Stratospheric ozone is important in the earth
system because it absorbs ultraviolet radiation from the sun, protecting
life on earth. Ozone is a relatively rare and unstable molecule composed
of three oxygen atoms O3. The normal oxygen molecule has two
oxygen atoms O2. It is the second most common gas in the atmosphere,
and it is relatively stable.
Ozone is found in two regions of the atmosphere:
- In the stratosphere at heights around 20–30
km, where it is produced by sunlight. This is good ozone.
It is critical for life because it protects all life on earth from
dangerous solar ultraviolet radiation, especially UVB, a band of
ultraviolet radiation with wavelengths from 280–320 nanometers
produced by the sun. Ultraviolet radiation with wavelengths from
320–400 nanometers, UVA, is not absorbed, and it is much less
dangerous to life.
- Close to the surface, where it is produced
by sunlight acting on atmospheric pollutants. It is produced
from nitrogen oxides and volatile carbon-based compounds when there
is intense sunshine, above all in the spring and summer. This is
bad ozone. It causes respiratory illness; it damages plants; and
it attacks rubber.
Remember: Good up high. Bad nearby.
Stratospheric Ozone Chemistry Up to 1984
Ozone concentration in the stratosphere is due to a balance between
production and destruction of ozone. Here is a somewhat oversimplified
overview of key reactions that were known up to 1984, leaving out many
other possible chemical reactions in the stratosphere.
- Production. Ultraviolet (uv) radiation
from the sun splits molecules O2 into two free oxygen atoms
O, which immediately combine with oxygen to produce ozone O3 with
the help of a random air molecule M (N2 or O2).
Eq 1) O2 + uv-light –> 2
Eq 2) 2 O + 2 O2+ M –> 2
Production is greatest high in the tropical atmosphere at heights near
40 km. The circulation in the stratosphere then carries the ozone to
- Destruction. Solar radiation of any
wavelength from near infrared to ultraviolet can destroy ozone. This
too is greatest high in the tropical atmosphere at heights near 40
Eq 3) O3 +
sunlight –> O2 + O
Eq 4) O + O + M –> O2+
Eq 5) O + O3 –> 2
This reaction is relatively weak because almost all
the O atoms combines with molecular oxygen to remake ozone (Eq 2).
These interactions among O, O2, and O3 are
called the Chapman reactions, described by Sydney Chapman in 1929
Chapman reaction of oxygen in
the stratosphere. From Environmental
Science Published for Everybody Round the Earth.
The most important reactions that remove ozone involve
nitric oxide NO, the hydroxyl radical HO, and the halogen atoms Cl
and Br. All catalyze a much faster destruction of ozone:
Eq 6) X + O3–> XO
Eq 7) O + XO –> X
Net result: O + O3–> 2 O2
Where X is NO, OH, Cl, or Br. Note that the reaction
converts ozone to molecular oxygen and regenerates the catalyst.
The catalyst then removes more ozone. Here is the reaction involving
Catalytic destruction of ozone by chlorine. From Twenty
Questions and Answers About the Ozone Layer: 2006 Update.
As a result, one chlorine atom can catalyze the destruction of about
100,000 ozone molecules before the chlorine atom is incorporated
into inert molecules of HCl (hydrochloric acid) and ClONO2 (chlorine
nitrate) through the reactions:
Eq 8) Cl + CH4 –> CH3 +
Eq 9) OH + ClO –> O2 +
Eq 10) H2O + Cl –> O2 +
Eq 11) ClO + NO2 + M –> ClONO2 +
Both HCl and ClONO2 are relatively stable and remain in
the air. Their concentrations gradually increase as CFCs are destroyed
by sunlight. Winds eventually carry HCl into the troposphere where
it is rained out. This is the primary pathway for removal of CFCs from
the atmosphere: Transport to the stratosphere, conversion of fluorine
and chlorine to acids (HF and HCl), the transport of the acids to the
troposphere, and removal of HF and HCl from the troposphere by precipitation.
Notice that there are two types of chlorine molecules in the stratosphere.
- Active, ozone destroying molecules: Cl and ClO.
- Non-ozone destroying molecules: HCl and ClONO2.
Our understanding of how chlorine can remove ozone is the result of
- Paul Creutzen showed in 1970 that NO and NO2 are catalysts
able to destroy stratospheric ozone.
- Richard Stolarski and Ralph Cicerone showed in 1973 that chlorine
is even more effective than nitrogen oxides in destroying stratospheric
ozone. Each free chlorine atoms in the stratosphere can rapidly destroy
thousands of ozone molecules.
- Building on this work, Mario Molina and John Rowland showed in 1974
that man-made chlorofluorocarbon (CFC) gases are the most important
source of free chlorine in the stratosphere, and that they would cause
significant reduction in ozone levels. Ultraviolet light splits chlorine
atoms from the chlorofluorocarbon molecules producing free chlorine.
The production of chlorine from CFCs caused much concern because CFCs
have long lifetimes (50 – 500 years) in the atmosphere:
- They do not dissolve in water, so they are not rained out
of the atmosphere as are many other pollutants.
- They are chemically inert, they do not react easily with other
- They are broken down mostly in the stratosphere, by ultraviolet
radiation. The breakdown produces Cl atoms in the stratosphere.
- Cl reacts to produce HCl (hydrochloric acid), which enters the
troposphere and rains out.
For their work, Creutzen, Molina, and Rowland were awarded the 1995
Nobel Prize in Chemistry.
Distribution of Stratospheric Ozone
Ozone concentrations are greatest at heights near 25 km and in polar
regions in winter. These regions have less production, but much less
destruction, and ozone molecules accumulate in higher concentrations.
Transport from areas of high production in the tropics is also important.
Average ozone concentration in June (red
line) and ozone pressure (in nano bars, where one
bar is approximately atmospheric pressure at sea level) on 23 June
2006 (black line), as a function of height in kilometers above the
Swiss Payerne station. Click on the image for a zoom.
From Swiss Federal Office of Meteorology and Climatology, ozone
Global map of all the ozone in a column
of air extending from the surface to the top of the stratosphere
measured in Dobson
units on June 2002 by the Global Ozone Monitoring Experiment
GOME. From GOME
Fast Delivery Service.
The Ozone Hole
Ozone concentration in the stratosphere over Antarctica in the southern
hemisphere in Spring has become much less than it was in the period up
to 1980. The large area of low ozone concentration is called the ozone
The first measurements of ozone concentration in the stratosphere above
Antarctica were made by the British Antarctic Survey at Halley Bays starting
in 1965. Global measurements began in 1978 when the Total Ozone Mapping
Spectrometer TOMS was launched into space on Nimbus-7. Scientists from
the Survey first noticed that ozone over Antarctica was slowly decreasing
in the 1960s. Then Joseph Farman, Brian Gardiner and Jonathan Shanklin
measured extraordinarily low ozone in October 1984 during the Antarctic
spring. They published their measurements in May 1985. At the same time,
the TOMS team also measured very low values, but they were slower to
report their values. Both teams at first thought their instruments were
wrong because the ozone values were far too low. Clearly, something was
destroying ozone in the Spring over Antarctica much faster than anyone
The very low ozone measurements in 1984 were the first indication of
an ozone hole above Antarctica. Since then, ozone values continued to
decrease, and the area of the ozone hole continued to expand from about
5 million square kilometers in 1984 to 28 million square kilometers in
2006. For comparison, 24 million square kilometers is about the size
of North America.
Average size of the Antarctic ozone hole.
From NASA Total Ozone Mapping Spectrometer Ozone
Ozone hole over Australia on 4 October 2004
as measured by the NASA Ozone Monitoring Instrument on the Aura satellite.
Values are in Dobson Units. The edge of the hole is defined by the
220 Dobson Unit contour. From NASA
Ozone Hole Watch.
Antarctic Ozone Theory
The reactions described by Chapman, Creutzen, Molina, and Rowland (Eq
1 to Eq11) cannot explain the ozone hole. The reaction predict only a
small global reduction of ozone. Why was the reduction so large? Why
in Antarctica? Why in the Spring? A series of field experiments that
included flying instruments on airplanes in the Antarctic stratosphere
and new detailed satellite measurements of different types of molecules
in the stratosphere answered the questions.
The ozone hole is the result of a series of processes:
- During the winter, the very cold air over Antarctica is surrounded
by warmer air at lower latitudes. This creates a low pressure region
with strong winds blowing around the region at the boundary between
warm and cold air. The rotating air, a strong polar vortex, isolates
the the stratosphere above Antarctica from rest of the stratosphere.
From Climate & Society Lectures at Columbia University Department
of Earth and Environmental Sciences: Climate and Society: Stratospheric
- As the air cools, Polar stratospheric Clouds form inside the vortex.
When temperatures drop to 195 K, nitric acid, sulfuric acid and water
condense to form Type I Polar Stratospheric Clouds. Then, as temperatures
drop to 188 K, H2O molecules condense on the Type I cloud
particles to form Type II Polar Stratospheric Clouds. Type II particles
are large enough (10 microns in diameter) that they fall out of the
stratosphere, removing nitric acid and water from the stratosphere.
Type I cloud particles are so small (1 micron in diameter) that they
remain in the stratosphere.
Because the Antactic stratosphere is much colder than the Arctic stratosphere,
the clouds are most important in the Antarctic. They form early in
the Winter, and they persist into the Spring. This answers the question:
Temperature in the stratosphere above
Antarctica from Fall through Spring. PSC = Polar Stratospheric
Clouds. From Antarctic
Ozone Bulletin No 7/2006.
Polar Stratospheric Clouds (PSCs) at dusk
over the Arctic region of Sweden. From NASA Looking
at Earth SAGE.
- Polar stratospheric clouds are important for two reasons.
- Chlorine nitrate ClONO2 and hydrochloric acid molecules in
the air strike the cloud particles and are become attached to
the surface of the particles. Chemical reactions on the particle
surface converts the non-ozone destroying molecules HCl and ClONO2 into
Cl2, building up a reservoir of Cl2 during
the winter. The important reactions are:
Eq 12) HCl + ClONO2 –> Cl2 +
Eq 13) ClONO2 +
HOCl + HNO3
Eq 14) HCl + HOC –> H2O
The nitric acid (HNO3) becomes incorporated into the
Chemical reactions on the surface of
polar stratospheric clouds. From Stratospheric
Ozone: An Electronic Textbook Figure 11.46 Chapter 11, Section
- The clouds remove most of nitrogen oxides from the air inside
the vortex so the nitrogen oxides can no longer contribute to
the destruction of ozone.
- The nitric acid produced by the chlorine reactions on
the particle surface(Eq 12 and Eq13) remains on the particle.
- In addition, nitrogen oxides condense to form the clouds.
- Other nitrogen oxides react with H2O on the
ice surface to produce nitric acid.
- The cloud particles fall out of the stratosphere, removing
nitric acid (HNO3) and other nitrogen-containing
molecules from the polar vortex. Type II Polar Stratospheric
Cloud particles have a diameter of about 10 micrometers,
and they fall at a rate of ~1.5 km/day.
- In Spring, the first sunlight warms the cloud particles, releasing
large amounts of Cl2 built up during the winter. Ultraviolet
light quickly splits Cl2 into two chlorine atoms which begin
the ozone-destroying reactions described above. The chlorine rapidly
destroys ozone within the polar vortex, leading to the ozone hole.
This amswers the questions: Why was the reduction so large? Why in
Efforts to Reduce Chlorofluorocarbons in the Atmosphere
The work by Mario Molina and John Rowland in 1974 convinced the US and
other governments to ban the use of CFCs as a propellant in aerosol cans
in 1977. The legislation was easily passed because other gasses could
be substituted for CFCs. At that time the threat to stratospheric ozone
was predicted to relatively small even if CFC continued to increase in
the atmosphere from other uses of CFCs, primarily for refrigeration.
The discovery of the Antarctic Ozone Hole in 1985 showed the destruction
of ozone was much worse that expected. The dramatic growth of the ozone
hole from 1980 to 1990 and the threat that low ozone posed for life on
earth led to an international effort to ban the production and use of
CFCs. The Montreal Protocol of 1987 and later amendments at meetings
in London (1990), Copenhagen (1992) Montreal (1997) and Beijing (1999)
banned the manufacture of most ozone depleting gasses.
It [the protocol plus amendments] requires
each of the 191 Parties [governments] that have ratified the Montreal
Protocol on Substances that Deplete the Ozone Layer virtually to
eliminate in accordance with agreed timelines the production and
import of nearly 100 chemicals that have ozone depleting properties...
Whereas in 1987 production of controlled ozone-depleting substances
exceeded 1.8 million tonnes annually, by the end of 2005 it had been
reduced to some 83,000 tonnes.
From United Nations Environmental Program Ozone
Secretariat 20th Anniversary Information Kit.
Total stratospheric chlorine (ppb - parts
per 109 molar) from the major ozone depleting substances
comprising CFCs, chlorinated solvents, halons, methyl bromide, methyl
chloride and Halogenated Chlorofluorocarbons (HCFCs) in the stratosphere
has begun a slow decline after reaching a peak in the mid 1990s.
The slow reduction is the result of the Montreal Protocol of 1987
and later amendments. The decline is now about 1% per year. From
Australian Government Department of the Environment Indicators
of Environmental Change.
- The University Center for Atmospheric Research web site
provide detailed background
information and the environmental effects associated with reduced
ozone concentrations in the Polar Regions in the Antarctic and Arctic.
- More stratospheric ozone information can also be found at the EPA cite on
science and the science
of ozone depletion.
- There is a nice virtual
tour on the history of the discovery of the ozone hole provided
by Cambridge University’s Center of Atmospheric Science.
observations made at the British Antarctic Survey’s Halley
Research Station suggest that the 2003 ozone hole could be one
of the biggest on record. There is a very nice animated
graphic at bottom of their page on ozone status.(This is a 2.3
- The Goddard Space Flight Center and NASA monitor the
ozone using the Total Ozone Mapping Spectrometer (TOMS). It too has nice
animation and graphics comparing 2002 and 2003 ozone holes side-by-side
from September 22 through October 6 for each year (click on the MPEG
or Quicktime movies).
23 December, 2008