone layer is vital for life because it shields
the surface from high-energy ultraviolet radiation
 Currently, concerns about the thinning of the
ozone layer are being raised by the world’s
nations
 Ozone is three atoms of oxygen bonded together
as a molecule (O3)

4. Creation of Stratospheric Ozone
 Stratospheric ozone is created differently than the ozone
found near the surface
 Photodissociation – when a photon of sunlight strikes a
molecule with enough energy to break the molecular bonds
apart
 Diatomic oxygen is photodissociated by ultraviolet sunlight
(wavelengths 0.25 micrometers or less)
 The reactions that create ozone are:
O2 + UV light —> O + O
2 ( O + O2 + M —> O3 + M )
Net Reaction: 3O2 —> 2O3
Where M is a mediator molecule to allow the reaction

5. Destruction of Stratospheric Ozone
 The same photochemical process that creates
ozone also destroys it (a good thing because all
oxygen would be converted to ozone eventually
without it!)
 The photochemical destruction of ozone is:
O3 + UV —> O + O
O + O3 —> O2 + O2
 The total amount of oxygen in all forms is
conserved
 “Odd oxygen” always wants to pair up evenly

6. Budget of Stratospheric Ozone
 At any given moment, the stratosphere
holds about 3 Gigatons of ozone
 The creation of ozone by the photolysis of
diatomic oxygen by UV light is 1 Gigaton per
year
 The destruction of ozone by UV light also
equals 1 Gigaton per year
 If all the ozone were to disappear suddenly,
it would take 3 years to replenish it

7. Vertical Ozone Layer Distribution
Figure showing the vertical distribution of
ozone in the atmosphere. Peak values are
found between 20 and 30 kilometers, with
rapidly decreasing amounts above and
below that level

8. Measurement of Ozone
 Ozone column – the number of ozone molecules
contained in a column of air 1 square centimeter in
area that extends vertically from the surface up
through the atmosphere
 1 Dobson Unit = 2.7 x 1016 molecules in the ozone
column
 The average amount in the atmosphere is 300
Dobson Units
 If you took all the ozone and brought it down to the
surface, the ozone layer would be about 3 mm
thick

9. Variations in Ozone Distribution
 Every point on the globe has at least 250
DU of ozone overhead
 Ozone distribution is zonally symmetric
(parallel contours of DU with latitude circles)
 Ozone increases as you go from the tropics
to the poles (destruction stronger at tropics)
 Seasonal variations are higher over high
latitudes and small over tropics, with the
most ozone found in late-winter/early-spring

10. Latitudinal Ozone Distribution
Figure showing the global distribution of total
ozone in the Earth’s atmosphere

11. The Ozone Hole
 A loss of half the typical amount of ozone
has been observed over Antarctica
 The culprit of the ozone hole is believed to
be CFC’s: chlorofluorocarbons (contains
chlorine, fluorine, and carbon)
 A series of meteorological circumstances
results in the ozone hole over Antarctica

12. Chlorofluorocarbons
 Man-made compounds created to act as propellants and
refrigerants
 In the old days, refrigerators used hazardous materials like
Ammonia, and leaky refrigerators could result in death!
 Chlorofluorocarbons are stable compounds, do not break
down easily, and are nontoxic
 Because of their stability, they remain in the atmosphere
for a long time (allowing their long travel to stratosphere)
 Eventually, they leak into the stratosphere (by overshooting
tops in thunderstorms) where they do break down in the
presence of ice and ultraviolet light
 Chlorine in particular is the problem that creates the hole

13. Chlorine’s Attack on Ozone
 First, CFC’s are broken down by light
CFCl3 + UV —> CFCl2 + Cl
 The free chlorine then reacts with ozone:
O3 + Cl —> O2 + ClO
ClO + O —> Cl + O2
 Note that ozone is destroyed into diatomic oxygen
 Also note that the chlorine that started it all ends up being
released at the end
 Catalytic Reaction – a repetitive reaction sequence where
chlorine is produced at the end, able to react yet again
 As a result, one chlorine atom can destroy hundreds of
thousands of ozone molecules

14. Chlorine’s Lifetime
 Chlorine is locked into the CFC molecule, which is
very stable
 It takes about 50 years for a CFC molecule to
make its way up to the stratosphere
 Once in the stratosphere, high energy UV
radiation breaks it apart
 Chlorine can remain in the stratosphere for 100
years (all the while destroying ozone molecules)
 To remove the stray chlorine atom, it must react
with hydrogen to produce hydrochloric acid (HCl)
or combine to form ClONO2

15. Meteorology of the Ozone Hole
 The reason why the ozone hole has been observed over
Antarctica and nowhere else is due to meteorological
circumstances:
– Fast winds (polar vortex) develops in the Stratosphere around
Antarctica in the winter, isolating the stratospheric air
– Extremely cold temperatures allowing the formation of clouds in the
stratosphere (ice allows the CFCs to become unstable)
– With the first rays of Sun in the spring, UV light breaks up the CFCs
and releases the chlorine
– Ozone hole develops as catalytic reaction takes over in the
Antarctic spring
– Once the polar vortex weakens, mixing of stratospheric air from
lower latitudes heals the ozone hole

16. Ozone Layer Anomalies
Figure showing stratospheric ozone
concentrations globally, with the ozone hole
evident over Antarctica. An ozone “dent”
appears over the North Pole. Reductions of
15% are found over the North Pole, but the
same meteorological conditions that are
found over Antarctica are not found there.

17. Thinning Ozone = More UV
 The ozone hole is alarming because higher ultraviolet radiation
amounts are a serious threat to almost all living organisms
 Ultraviolet light comes in four forms:
– Far UV: Wavelengths < 250 nm, entirely absorbed by oxygen in the
upper atmosphere
– Ultraviolet C: Wavelengths of 250-290 nm, extremely hazardous and
currently is completely absorbed by the ozone layer (organisms have
no natural protection)
– Ultraviolet B: Wavelengths of 290-320 nm, biologically hazardous,
organisms produce melanin (skin pigments) that can absorb UV-B and
prevent damage to deeper tissues. Can cause skin cancer and
cataracts over time. A small amount of UV-B reaches the surface
– Ultraviolet A: Wavelengths >320 nm, tanning beds use this type,
suspected of having health consequences. 100% reaches the ground
 For every 1% decrease in total ozone, the UV-B radiation dose
increases 2-4%

18. Penetration of UV in the Atmosphere
Figure showing the depth of penetration for far
UV, UV-C, B, and A in our atmosphere.

19. Skin Cancers and Latitude
Figure showing that skin cancers decrease as
you go to higher latitudes (because sunlight
is weaker). In addition, recall that the ozone
layer is weaker in the tropics, so tropical
latitudes get a double-whammy (high
insolation and a weak ozone layer)

20. Other Ways to Destroy Ozone
 Jets flying in the stratosphere release NO, which
also has a catalytic reaction to destroy ozone:
O3 + NO —> NO2 + O2
NO2 + O —> NO + O2
Net reaction: O3 + O —> 2O2
 Other sources of chlorine that can later reach the
stratosphere are methyl chloride (CH3Cl), released
by biological activity, and hydrochloric acid (HCl)
released by volcanic eruptions

21. Can We Repair the Hole?
 Planes flying in the troposphere can create ozone by
emitting NO and going through the photochemical
reactions of smog. However, ozone doesn’t last long for
this to fix the hole
 Lasers may allow us to break down the CFCs before they
reach the stratosphere, but such lasers would fry birds that
flew in the beam and pose risks to aircraft
 Another idea involved charging chlorine by having it obtain
extra electrons and have these charged atoms accelerated
out of the atmosphere by Earth’s normal electrical field.
Many problems exist with this idea, including the fact that
an airplane would need to fly up there to emit electrons
 In general, the problem is so large that any attempt to
engineer a solution would be feeble

22. The Montreal Protocol
 The only real solution today is to limit the
production and emission of CFCs
 The Montreal Protocol (1987) and later
amendments phased out CFC use
 Nearly 150 countries have signed the treaty
 New compounds to perform the tasks of
CFCs are being designed that are safe
 Third-world countries require assistance in
technology to phase out their CFCs

23. Future Outlook
 Because of the long residence times, the ozone
hole will continue getting worse before it gets
better
 Levels of chlorine in the atmosphere have leveled
off in response to the protocols
 Projections in chlorine amount have us returning
to pre-1960s levels of 2 ppbv by 2060-2080
 Reduction of ozone would have doubled between
1990 and 2030 without the Montreal Protocol

24. Chlorine Levels in the Future
Figure showing the forecasted chlorine
concentration in our atmosphere given the
Montreal Protocol and the subsequent
London and Copenhagen amendments.
The Montreal Protocol would not have
reduced chlorine levels, which is why we
had the subsequent amendments.

25. Other Increases in Radiation
Dosage
 Earth’s magnetic field keeps us protected from many
charged particles that would be hazardous to our health
and our atmosphere
 The magnetic field is created by hot, liquid iron flowing in
our outer core
 Our magnetic field has weakened and reversed polarity in
the past, and is forecast to do so again (we are overdue)
 Weakening of the field during reversal would increase our
radiation dosage
 Without our magnetic field, the atmosphere would be
stripped from our planet (like Mars, which currently doesn’t
have a magnetic field)