1. A tour of the ozone hole
Courtesy of the Centre for Atmospheric Sciences,
Cambridge University
and
plus Claire Cosgrove and Peter Webster (EAS)
with liberal use of Rich Turco’s
“Earth Under Siege”
Modified for use in Lisa Devillez’s Chemistry Classes

2. History of the Ozone Discovery
Dramatic loss of ozone in the lower stratosphere over Antarctica was first noticed in the 1970s by a research group
from the British Antarctic Survey (BAS) who were monitoring the atmosphere above Antarctica
Video: Discovery of the ozone hole
Dramatic loss of ozone in the lower stratosphere over Antarctica was first noticed in the 1970s by a research group from the British Antarctic Survey (BAS) who were monitoring the atmosphere above Antarctica from a research station much like the picture to the right.
Folklore has it that when the first measurements were taken in 1985, the drop in ozone levels in the stratosphere was so dramatic that at first the scientists thought their instruments were faulty. Replacement instruments were built and flown out, and it wasn't until they confirmed the earlier measurements, several months later, that the ozone depletion observed was accepted as genuine.
Another story goes that the TOMS satellite data didn't show the dramatic loss of ozone because the software processing the raw ozone data from the satellite was programmed to treat very low values of ozone as bad readings! Later analysis of the raw data when the results from the British Antarctic Survey team were published, confirmed their results and showed that the loss was rapid and large-scale; over most of the Antarctica continent.

3. What is the ozone hole?
News media confuses it with the problem of global warming
ozone contributes to the greenhouse effect
over Antarctica (and the Arctic), stratospheric ozone depleted over past 15 years at certain times of the year
hole presently size Antarctica, 10km altitude - lower stratosphere

4. What is ozone?
Ozone forms a layer in the stratosphere, thinnest in the tropics (around the equator) and denser towards the poles
Ozone (O3 : 3 oxygen atoms) occurs naturally in the atmosphere.
The earth's atmosphere is composed of several layers. We live in the "Troposphere" where most of the weather occurs; such as rain, snow and clouds. Above the troposphere is the "Stratosphere"; an important region in which effects such as the Ozone Hole and Global Warming originate. Supersonic jet airliners such as Concorde fly in the lower
stratosphere whereas subsonic commercial airliners are usually in the troposphere. The narrow region between these two parts of the atmosphere is called the "Tropopause".
Ozone forms a layer in the stratosphere, thinnest in the tropics (around the equator) and denser towards the poles. The amount of ozone above a point on the earth's surface is measured in Dobson units (DU) - typically ~260 DU near the tropics and higher elsewhere, though there are large seasonal fluctuations.

5. How is ozone formed?
UV radiation strikes the O2 molecule and splits it, atomic oxygen associates itself with another O2 molecule – simplistic version
It is created when ultraviolet radiation (sunlight) strikes the stratosphere,
dissociating (or "splitting") oxygen molecules (O2) to atomic oxygen (O). The atomic oxygen quickly combines with further oxygen molecules to form ozone:
O2 + hv -> O + O (1)
O + O2 -> O3
(2)
(1/v = wavelength < ~ 240 nm)

6. “Chapman Reactions”
Ozone is formed by:
O2 + hv -> O + O
(1)
Ozone can reform resulting in no net loss of ozone:
O3 + hv -> O2 + O
(3)
O + O2 -> O3
(2)
Ozone is also destroyed by the following reaction:
O + O3 -> O2 + O2
(4)
It is created when ultraviolet radiation (sunlight) strikes the stratosphere,
dissociating (or "splitting") oxygen molecules (O2) to atomic oxygen (O). The atomic oxygen quickly combines with further oxygen molecules to form ozone:
O2 + hv -> O + O (1)
O + O2 -> O3
(2)
(1/v = wavelength < ~ 240 nm)
The reactions above, labelled (1)-(4) are known as the "Chapman reactions". Reaction (2) becomes slower with increasing altitude while reaction (3) becomes faster. The concentration of ozone is a balance between these competing reactions. In the upper atmosphere, atomic oxygen dominates where UV levels are high. Moving down through the stratosphere, the air gets denser, UV absorption increases and ozone levels peak at roughly 20km. As we move closer to the ground, UV levels decrease and ozone levels decrease. The layer of ozone formed in the stratosphere by these reactions is sometimes called the 'Chapman layer'.
But there was a problem with the Chapman theory. In the 1960s it was realised that the loss of ozone given by reaction (4) was too slow. It could not remove enough ozone to give the values seen in the real atmosphere. There had to be other reactions, faster reactions that were controlling the ozone concentations in the stratosphere.
So called after S. Chapman who is famous for his paper 'A theory of upper-atmosphere ozone, Mem. Roy. Meteorol. Soc.' in 1930 which set out the first theoretical explanation of the ozone layer in the stratosphere.
Ozone is in a fluid state of creation and destruction

7. How ironic . . . at ground level, ozone is a health hazard
major constituent of photochemical smog
in the stratosphere, it absorbs potentially harmful ultra-violet (UV – nm harmful) radiation
Protects from skin cancer, etc
at ground level, ozone is a health hazard - it is a major constituent of photochemical smog. However, in the stratosphere we could not survive
without it. Up in the stratosphere it absorbs some of the potentially harmful ultra-violet (UV) radiation from the sun (at wavelengths between 240 and 320
nm) which can cause skin cancer and damage vegetation, among other things.

8. Chemical processes  ozone depletion
Video: Ozone Hole
The main long-lived inorganic carriers (reservoirs) of chlorine are hydrochloric acid (HCl) and chlorine nitrate (ClONO2). These form from the breakdown products of the CFCs. Dinitrogen pentoxide (N2O5) is a reservoir of oxides of nitrogen and also plays an important role in the chemistry. Nitric acid (HNO3) is significant in that it sustains high levels of active chlorine
One of the most important points to realise about the chemistry of the ozone hole is that the key chemical reactions are unusual. They cannot take place in the atmosphere unless certain conditions are present:
The central feature of this unusual chemistry is that the chlorine reservoir species HCl and ClONO2 (and their bromine counterparts) are converted into more active forms of chlorine on the surface of the polar stratospheric clouds.
The most important reactions in the destruction of ozone are:
HCl + ClONO2 -> HNO3 + Cl2
(1)
ClONO2 + H2O -> HNO3 + HOCl
(2)
HCl + HOCl -> H2O + Cl2
(3)
N2O5 + HCl -> HNO3 + ClONO
(4)
N2O5 + H2O -> 2 HNO3
(5)
It's important to appreciate that these reactions can only take place on the surface of polar stratospheric clouds, and they are very fast. This is why the ozone hole was such as surprise. Heterogeneous reactions (those that occur on surfaces) were neglected in atmospheric chemistry (at least in the stratosphere) before the ozone hole was discovered.
The nitric acid (HNO3) formed in these reactions remains in the PSC particles, so that the gas phase concentrations of oxides of nitrogen are reduced. This reduction, 'denoxification' is very important as it slows down the rate of removal of ClO that would otherwise occur by the reaction:
ClO + NO2 + M -> ClONO2 + M
(6)
(where M is any air molecule)
... and so helps to maintain high levels of active chlorine.

9. What causes the depletion?
release of manmade chemicals –
CFC - refrigerants, aerosol sprays, solvents and foam-blowing agents
halogen compounds - Fire fighters used bromine-containing halogens to put out fires
NOx

10. Final stage
Catalytic cycle – molecules significantly changes or enables a reaction cycle without being altered by the cycle itself
Video: When CFCs meet ozone
The answer to this question lies in what are known as 'catalytic cycles'. A catalytic cycle is one in which a molecule significantly changes or enables a reaction cycle without being altered by the cycle itself.
The production of active chlorine requires sunlight, and sunlight drives the following catalytic cycles thought to be the main cycles involving chlorine and
bromine, responsible for destroying the ozone:
(I)
Low temperatures in the polar vortex during winter are important. It is thought to be responsible for most (70%) of the ozone loss in Antarctica.
ClO + ClO + M -> Cl2O2 + M
Cl2O2 + hv -> Cl + ClO2
ClO2 + M -> Cl + O2 + M
then:
2 x (Cl + O3) -> 2 x (ClO + O2)
=============================
net:
2 O3 -> 3 O2
and
In the warmer Arctic a large proportion of the loss may be driven by Cycle (II).
(II)
ClO + BrO -> Br + Cl + O2
Cl + O3 -> ClO + O2
Br + O3 -> BrO + O2
==============================
The dimer (Cl2O2) of the chlorine monoxide radical involved in Cycle (I) is thermally unstable, and the cycle is most effective at low temperatures. Hence, again low temperatures in the polar vortex during winter are important. It is thought to be responsible for most (70%) of the ozone loss in Antarctica. In the warmer Arctic a large proportion of the loss may be driven by Cycle (II).

11. Ozone loss over Antarctica
most dramatic in the lower stratosphere
nearly all the ozone depleted
area the size of Antarctica
many km thick
most pronounced in spring/October
persists two months
December – moves  Falklands, S Georgia, S Am
Over Antarctica (and recently over the Arctic), stratospheric ozone has been depleted over the last 15 years at certain times of the year. This is mainly due to the release of manmade chemicals containing chlorine such as CFC's (ChloroFluoroCarbons), but also compounds containing bromine, other related halogen compounds and also nitrogen
oxides (NOx). CFC's are a common industrial product, used in refrigeration systems, air conditioners, aerosols, solvents and in the production of some types of packaging. Nitrogen oxides are a by-product of combustion processes, eg aircraft emissions.
[CFC - A common industrial product, used in refrigeration systems, air conditioners, aerosols, solvents and in the production of some types of packaging. Although chemically inert in the lower atmosphere (troposphere), they are taken to very high altitudes where they are broken down into their components by the stronger sunlight (UV) at these altitudes. It is the chlorine formed in this process that cause the damage to ozone. The manufacture and use of CFCs in industry has been severely curtailed following the Montreal Protocol and subsequent amendments.]

12. Ozone loss recipe Why is the largest hole above Antartica?
Video: NASA: Exploring Ozone

13. Sources that harm ozone layer

14. Health Consequences Skin cancers, sunburn, eye damage, cataracts
estimated 10 % reduction ozone layer  25 % increase non-melanoma skin cancer -temperate latitudes by 2050
Suppress immune system
DNA mutation of existing disease bacteria and viruses

15. Biological Consequences
Biologically damaging young, new shoots
Southern Ocean - most productive marine ecosystem - less phytoplankton (8.5per cent decr)- food for microscopic animals - eaten by krill – sustain seals, penguins, and baleen whales
6 % ozone depletion  loss 7 million tons fish per year

16. What is being done?
First global agreement - restrict CFCs - Montreal Protocol
European Community countries have even stricter measures
Was anticipated - recovery of the ozone layer within 50 years of 2000
World Meteorological Organisation (WMO reports #25, #37)
The first global agreement to restrict CFCs came with the signing of the Montreal Protocol in 1987 ultimately aiming to reduce them by half by the year2000. Two revisions of this agreement have been made in the light of advances in scientific understanding, the latest being in Agreement has been reached on the control of industrial production of many halocarbons until the year The main CFCs will not be produced by any of the signatories after the end of 1995, except for a limited amount for essential uses, such as for medical sprays.
The countries of the European Community have adopted even stricter measures than are required under the Montreal Protocol agreements. Recognising their responsibility to the global environment they have agreed to halt production of the main CFCs from the beginning of Tighter deadlines for use of the other ozone-depleting compounds are also being adopted.
It was anticipated that these limitations would lead to a recovery of the ozone layer within 50 years of 2000; the World Meteorological Organisation estimated 2045 (WMO reports #25, #37), but recent investigations suggest the problem is perhaps on a much larger scale than anticipated.