Parent document
Environmental Physics / Lettner
VO 437-503

Ozone in the Atmosphere

Author: Maricela Yip
December 01, 2000

List of Contents:
  • Regions of Earth's atmosphere
  • The Stratosphere
  • Ozone (O3)
  • Ozone Destruction by CFC's
  • Factors influencing ozone concentration
  • Stratospheric ozone reduction
  • The Polar Vortex
  • The Situation in the Northern Hemisphere
  • Microholes
  • Efforts to protect the Ozone layer
  • References
Regions of Earth’s atmosphere: Scientists divide the atmosphere into several different layers according to temperature variation and composition. As far as visible events are concerned, the Troposphere is the most active layer (see fig.1). All the dramatic events of weather-rain, lightning, hurricanes-occur in this region. It is the thinnest layer of the atmosphere (10 km); yet it contains about 80 % of the total mass of air and practically all of the atmosphere’s water vapor. Temperature decreases almost linearly with increasing altitude in this region. Above the troposphere is the stratosphere, which consists of nitrogen, oxygen, and ozone. In the stratosphere, the air temperature rises with altitude. This warming effect is the result of exothermic reactions triggered by UV radiation from the sun. One of the products of this reaction sequence is ozone (O3), which, serves to prevent harmful UV rays from reaching Earth’s surface. The concentration of ozone and other gases in the mesosphere above the stratosphere is low, and the temperature there decreases with increasing altitude. The upper most layer of the atmosphere is the thermosphere, which is also known as the ionosphere. The rise in temperature in this region is the result of the bombardment of molecular oxygen and nitrogen and atomic species by energetic particles, such as electrons and protons, from the sun. Typical reactions are:


Fig.1 The Earth’s Atmosphere (90kB)
The reverse of these processes liberates the equivalent amount of energy, mostly as heat. The ionized particles responsible for the reflection of radio waves on Earth. N2 2N ..............DH° = 941.4 kJ
N N+ + e- ........DH° = 1400 kJ
O2 O2+ + e- .....DH° = 1176 kJ
The Stratosphere: Variations in temperature and pressure divide the earth's atmosphere into layers, and mixing of gases between the layers happens very slowly. The altitudes on the diagram are logarithmic. The lowest 10% of the atmosphere holds 90% of the air (see fig. 1). This is because gases are compressible. In a huge pile of feathers the bottom-most feathers become compressed under the weight of the feathers above them. Likewise the lower levels of the atmosphere are filled with compressed air while the upper levels, such as the stratosphere, contain very 'thin' uncompressed air.
Although the stratosphere layer is over four times thicker than the lower atmosphere, the stratosphere holds so little gas that ozone is still considered one of the minor trace-gases of the overall atmosphere. In the lowest portion of the atmosphere, ozone represents less than one part in 100 million. The concentration of ozone in the stratosphere is up to 10ppm: this corresponds to about 12 ozone molecules per million air molecules. The ozone layer is located at an average altitude of 10 to 50 kilometers (see fig.1, 2). Ozone absorbs 97-99% of the sun's high frequency ultraviolet light in the 200 - 300 nm wavelength range which is potentially damaging to life on earth. Irradiation of these wavelengths is capable of destroying the complex molecules that make up biochemical systems. Ozone serves as a natural atmospheric filter to prevent this light from reaching the surface, thereby protecting Earth's life from damages. Every 1% decrease in the earth’s ozone shield is projected to increase the amount of UV light exposure to the lower atmosphere by 2%. Because this would cause more ozone to form in the lower atmosphere, it is uncertain how much of UV light would actually reach the earth’s surface (fig.2).


Fig.2 Polar stratospheric clouds (133kB)
Ozone: (Gr. ozein, to smell), ozone is an allotropic form of oxygen having three atoms in each molecule, formula O3. It is a pale blue, highly poisonous gas with a strong odor and it is an irritating, corrosive, colorless gas with a smell something like burning electrical wiring. Ozone boils at -111.9° C, melts at -192.5° C, and has a specific gravity of 2.144. Liquid ozone is a deep blue, and a strongly magnetic liquid. It is formed when an electric spark is passed through oxygen. The presence of ozone causes a detectable odor near electrical machinery. In fact, ozone is easily produced by any high-voltage electrical arc (spark plugs, Van de Graff generators, Tesla coils, arc welders, as well as photo-copiers, laser printers, CRT-tubes as used in TV and PC-sets, etc). The commercial method of preparation consists of passing cold, dry oxygen through a silent electrical discharge. Each molecule of ozone has three oxygen atoms and is produced when oxygen molecules (O2) are broken up by energetic electrons or high energy radiation. Ozone is chemically much more active than ordinary oxygen and is a better oxidizing agent. It is used in purifying water, sterilizing air, and bleaching certain foods. Ozone formed in the lower troposhere originates from nitrogen oxides and organic gases emitted by automobiles and industrial sources.
In the stratosphere at an altitude of 10 and 50 kilometers, the following reactions occur (fig.3):

  1. Step: the O2 molecule is broken down by the absorption of a photon of radiant energy (l = 240 nm).
  2. Step: diatomic oxygen molecules bond with single oxygen atoms to form ozone.
  3. Step: O3 absorbs a photon and is broken down into a diatomic oxygen molecule and an oxygen atom (occurs simultaneously to step 2) - Rate of O3 formation = rate of O3 destruction.
  4. Step: ozone combines with a single oxygen atom to form 2 diatomic oxygen molecules an O3 molecule lives 100-200 seconds before it is dissociated. The chapman cycle is not flawless; concentration levels of ozone are lower than those predicted from the simple chapman cycle. Although, the concentration of ozone in the stratosphere is very low, it is sufficient to filter out (absorb) solar radiation in the range of 200 - 300 nm.


Fig.3 Chapman cycle (110kB)
Dobson Units used to measure ozone concentration in die atmosphere: 1 Dobson Unit (DU) is defined to be 0.01 mm thickness at STP (O°C and 1 ATM pressure) and spreads out evenly over the area, it would form a slab approximately 3mm thick corresponding about 300DU. A Dobson unit is the most basic measure used in ozone research. The unit is named after G.M.B.Dobson, one of the first scientists to investigate atmospheric ozone (1920-1960). He designed the Dobson spectrometer which it is the standard instrument used to measure ozone from the ground. The Dobson spectrometer measures the intensity of solar UV-radiation at 4 wavelengths, two of which are absorbed by ozone and two of which are not (fig.4).


Fig.4 Dobson Unit (110kB)
Ozone destruction by CFC's: Under normal conditions, formation and destruction of ozone occurs naturally and is in a dynamic equilibrium maintaining a constant concentration of ozone in the stratosphere. Once this reaction involves CFC's, the natural balance is upset since more ozone is decomposed than formed. Since the mid-1970 scientists have been concerned about the harmful effects of certain chlorofluorocarbons (CFCs) on the ozone layer.
The CFCs, which are generally known by the trade name Freons, were first synthetized in the 1930s. Some of the common ones are CFCl3 (Freon 11), CF2Cl2 (Freon 12), C2F3Cl3, (Freon 113), and C2F4Cl2 (Freon 114).
for a detailed listing about CFC'S (click here)
Because these compounds are readily liquified, relatively inert, nontoxic, noncombustible, and volatile, they have been used as coolants in refrigerators and air conditioners, in place of highly toxic liquid sulfur dioxide (SO2) and ammonia (NH3).
Large quantities of CFCs are also used in the manufacture of disposable foam products such as cups and plates, as aerosol propellants in spray cans, and as solvents to clean newly soldered electronic circuit boards. In a peak year of production, nearly 1.5 x 106 tons of CFCs were produced in the USA. Most of the CFCs produced for commercial and industrial use are eventually discharged into the atmosphere (see fig.5).
Because of their relative inertness, the CFCs slowly diffuse unchanged up to the stratosphere, where UV radiation of wavelengths between 175 nm and 220 nm causes them to decompose:

Fig.5 CFCs use in the USA (1978 - 75kB)
The reactive chlorine atoms formed then undergo the following reactions (fig.6):

The oxygen atoms are supplied by the photochemical decomposition of molecular oxygen and ozone. The overall result is the net removal of an O3 molecule from the stratosphere:

CFCl3 CFCl22 + Cl
CF2Cl2 CF2Cl + Cl

Cl + O3 ClO + O2
ClO + O Cl + O2
O3 + O 2O2

Fig.6 Ozone depletion by CFC's (80kB)
The Cl atom plays the role of a catalyst in the reaction mechanism scheme. One Cl atom can destroy up to 100,000 O3 molecules before it is removed by some other reaction. The ClO species is an intermediate because it is produced in the first elementary step and consumed in the second step (see fig.7). The above mechanism for the destruction of ozone has been supported by the detection of ClO in the stratosphere in recent years.

Fig.7 Variation of ClO and O3 (100kB)
Production of Chlorine Radicals: A schematic illustrating the life cycle of the CFCs and how they are transported up into the upper stratosphere / lower mesosphere, how sunlight breaks down the compounds and then how their breakdown products descend into the polar vortex is shown in fig.8,13. The main long-lived inorganic carriers (reservoirs) of chlorine are hydrochloric acid (HCl) and chlorine nitrate (ClONO2). These originate 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 (see fig.8).


Fig.8 The life cycle of the CFCs
One of the most important points to realize 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.
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 (PSC). These reactions take place very fast.
The most important reactions in the destruction of ozone are:
  1. HCl + ClONO2 HNO3 + Cl2

  2. ClONO2 + H2O HNO3 + HOCl
  3. HCl + HOCl H2O + C2
  4. N2O5 + HCl HNO3 + ClONO
  5. N2O5 + H2O 2 HNO3
Factors influencing ozone concentrations: 1. Stratospheric sulfate aerosols: large explosive volcanoes are able to place a significant amount of aerosols into the lower stratosphere, as well as some chlorine. Because more than 90% of a volcanic plume is water vapor most of the other compounds, including volcanic chlorine, get ''rained-out'' of the stratosphere. The effects of a large volcano on global weather are nevertheless significant, which in turn can affect localized weather patterns such as the Antarctic ozone hole. Many observations have linked the 1991 Mt. Pinatubo eruption to a 20% increase in the ozone hole that following spring. The effects of a large volcanic eruption on total global ozone are more modest (less than 3%) and last no more than 2-3 years (see fig.9).


Fig.9 Popocatepetl Nov. 10th 2000 (11kB)
2. Stratospheric winds: every 26 months the tropical winds in the lower stratophere change from easterly to westerly and then back again, an event called the Quasi-biennial Oscillation (QBO). The QBO causes ozone values at a particular latitude to expand and contract roughly 3%. Since stratospheric winds move ozone, not destroy it, the loss of one latitude is the gain of another and globally the effects cancel out.

3. Influences from the hydrosphere: As can be seen in fig.10, the polward movement of air is promoted by the Hadley-cells in both hemispheres; the load of CFC's is transported along this stratospheric system of conveyor belts towards the polar regions.
Both wind and water currents in the south polar region circulate around the antarctic continent (in contrast, the north polar region is not encircled by a circumpolar water current, because the landmass of Greenland efficiently blocks this pattern).
In the south polar region, this circulating pattern facilitate the formation of a polar vortex (fig.13). This enables the high pressure system to suck very cold air from the upper layers of the atmosphere into the layers of the stratosphere (fig.10).

4. Greenhouse gases: to the degree that greenhouse gases might heat the planet and alter weather patterns, the magnitude of the stratospheric winds will certainly be affected. Some of the more popular scenarios of global warming predict cooler stratospheric temperatures, leading to more polar stratospheric clouds and more active chlorine in the area of the Antarctic ozone hole.

Fig.10 Atmospheric and hydrospheric influences aiding in Antarctic ozone depletion (160kB)
5. Sunspot cycle: ozone is created by solar UV radiation. The amount of UV radiation produced by the sun is not constant but varies by several percent in a rougly 11-year cycle. This 11-year cycle is related to magnetic changes within the sun which increase the solar UV output, and is heralded by an increase in sunspots which appear on the surface of the sun. Comparisons of yearly ozone concentrations show a small 11-year variation in global ozone of about 2%. Episodes of unusual solar activity, solar storms and large solar flares, could certainly alter this value.

6. Stratospheric chlorine, coming mostly from man-made halocarbons. Careful subtracting of other natural factors yields a net decrease of 3% per decade in global stratospheric ozone, in the period between 1978-1991; due most likely to catalytic degradation by stratospheric chlorine (fig.5, 6, 13)

Stratospheric ozone has decreased by about 3 % since 1978: Since measurement of atmospheric composition were first made, measurable decreases in stratospheric ozone have been recorded. These changes are seasonal being most severe in the winter. They show extreme variations with latitude: large reductions in the ozone layer over the Antartic were noticeable as early as 1978. Satellite measurements confirmed that total ozone content in this part of the atmosphere in 1987 was less than half of its usual value, and by 1994 it had dropped further, to less than one-third of normal. Localized regions of the Antartic had no ozone layer above them at all: an ozone hole was observed. Ozone amounts north of the artic Circle sank to 45 % of normal, the lowest readings so far were recorded in the winter of 1996.

Video of Ozone levels '97/98 (1MB)
Stratospheric clouds, which can form only in the extreme cold of the polar regions, appeared to correlate with ozone-hole formation (fig.13). Reduction in total ozone above the temperate regions of the Northern Hemisphere currently average 3 %, approximately the worldwide mean, but 6 % depletion is measured at 40°N latitude during the winter months. Epidemiological studies suggest 1-3 % increase in skin cancers over the next decades. As a consequence, considerable effort has been made to identify the causes of ozone depletion. Are CFC's the only responsible substances? Are other natural or manmade substances contributing to it as well?

Fig.11 Antarctic O3 distribution (1995 - 110kB)
The Polar Vortex: During the winter polar night, sunlight does not reach the south pole. A strong circumpolar wind develops in the middle to lower stratosphere. These strong winds are known as the 'polar vortex'. This has the effect of isolating the air over the polar region. Since there is no sunlight, the air within the polar vortex can get very cold. So cold that special clouds can form once the air temperature gets to below about -80C (fig.2). These clouds are called Polar Stratospheric Clouds (or PSCs for short) but they are not the clouds that you are used to seeing in the sky which are composed of water droplets. PSCs first form as nitric acid trihydrate. As the temperature gets colder however, larger droplets of water-ice with nitric acid dissolved in them can form. However, their exact composition is still the subject of intense scientific scrutiny. These PSCs are crucial for ozone loss to occur. 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. 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 (fig.11,13).
Wind speeds around the vortex may reach 100 metres per second. The vortex establishes itself in the middle to lower stratosphere. It's important because it isolates the very cold air within it.
When the sun rises after the long winter night, its light triggers the wholesale destruction of ozone by chlorine monoxide. In the late winter and early spring of 1987 and 1991, the loss was as much as 40 % of the ozone layer.
Fig.11 shows the Ozone distribution of the southern hemisphere for the years of Oct.1980 to 1991. The ozone levels reached a min. of about 120 Dobson units, far below the 220 Dobson units typically seen over Antartica before the hole forms.



Fig.12 Antarctic Ozone distribution (1980-91, 70kB)

Fig.13 The polar vortex (120kB)

The Situation in the Northern Hemisphere: The situation in the North, near the Arctic Circle, was considered to be less severe because its polar vortex is not as well defined and the Arctic stratosphere is warmer than its Antarctic counterpart. Studies showed that ozone levels in this region had decline between 4 and 8 % in the past decade. However, measurements in 1992 revealed a surprising high level of ClO over the northermost parts of the USA, Canada, and Europe (see fig.14). This evidence implies that a large ozone hole, like the one over Antarctica, will probably open up near the North pole in the years to come. What caused this sudden change? The culprint is believed to be the tiny particles and sulfuric acid aerosols from volcanic eruptions, the most recent of which was at Mount Pinatubo in the Philippines in 1991. Apparently, these particles can perform the same catalytic function as the ice crystals at the South Pole.



Fig.14 South Pole image (100kB)
Microhole Formation: Ozone "Microholes" affects Chile and Argentina. The hole in the atmosphere's ozone layer over Antarctica has spread north into southern Chile and lasts longer each year, it has spread as far as the south-central Chilean city of Puerto Montt (see fig.14, 16, and video).
Earlier studies showed the ozone hole used to appear for only days or weeks during the southern hemisphere spring; it now appears in September and lasts until November. That study found unusually high UV radiation levels in central Chile during the weeks of heaviest ozone depletion in Antarctica. A huge gap in the Earth’s ozone layer opened over major cities for the first time this month when the atmosphere’s protective covering against ultra-violet radiation thinned out across the whole continent of South America, from Santiago de Chile on the Pacific coast to Buenos Aires, the capital city of Argentina on the Atlantic.

Video of Chilenian Microhole

Global efforts to protect the ozone layer:
  • In 1978 the USA banned the use of CFCs in hair sprays and other aerosols. International conference to control these chemicals signed by U.S. and 22 other countries. Limited production and use of CFC's.
50% reduction in CFC production worldwide by 2000 now signed by over 90 nations, revised to require the virtual phase out of CFC production by 1996.

  • In 1987, an international treaty, The Montreal protocol, was signed by most industrialized nations; in which it sets targets for cutbacks in CFC production and the complete elimination of these substances by the year 2000. While some progress has been made in this respect, it is doubtful that poorer nations such as China and India can strictly abide the treaty because of the importance of CFCs to their economies.
  • The Vienna Convention for the Protection of the Ozone Layer. In 1981 the Governing Council set up a working group to prepare a global framework convention for the protection of the Ozone Layer. Its aim was to secure a general treaty to tackle ozone depletion. First, a general treaty resolved in principle to tackle a problem; then the parties got down to the more difficult task of agreeing protocols that established specific controls.


Fig.15 North Pole image (190kB)


Fig.16 Global ozone image (100kB)
  • 1990 In U.S. "Clean Air Act Amendment" passed.
  • CFCs gone by January 1, 1996.
  • May 15th, 1993 Warning labels are printed on all products with ozone-depleting substances.
  • Recycling could play a significant supplementary role in preventing CFCs already in appliances from escaping into the atmosphere.
  • There are efforts to find substitutes that are not harmful to the ozone layer. One is the hydro-chloro-fluorocarbon-123 or HCFC-123 (CF3CHCl2). The presence of the hydrogen atom makes the compound more susceptible to oxidation in the lower atmosphere, so that it never reaches the stratosphere. Unfortunately, the hydrogen also makes the compound more active biologically than the CFCs. Laboratory tests have shown the HCFC-123 can cause tumors in rats, although its toxic effect on humans is not known.
  • By reducing the Cl atoms, some chemists suggested sending a fleet of planes to spray 50,000 tons of ethane (C2H6) or propane (C3H8) high over the South Pole in an attempt to heal the hole in the ozone layer. Been reactive species, the chlorine atom would react with the hydrocarbons:
The products of these reactions would not affect the ozone concentration.
  • A less reactive plan is to rejuvenate the ozone layer by producing large quantities of ozone and releasing it into the stratosphere from airplanes. Technically this solution is feasible, but it would be very costly and it would require the collaboration of many nations.
  • As proofed by Greenpeace's alternative refrigerating technology, there are several alternatives; e.g. greenfreeze, a mixture of propane (R290) and isobutane (R600a).


Cl + C2H6 HCl + C2H5
Cl + C3H8 HCl + C3H7
  1. Chang, R.; (1994); Chemistry5thed.; McGraw-Hill, INC.; New Jersey, USA.
  2. Lutgens, F. & Tarbuck, E.; (1998); The Atmosphere3rded.; Prentice Hall; New Jersey, USA.
  3. Reichl, F.; (1997); Taschenatlas der Toxikologie; Thieme Verlag; Stuttgart, GER
  4. Vollhardt, K. P., Schore, N. E.; (2000); Organic Chemistry, Structure and Function; 3rded.; W.H. Freeman & Co., New York, USA.
  5. Internet addresses:

    WWW: United Nations Environmental Programme
    WWW: The 1985 Vienna Convention for the Protection of the Ozone Layer
    WWW: Montreal Protocol/
    WWW: Dobson Units
    WWW: Chemistry of the Ozone Layer
    WWW: US Environmental Protection Agency
    WWW: Questions and Answers on Ozone Depletion
    WWW: Global Warming Potentials of ODS Substitutes
    WWW: Stratospheric Ozone Depletion
    WWW: Glossary
    WWW: Ozone Greenpeace Crises
    WWW: Greenfreeze: Superior Refrigeration Technology
    WWW: The Ozone Hole Tour