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What is the Ozone Layer

 

AtmosphereThe ozone layer refers to a region of Earth’s stratosphere that absorbs most of the Sun’s ultraviolet radiation. It contains high concentrations of ozone (O3) relative to other parts of the atmosphere, although it is still very small relative to other gases in the stratosphere. Out of every 10 million air molecules, about 2 million are normal oxygen, but only 3 are ozone.(1)

The ozone layer contains less than ten parts per million of ozone, while the average ozone concentration in Earth’s atmosphere as a whole is only about 0.3 parts per million. The ozone layer is mainly found in the lower portion of the stratosphere, from approximately 20 to 30 kilometres above the Earth, though the thickness varies seasonally and geographically.(2) Even though the concentration of ozone in this layer is very small it is vital to life as it absorbs biologically harmful UV radiation coming from the sun. The ozone layer absorbs 97–99% of UV radiation in the wavelength range 200 to 310 nm, which means (in combination with O2 absorption below 200nm) all UVC (100 to 280 nm) and nearly all UVB (280 to 315 nm) radiation is absorbed and so does not reach the Earth’s surface.(3) See the page Effects of UV radiation for more information.

Ozone is a molecule made up of three oxygen atoms which is naturally formed by the photolysis of oxygen molecules (O2) by UV solar radiation at wavelengths below 242.5 nm in the stratosphere. A certain amount of ozone is also produced in the troposphere in a chain of chemical reactions involving hydrocarbons, nitrogen-containing gases and sunlight. This is referred to as photochemical “smog”, a familiar pollution problem in the atmosphere of many cities around the world. Higher amounts of surface-level ozone are increasingly being observed in rural areas as well.

The ozone layer was discovered in 1913 by the French physicists Charles Fabry and Henri Buisson. Its properties were explored in detail by the British meteorologist G. M. B. Dobson, who developed a simple spectrophotometer (the Dobsonmeter) that could be used to measure stratospheric ozone from the ground. Between 1928 and 1958 Dobson established a worldwide network of ozone monitoring stations, which continue to operate to this day. The “Dobson unit”, a convenient measure of the amount of ozone overhead, is named in his honour. One Dobson unit (DU) is the thickness in the units of hundredths of a millimetre that the ozone column would occupy at standard temperature and pressure (0°C and 1 atmosphere). The ozone column compressed in this way would typically be 3 mm thick, equivalent to 300 DU.

The Ozone hole over Antartica, on 24 September 2006

The Ozone hole over Antartica, on 24 September 2006

To determine the concentration of atmospheric Ozone (not ground level Ozone) the spectrum of the UV radiation from the sun is analysed using a spectrophotometer. In fact, what is measured is ‘Total Column Ozone’ which is a sum of all the ozone in the atmosphere, in a column between the instrument and the sun in Dobson Units (DU). The global average value is about 300 DU and the boundary of an ozone hole is normally defined as 220 DU.

Depletion of ozone and the ozone hole

The ozone layer can be depleted by natural and man-made free radical catalysts, including nitric oxide (NO), nitrous oxide (N2O), hydroxyl (OH), atomic chlorine (Cl), and atomic bromine (Br). The concentrations of chlorine and bromine have increased due to the release of large quantities of man-made organohalogen compounds, such as halocarbon refrigerants, solvents, propellants, and foam-blowing agents (CFCs, HCFCs, freons, halons). These compounds are transported into the stratosphere after being emitted at the surface, and survive there for 50 to over 100 years. Cl and Br radicals are liberated by the action of ultraviolet light. Each radical is then free to initiate and catalyse a chain reaction capable of breaking down over 100,000 ozone molecules. The breakdown of ozone in the stratosphere results in a reduction of the absorption of ultraviolet radiation and more harmful ultraviolet radiation is able to reach the Earth’s surface.

The ozone hole is an annual thinning of the ozone layer over Antartica, caused by stratospheric chlorine. This was discovered in 1985 and announced in a Nature paper by Joe Farman, Brian Gardiner, and Jonathan Shanklin.(4) An international treaty, the Montreal Protocol was signed in 1987, sharply limiting CFC production, and phasing out production by 1996. Recently in 2014 scientists have seen signs that the ozone layer is beginning to recover (5), but it will be many years before it is back to its original state.

 

References:

  1. http://www.epa.gov/ozone/science/sc_fact.html Retrieved 2014-09-16.
  2. “Science: Ozone Basics”. Retrieved 2014-09-16.
  3. “Ozone layer”. Retrieved 2014-09-16.
  4. Farman, J. C.; Gardiner, B. G.; Shanklin, J. D. (1985). “Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction”. Nature 315 (6016): 207. doi:10.1038/315207a0
  5. https://www.wmo.int/pages/mediacentre/news/pr_1003_en.html and http://www.bbc.com/news/science-environment-29152028

 

Further reading:
http://www.atm.ch.cam.ac.uk/tour/index.html
http://www.wmo.int/pages/prog/arep/gaw/ozone/index.html
http://www.wmo.int/pages/prog/arep/gaw/ozone_2014/documents/2014%20Twenty%20Questions_Final.pdf
http://www.wmo.int/pages/prog/arep/gaw/ozone_2014/documents/Full_report_2014_Ozone_Assessment.pdf