Ozone
"There are strange things done in the midnight
sun...".
- Robert Service, "The Cremation of Sam McGee"
Suggested Readings:

Development of the Antarctic Ozone
Hole in 1997, as seen by the Total
Ozone Mapping Satellite, TOMS
(NASA Goddard Space Flight Center)




Scientific American Readings,
"Managing Planet Earth", W. H.
Freeman, New York, 1990.
James C. White., "Global Climate
Change Linkages", Elsevier, 1989.
Richard P. Wayne, "Chemistry of
Atmospheres", Oxford, 1991.
Jonathan Weiner. "The Next One Hundred Years", New Sciences, Bantam,
1991.
Sayed Z. El-Sayed, "Fragile Life under the Ozone Hole, Natural History, October
1988, pp 73-80.
We wish to study:




Which gases provide greenhouse warming and what is happening to them?
What chemistry controls the ozone layer?
How might the ozone layer react to anthropogenic changes?
How does society react to alarming scientific findings?
Jump to: [Compendium of Trace Gases] [Stratospheric Ozone] [Ozone Depletions] [Northern Hemisphere
Ozone] [Summary]
1. Compendium of Trace Gases
In the past twenty years, it has become increasingly evident that certain trace gases play a major
role in determining the climate system - far in excess of what might be thought based on their
small numbers. Carbon Dioxide is perhaps the principal culprit for potential global warming, but
it is by no means the only one.
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Figure 1 shows the relative contribution to
tropospheric warming due to the greenhouse effect
of various gases. This plot, taken from model
calculations, contains two surprises. Firstly, the
Chloro-fluorocarbons (CFC's) taken as a whole
(there are several members of this family of gases)
represent the second most important gas for global
warming - even though their concentrations are
measured in the parts per trillion, as opposed to
parts per billion for carbon dioxide and methane.
The CFC's are entirely of anthropogenic origin.
Secondly, we see that both ozone and nitrous oxide
(N2O or "laughing gas") are significant greenhouse
gases. In fact, most gases that are made up of three
Figure 1. Relative importance of the
or more atoms are effective greenhouse gases. This
five most important greenhouse gases is because they have the ability to absorb and emit
infra-red radiation via processes of rotational and vibrational excitation (think, for example, of
the three atoms making up CO2 as being connected by springs - infra red light is emitted and
absorbed in association with the jiggling and spinning of the springed molecule).
For a full study of the issues relating to Global Change, therefore, we need to account
quantitatively for the sources and sinks of all these greenhouse gases, incorporating a discussion
of the extent to which their presence in the atmosphere can be attributed to human activities and
a projection of their future abundances.
Tables 1 and 2 provide more detailed summaries of some of the attributes of important trace
gases that are found in the Earth's atmosphere. Table 1 lists the major anthropogenic sources for
each trace gas, as well as the mean residence time and the projected change in abundance with
time. The last column of Table 1 provides an estimate for the projected concentration of the gas
in the year 2030 in parts per billion (ppb), based on a conservative assumption for future global
industrial development.
Table 2 provides information on the two principal concerns we always have when discussing a
trace gas, namely:
1. what is its the "Greenhouse Potential" (GP)?
2. what is its the Ozone Depletion Potential (ODP)?
For convenience, both GP and ODP are measured on a per molecule basis, using as reference the
potentials of specific CFC molecules. Thus, for example, we see from Table 2 that a molecule of
methane has only 0.001 times the effectiveness of a molecule of CFC-12 for greenhouse
warming. Similarly, we see that Carbon Dioxide is not a particularly effective greenhouse gas on
a per molecule basis (GP = 0.00005), but since it is much more abundant than the others, it still
comes out on top (see Figure 1).
Table 1. Compendium of Trace Gases in the Atmosphere
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ANTHROPOGENIC/
AVERAGE
AVERAGE
APPROXIMATE
PROJECTED
MAJOR
TOTAL EMISSIONS RESIDENCE CONCENTRATION
CURRENT
CONCENTRATION
ANTHROPOGENIC
PER YEAR (MILTIME IN
100 YEARS AGO CONCENTRATION
IN YEAR 2030
SOURCES
LIONS OF TONS) ATMOSPHERE
(PPB)
(PPB)
(PPB)
GAS
Fossil-Fuel
CARBON
Combustion,
MONOXIDE
Biomass
(CO)
Burning
700/2,000
Months
100-200, N.
?, N. Hem.
Hem.
40-80, S. Hem.
40-80, S. Hem.
(Clean
(Clean
Atmospheres)
Atmospheres)
Probably
increasing
CARBON
DIOXIDE
(CO2)
Fossil-Fuel
Combustion,
Deforestation
5,500/~5,500
100 Years
290,000
350,000
400,000550,000
METHANE
(CH4)
Rice Fields,
Cattle,
Landfills,
Fossil-Fuel
Production
300-400/550
10 Years
900
1,700
2,200-2,500
NOX
GASES
Fossil-Fuel
Combustion,
Biomass
Burning
20-30/30-50
Days
.001 to ?
(Clean to
Industrial)
.001-50
(Clean to
Industrial)
.001-50
(Clean to
Industrial)
NITROUS
OXIDE
(N2O)
Notrogenous
Fertilizers,
Deforestation,
Biomass
Burning
6/25
170 Years
285
310
330-350
SULFUR
DIOXIDE
(SO2)
Fossil-Fuel
Combustion,
Ore Smelting
100-130/150200
Days to
Weeks
.03 to ?
(Clean to
Industrial)
.03-50
(Clean to
Industrial)
.03-50
(Clean to
Industrial)
CHLOROFLUOROCARBONS
Aerosol
Sprays,
Refrigerants,
Foams
-1/1
60-100
Years
0
About 3
(Chlorine
atoms)
2.4-6
(Chlorine
atoms)
Table 2. Ozone Depletion Potential and Greenhouse Potential for various gases
Average
Life in
ODP*
Atmosphere
(Years)
Trace Gas
Formula
Primary
Source
CFC-11
CFCl3
Refrigerant/AC,
Plastic Foams,
Aerosols
75
1.0
0.40
CFC-12
CF2Cl2
Refrigerant/AC,
Plastic Foams,
Sterilants
110
1.0
1.00
CFC-113
C2F3Cl3
Solvents
90
0.8
0.3-0.8
Halon 1211
CF2ClBr
Fire
Extinguishers
25
3.0
?
Halon 1301
CF3Br
Fire
Extinguishers
110
10.0
0.80
Carbon
CCl4
Industrial
67
1.1
0.05
GP**
3
Tetrachloride
Processes
Methyl
CH3CCl3
Chloroform
Industrial and
Natural
Processes
8
0.1
0.01
Nitrous
Oxide
N2O
Fossil Fuels
150
--
0.016
Methane
CH4
Biogenic
Activity, Fossil
Fuels
11
--
0.001
Carbon
Dioxide
CO2
Fossil Fuels
7
--
0.00005
Carbon
Monoxide
CO
Motor Vehicles
0.4
--
--
*
ozone depletion potential (CFC-11 = 1.0)
**
greenhouse potential (CFC-12 = 1.0)
There are many other things to note about the data of these tables. Notice, for example, the long
atmospheric lifetimes of some of the gases - 60-100 years for CFC's, 170 years for N2O, etc.
Clearly, climate changes induced by anthropogenic effects via these gases will take a long time
to undo.
Also, all the trace gases of Tables 1 and 2 have significant anthropogenic (human) sources. In
many cases, it is via fossil fuel burning - but cattle raising, rice production, deforestation,
fertilizer application, ore smelting, motor vehicle emissions, aerosol sprays, etc., also all play a
role. The production of trace gases is seen to be part and parcel of our industrial and agricultural
civilization - it will not be politically easy to make great changes. In fact, the data of Table 1
predict continuing accumulation of these gases in the atmosphere.
Notice also that it is the chlorine- or bromine-bearing (CFC-family-member) gases that have the
greatest ozone-destroying potential - more on that below.
2. Stratospheric Ozone
From the above discussion, we can see that ozone not only protects us from UV light - it is also a
greenhouse gas in its own right. We next focus on chemistry of ozone - how is it produced and
how is it destroyed?
Stratospheric Ozone Abundance
Figure 2 is a reminder of the measured
abundance of ozone in the stratosphere. Ozone
occurs in a layer, centered at around 30 km
altitude, reaching a peak abundance of ~10 parts
per million. Even at the peak of the ozone layer,
however, it is still very much a trace constituent
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- two orders of magnitude down from CO2 and 5 or 6 orders down from O2 and N2. If we were to
take all the ozone in a column overhead and bring it down to sea level (room temperature and
pressure) it would occupy a layer of only 3 mm in thickness!
It is interesting to notice how different the ozone distribution is from most of the other gases
shown in Figure 2. Ozone occurs in a layer, while the other gases have simple exponential dropoffs with altitude (straight lines on this logarithmic plot).
Why does stratospheric ozone exist is a layer? To answer this question, we need to understand
the production mechanism for ozone.
Ozone Production
Ozone is a deep blue, explosive, and poisonous gas. It is made in the atmosphere by the action of
sunlight on molecular oxygen. In the stratosphere, UV light is available that can split up ordinary
molecular oxygen into two atomic oxygen atoms.
O2 + UV photon --> O + O
Now, atomic oxygen is a very reactive species - so much so that it is very hard to make in the
laboratory - it immediately combines with something else. In the stratosphere, atomic oxygen
can quickly combine with molecular oxygen (in the presence of a third body) to yield the almost
equally reactive other allotrope of oxygen: ozone or O3.
O + O2 + third body --> O3 + third body
The combination of these two reactions, mediated by sunlight, converts molecular oxygen into
ozone. Thus ozone is continually being created in the stratosphere by the combination of
molecular oxygen and sunlight.
Ozone Layering
We can now explain why ozone is created in a layer in the stratosphere. Figure 3 illustrates the
altitude dependence of the ozone production rate.
Figure 3. Production of the Ozone Layer in the Stratosphere. The two ingredients for stratospheric ozone
production are molecular oxygen and UV sunlight.
On the topside of the layer, production is limited by the availability of molecular oxygen (which
is dropping off exponentially with altitude as shown in Figure 2).
On the bottomside of the layer, production is limited by the availability of UV sunlight (which
gets rapidly absorbed by ozone itself).
The net effect of these two factors is to produce the characteristic layer for ozone.
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Ozone Loss
Ozone is lost through the following pair of reactions:
O3 + UV photon --> O2 + O
O + O3 --> 2O2
The first of these two reactions serves to regenerate atomic oxygen for the second reaction which
converts the ozone back to molecular oxygen. This second reaction is very slow. It can be
enormously accelerated, however, by catalytic reactions (see below). In the absence of such
catalytic reactions, ozone can survive for 1-10 years in the stratosphere.
Catalytic Destruction of Ozone by Chorine from CFC's
Catalysis refers to the acceleration of a particular chemical reaction by a catalyst, a substance
that is not destroyed in the reaction, enabling it to continue having the same accelerating effect
time and time again.
Rapid catalytic destruction of ozone is best explained by reference to the famous example of
CFC's (also known as freons) in the stratosphere.
Chlorofluorocarbons (CFC's) were developed to be colorless, odorless, non-staining, chemically
inert, non-toxic, non-flammable, and to have certain other properties that make them excellent
refridgerants, solvents, propellants for aerosol cans, and foam-blowing agents. These same
properties make them essentially inert in the troposphere.
In the stratosphere, however, the CFC's can be broken apart into more reactive fragments under
the action of UV light. When this splitting occurs, free chlorine is liberated which can
catalytically destroy ozone. The process occurs in two steps:
Step 1.
"Photolysis" (splitting by sunlight) of CFC's in the stratosphere
Cl2CF2 + UV light --> ClCF2 + Cl
Step 2
Catalytic destruction of ozone
Cl + O3 --> ClO + O2
ClO + O3 --> Cl + 2O2
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Notice that the net effect of this pair of fast reactions is to turn two ozone molecules into three
normal molecules of oxygen. The (catalyst) atomic chlorine is recovered in the second reaction,
making it available to start over. In fact, each chlorine atom can destroy hundreds of thousands
of ozone molecules!
These two steps turn a very unreactive chemical into a devastatingly effective destoyer of ozone.
Whenever free chlorine atoms exist in the stratosphere, ozone is quickly depleted. Other species
(such as bromine and fluorine) can also act as ozone-destroying catalysts.
Given this chemistry, it is useful to consider a typical life history of CFC's in the atmosphere:
1. Spray starch aerosol can is emptied in Ann Arbor
2. The CFC is rapidly dispersed until it is uniformly distributed throughout the
troposphere. It takes about a year to mix across into the southern hemisphere as
well, carried by weather patterns
3. After a few years, some of the CFC leaks into the stratosphere. At a sufficiently
high altitude (~30 km), the available UV light can photolyze the CFC, liberating
chlorine.
4. Each atom of chlorine participates in the catalytic destruction of thousands of
molecules of ozone.
5. Eventually the chlorine atom reacts with methane to produce HCl, a molecule of
hydrochoric acid.
6. Some of the HCl reacts with OH to liberate Cl again, but a small fraction of it
mixes down into the troposphere where it can dissolve in rainwater and be lost to
the atmosphere through precipitation.
7. The time scale for this process is ~100 years!
Potential Effects of Depleted Ozone
Of primary concern are the enhanced levels of UV radiation that reach the Earth's surface for a
depletion in stratospheric ozone. It is customary to break up the UV spectrum into two parts:
UV-A: 400 - 320 nm
UV-B: 320 - 290 nm
The more energetic UV-B portion of the spectrum is responsible for sunburn, cataracts, potential
ecological damage, and skin cancer. It can be absorbed by glass as well as by sunscreens and
hats.
Relatively little is known or understood about the consequences of enhanced UV-B levels. We
do know, however, that a 1% decrease in ozone abundance causes a ~2% increase in UV-B.
Table 3 summarizes some of the potential effects of UV-B increases.
Table 3. Potential Effects of UV-B Increases.
Effects
State of
Knowledge
Potential
Global
Impact
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Plant Life
Low
High
Aquatic Life
Low
High
Skin Cancer
Moderate
to High
Moderate
Immune
System
Low
High
Cataracts
Moderate
Low
Climate
Impacts*
Moderate
Moderate
Tropospheric
Moderate
Ozone
*
Low**
Contribution of both stratospheric ozone depletion itself and gases causing such depletion to
climate changes.
**
Impact could be high in selected areas typified by local or regional scale surface-level ozone
pollution problems.
Our best understanding of potential effects is in the area of skin cancers, for which detailed
epidemiological records and studies exist. It is known, for example, that more than 90% of nonmelanoma skin cancers are related to UV-B exposure. A 2% increase in UV-B is linked with a 25% increase in basal-cell cancer cases and a 4-10% increase in squamous-cell cancer cases.
In 1990, there were ~500,000 cases of basal-cell cancer in the U.S. and ~100,000 cases of
squamous-cell cancer. A 1% depletion of ozone would cause an increase in skin cancer cases of
~20,000 per year.
To put this rather alarming figure in context, it is necessary to discuss briefly the geographical
prevalence of skin cancer in the U.S. Figure 4 illustrates the rate of skin cancer as a function of
latitude. While the data has some scatter, the trend is clear. A decrease of ~110 in latitude results
in an increase of a factor of 2 in skin cancer occurrence. This occurs because the UV-B exposure
increases towards the equator (~ a factor of 50 from pole to equator).
An increase of ozone of 1% gives an increase of
~20,000 cases of skin cancer per year.
This is equivalent to a southward shift in the
average latitude of the U.S. population by only ~12
miles.
Actual ozone depletions at the latitude of the U.S.
are ~1-3% already - primarily caused by chlorine
catalytic chemistry.
Occurrence of non-melanoma skin
cancers as a function of state and
latitude.
Figure 5 illustrates the global ozone reductions
estimated from satellite data as a function of
latitude and season. Notice, the large reduction in
the southern polar region - more on this below. This
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represents the so-called Antarctic Ozone Hole.
We can see from this figure that we live in an ozone depleted world already. People living in the
southern hemisphere are already well aware of this. Children growing up in Teirra del Fuego and
New Zealand are very conscious of the need to wear hats in the midday sun. Daily news reports
quote the ozone levels so that people can adjust their exposure to sunlight accordingly.
It is thought that ozone levels have been already
depleted globally due to the CFC emissions and
consequent chlorine catalysis.
The depletions occur at all latitudes and seasons,
but are most dramatic in the southern polar
region in austral springtime (October). This
depletion is the famous Antarctic Ozone Hole.
Figure 5. Global Ozone depletions.
The Montreal Protocol was followed by
additional strengthening of international controls
on ozone-depleting chemicals - the London and
Copenhagen Protocols.
In 1996, Molina, Rowland and Crutzen received the first Nobel Prize (for Chemistry) to ever be
awarded in Atmospheric Sciences. These scientists were instrumental in predicting the problem
and in developing the needed scientific case for governmental action.
3. Ozone Depletions
Theoretical models have been developed to
predict future changes in ozone abundance.
Figure 6 shows the results of one such projection
into the future.
Figure 6. Percentage change in global
ozone predictions with and without
compliance with the Montreal Protocol,
and international agreement which
phases out CFC production by 2000
The Montreal Protocol was signed in 1987 and
has since been strengthened. It commits to phase
out production of the CFC's (first invented in
1930) by the turn of the century.
Without the Montreal Protocol, we would be
looking at a disastrous reduction in ozone levels.
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The Antarctic Ozone Hole
The famous Antarctic Ozone Hole was discovered by British scientists who made systematic
obseravtions of ozone using a simple ground instrument - the Dobson Meter. They published this
famous figure which illustrated the downward trend of total ozone over Halley Bay, Antarctica
in the month of October (austral Spring)..
These measurements of Farman et al., provided a
wake-up call to the atmospheric science community.
They were quickly verified by satellite observations
and several campaigns were organized to find out
what was happening in this region and during this
particular time of the year.
The Farman et al., paper, published in 1985 showed
a dramatic decrease in ozone. The decline from year
to year has continued, more-or-less to this day.
Measured ozone total colum
The figures shown below illustrate the satellite
abundance for the month of October in view of the same phenomena for the years
the years 1958-1984.
leading up to the present. There are now
several satellite missions that are dedicated to unraveling the chemistry and dynamics
of ozone. These include the Total Ozone Mapping Satellite, TOMS< and the Upper
Atmosphere Research Satellite, UARS.
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Satellite measurements of the Antarctic Ozone Hole from 1979 to 1997, as
measured by the TOMS spacecraft
The hole deepens and becomes enlarged from year to year, as well as deeper although not
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monotonically.
The Antarctic Ozone Hole is now well understood. Briefly what happens can be summarized as
follows:
 The Antarctic Ozone hole is limited in space and time, occurring at the time of year
when the Sun first appears above the horizon after the long polar night.
 During Polar Winter, a polar vortex forms and the polar air mass in the stratosphere
becomes separated from other air masses. The temperature drops and drops, ultimately
leading to the stratospheric air trapped in the vortex becoming very cold - in fact the
coldest air to be found in any part of the Earth's stratosphere.
 In this cold vortex, polar stratospheric ice crystal clouds form. Gas phase HCl
dissolves in the surfaces or clings to the surfaces of the clouds. The CFC's react with
the HCl ice, converting relatively unreactive chlorine to the more active species, Cl2,
ClONO2, and HOCl.
 At sunrise, in October, the chlorine-bearing
compounds are photolyzed, releasing the highly
reactive Cl atoms whcih attack ozone.
 Ozone densities drop rapidly, only to recover
when the polar vortex breaks up, mixing warmer
air in and releasing the ozone-depleted air to
move away from the polar region.
 The ozone loss is felt globally!
4. Northern Hemisphere Ozone
The Northern hemisphere is not immune from Ozone
Holes. In the north, the stratospheric polar vortex is not
as well formed as in the south. This is because of the
larger contrast between land and water in the northern
hemisphere. The existence of land masses tends to
break up the symmetry of the polar vortex in the north. However, the same processes operate as
in the south and satellite data show the effect occurring in March (Springtime in the northern
hemisphere).
Sooner or later, we will see colder than usual northern polar stratospheric temperatures in the
early Spring and heavily populated areas will be warned of unusually low ozone levels. Since
ozone depling compounds will be in the atmosphere for many tens of years, we have to live with
these effects. Ultimately, chlorine compounds will cleanse themselves from the stratosphere and
the Earth's ozone shield will return to normal - for our grandchildren's children.
For a movie showing the latest in Northern Hemisphere ozone hole formation, click here. For a
movie showing the 1997 hole formation, click here
Summary
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Various greenhouse gases are accumulating in the atmosphere due to human activities, these
include CFC's, N2O, methane and CO2.
Some of these gases also affect stratospheric ozone concentrations. Ozone depletions occur when
catalytic chemistry accelerates natural loss processes. An important catalytic loss reaction chain
involves chlorine, made available in the stratosphere by the photolysis of CFC's.
Ozone reductions (estimated to be ~1-3% globally) can cause environmental problems due to the
increase in UV-B radiation. One such problem involves inceases in skin cancer. A recovery to
natural ozone levels would take ~100 years in the complete absence of further CFC emissions.
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