Earth's sunscreen – the ozone layer
Glossary
chlorofluorocarbons (CFCs). Organic compounds made up of atoms of chlorine, fluorine and carbon. They were commonly used as refrigerants in refrigerators and air conditioners, as blowing agents in foam plastics, and as cleaners for computer circuit boards. CFCs do not occur naturally – their increase in the atmosphere is entirely the result of human activity. Beginning in the 1940s there was a rapid increase in the rate of manufacture, and hence the escape, of CFCs.
DNA (deoxyribonucleic acid). The nucleic acid forming the genetic material of all organisms with the exception of some viruses which have RNA. DNA is present in the nucleus and other organelles such as mitochondria and chloroplasts.
electromagnetic radiation. Electromagnetic radiation is simply energy which travels through space at about 300,000 kilometres per second – the speed of light. We imagine radiation moving like a wave. The distance between two adjacent wave crests is called a wavelength. The shorter the wavelength, the more energetic the radiation is said to be. Also, the shorter the wavelength, the greater the frequency of the radiation. Other than wavelength, frequency and energy there is no difference between a radio wave, an X-ray and the colour green. They all possess the same physical nature. For more information see Back to Basics: Electromagnetic radiation (Australian Academy of Science) and Electromagnetic Spectrum (NASA's Observatorium, USA).
greenhouse gas. A gas that is transparent to incoming solar radiation and absorbs some of the longer wavelength infrared radiation (heat) that the Earth radiates back. The result is that some of the heat given off by the planet accumulates, making the surface and the lower atmosphere warmer. For more information see The greenhouse effect (CSIRO Atmospheric Research, Australia).
hydrochlorofluorocarbons (HCFCs). Organic compounds like CFCs but with extra hydrogen atoms, and a lower ozone-destroying potential. They have similar properties to CFCs and are being used as temporary substitutes for them.
Montreal Protocol. An intergovernmental document signed by many countries in 1987 (and regularly revised) which established restrictions for the manufacture and use of ozone-depleting substances in an international effort to reduce ozone depletion. More on the Montreal Protocol here.
ozone. Ozone (O3) is a form of oxygen. It is a colourless gas that has a very pungent odour. It exists naturally at low concentrations in the stratosphere where it absorbs ultraviolet radiation. In the troposphere it exists naturally at extremely low concentrations. But these concentrations increase when sunlight acts on various gases, coming mainly from vehicle exhausts, and ozone then becomes a pollutant in the troposphere. Ozone is a highly corrosive gas and is poisonous to most organisms. At concentrations as low as 0.00001 per cent (or 10 parts per hundred million) it can irritate the membranes lining the nose, throat and airways and can trigger or exacerbate asthma attacks.
ozone-depleting substances. Any substance that causes a net loss of ozone in the stratosphere. Such substances must be sufficiently stable to survive the time needed to mix into the stratosphere. Common ozone-depleting substances are the CFCs (there are more than one hundred different types), the HCFCs, carbon tetrachloride and methyl chloroform – all of which contain chlorine; as well as methyl bromide and oxides of nitrogen. Some ozone-depleting substances are naturally occurring, but by the far the greatest ozone-depleting potential comes from compounds synthesised and/or released as a result of human activity.
ozone formation and destruction. Ozone is formed when ultraviolet radiation causes oxygen molecules (O2 ) in the upper layers of the atmosphere to split apart. If a freed oxygen atom (O) bumps into an oxygen molecule (O2 ), the three oxygen atoms re-form as ozone (O3 ).
Ultraviolet radiation can cause ozone to break apart, resulting in an oxygen molecule (O2 ), and a single oxygen atom that is highly reactive. The oxygen molecule is quickly converted back to ozone. The reactive oxygen atom can play a part in breaking down more ozone molecules if ozone-depleting substances are present.
ozone 'hole'. The ozone 'hole' does not refer to a complete absence of ozone molecules but rather a general decrease in the number of ozone molecules scattered throughout a band of the stratosphere above certain regions of the Earth. The phenomenon is more like a carpet thinning.
polar stratospheric clouds (PSCs). Long faint clouds which form in the stratosphere only when the temperature falls below about -80°C. They are common above the poles in winter. These clouds appear to play a role in the depletion of stratospheric ozone. The ice particles in the cloud provide surfaces on which a reaction takes place to release free chlorine. The chlorine then reacts with ozone to form chlorine monoxide and oxygen.
stratosphere. The layer of atmosphere that lies about 15 to 50 kilometres above the Earth's surface. In the stratosphere, the temperature rises with increasing height, which is the opposite of the situation in the lower atmosphere. Ozone occurs in minute quantities throughout the full depth of the atmosphere, but its concentration peaks within the stratosphere at an altitude of about 35 kilometres. This is referred to as the ozone layer. There is little up-and-down air movement in the stratosphere, so the ozone layer stays in position.
ultraviolet (UV). A form of electromagnetic radiation. UV radiation has shorter wavelengths than visible light and it therefore carries more energy. It is divided into three broad categories: A, B and C. UV-A has the longest wavelength and is the least damaging form, although sufficient exposure will cause sunburn. UV-B damages proteins in unprotected organisms and can cause cancer, while UV-C is extremely dangerous because it can cause mutations in DNA.
Posted February 1997.