Stratospheric Ozone

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:

1. 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.
2. 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.

1. 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 O
   
   Eq 2) 2 O + 2 O₂+ M –> 2 O₃+ M
   
   Production is greatest high in the tropical atmosphere at heights near 40 km. The circulation in the stratosphere then carries the ozone to other regions.
   
2. 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 km.
   

   Eq 3) O₃ + sunlight –> O₂ + O

   Eq 4) O + O + M –> O₂+ M
   
   Eq 5) O + O₃ –> 2 O₂

   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 and 1930.

   
   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 + O₂
   
   Eq 7) O + XO –> X + O₂
   ______________________
   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 chlorine:
   
   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:

Chlorine deactivation:

Eq 8) Cl + CH₄ –> CH₃ + HCl

Eq 9) OH + ClO –> O₂ + HCl

Eq 10) H₂O + Cl –> O₂ + HCl

Eq 11) ClO + NO₂ + M –> ClONO₂ + M

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.

1. Active, ozone destroying molecules: Cl and ClO.
2. Non-ozone destroying molecules: HCl and ClONO₂.

Our understanding of how chlorine can remove ozone is the result of three studies:

1. Paul Creutzen showed in 1970 that NO and NO₂ are catalysts able to destroy stratospheric ozone.
2. 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.
3. 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:
   1. They do not dissolve in water, so they are not rained out of the atmosphere as are many other pollutants.
   2. They are chemically inert, they do not react easily with other molecules.
   3. They are broken down mostly in the stratosphere, by ultraviolet radiation. The breakdown produces Cl atoms in the stratosphere.
   4. 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 page.

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 hole.

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 had anticipated.

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 Hole Monitoring.

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:

1. 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 Ozone.
   
   
2. 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: Why Antarctica?
   
   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.
   
   
3. Polar stratospheric clouds are important for two reasons.
   1. 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 + HNO₃
      
      Eq 13) ClONO₂ + H₂O –> HOCl + HNO₃
      
      Eq 14) HCl + HOC –> H₂O + Cl₂
      
      The nitric acid (HNO₃) becomes incorporated into the cloud particle.
      
      Chemical reactions on the surface of polar stratospheric clouds. From Stratospheric Ozone: An Electronic Textbook Figure 11.46 Chapter 11, Section 5.
      
      
   2. 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.
      1. The nitric acid produced by the chlorine reactions on the particle surface(Eq 12 and Eq13) remains on the particle.
      2. In addition, nitrogen oxides condense to form the clouds.
      3. Other nitrogen oxides react with H₂O on the ice surface to produce nitric acid.
      4. 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.
         
         
4. 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 the Spring?

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.

Other Information

1. 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.
2. More stratospheric ozone information can also be found at the EPA cite on ozone science and the science of ozone depletion.
3. 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.
4. Recent 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 MByte file).
5. 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).

Revised on: 23 December, 2008