OZONE AND THE GREENHOUSE EFFECT
Ground-Level Ozone
Most of the free oxygen molecules in the earth’s atmosphere contain two oxygen atoms. This is known as diatomic oxygen and it makes up 20,95% our atmosphere. It is this type of oxygen that we need to breathe. However, the two atoms can be split up by solar radiation and when each of these then joins with diatomic oxygen, the result is a three-atom molecule of oxygen. This triatomic form of oxygen is called ozone. The earth’s ozone is found mainly in two areas: either at ground level or high above our heads. The presence of ozone can be a good or a bad thing … depends on where it is.
Ozone is poisonous and damaging – this makes ground-level ozone a problem. Although it does occur naturally, human activities are increasing the amount of ozone that we breathe. Action is needed to reduce ozone on the earth’s surface.
About 90% of the ozone in the earth’s atmosphere is located in the stratosphere – a band 15-50 km above our heads. This fragile layer, known as the ozone layer, is crucial to our life on the planet. It absorbs 99 per cent of the ultraviolet (UV) radiation of the sun. Without the ozone layer, this radiation would probably kill most of us. It is the damage being caused to the ozone layer that is worrying people. We need to take measures to protect it.
These two problems can’t balance each out. We need to find different solutions for both issues.
Some human activities indirectly produce ozone and these levels can be high enough to cause damage to our health, and to animals, trees, plants, crops and everyday materials. Increasing levels of ground-level ozone also adds to the acid rain and greenhouse effect problems.
Ozone does not come directly out of car exhausts or chimneys. It is formed when other pollutants react in sunlight. The other pollutants need to be present before ozone can be produced and, therefore, ozone is known as a secondary pollutant (the others are called primary pollutants). The main pollutants causing ozone are nitrogen oxides and unburned hydrocarbons. An important source of both pollutants is road traffic. Ozone isn’t formed straight away: there is usually a delay until the other pollutants have reacted in sunlight – this may take several hours.
High concentrations of ozone form mainly during sunny days in or near towns which have a lot of road traffic. One survey has shown that towns with a population of 100,000 or more can cause a build up of ozone in neighbouring countryside. You are not safe if you live well away from a big town. Ozone can be transported long distances – sometimes over 1000 km.
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If there is a wind carrying the primary pollutants away from the town, the highest levels of ozone may not occur until they have had time to react. This may be a long way from the source of pollution. That’s the trouble with this type of pollution … it has no respect for neighours’ rights.
Increased amounts of ozone at ground level are usually caused by a combination of pollutants and local weather. On days when there is a daytime temperature inversion they are stuck in a band air close to the ground. They can’t escape and their levels can build up dangerously.
Ozone is poisonous and can damage people’s health. Even at low concentration it can cause irritation of eyes, nose, throat and chest. Children and the aged are most at risk, particularly if they already suffer from chest complaints and blood diseases. Ozone alerts have been broadcast in many countries – such as the United States, Japan and the UK – for several years. Groups that are «at risk» are advised to go inside and avoid exercise when there is ozone alert.
Unfortunately, just because ozone levels don’t remain high all the time it doesn’t mean that we are safe. Short bursts of breathing ozone are just as damaging as prolonged exposure. Hiding indoors may not even be enough … ozone can be produced by some electrical equipment. For example, badly maintained photocopiers can produce quite high levels of ozone, particularly when they are put in small, poorly ventilated rooms.
Natural and man-made materials are also affected. Ozone can be an important link in the build up of acid air pollution – a huge problem around the world. It can also weaken textiles and cause paints and pigments to fade. Many museums and art galleries have installed air-conditioning equipment to prevent damage to paintings and valuables. Rubber is particularly prone to ozone damage which causes it to harden and crack. Many car tyres and insulating materials are now treated with chemicals to prevent attack by ozone. Worldwide, up to 30% of air pollution damage to man-made materials could be caused by ozone.
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Ground-level ozone damages plants – in certain conditions it may kill them. It attacks cell membranes and internal structures of the leaf, affecting photosynthesis and respiration of the plant. The first sign of visible damage is that the plant’s leaves start to go blotchy and eventually drop off. Ozone damaged plants may be more sensitive to climatic change and attack by pests and diseases.
In recent years forests throughout the world have been showing increasing signs of damage and ozone is thought to be one of the main causes in some areas – including Western Europe and North America. It is particularly damaging when combined with other air-borne pollutants – to make the so-called pollutant cocktail. Agricultural crops which can be damaged by ozone include potatoes, tomatoes, soya bean and spinach. In the USA ozone damage may cause a 20% reduction in crop yields.
High-Level Ozone
Must of the earth’s ozone is found in the stratosphere – a layer in the atmosphere 15-50 km above the ground. Unlike ground-level ozone, the high-level ozone does a useful job. It protects animals and plants from the sun’s harmful ultraviolet rays and it also helps stabilize the earth’ climate.
We know that ozone is formed when ultraviolet radiation splits up diatomic oxygen. This reaction absorbs part of the sun’s ultraviolet radiation. However, the story doesn’t end there. The ozone is then split up; again by another part of the sun’s ultraviolet radiation. Between them, the two sets of reactions help screen out most of the harmful UV rays they get to the earth’s surface. As long as there is enough oxygen and ozone, the cycle of reactions should ensure that we are protected.
Although the high-level ozone occupies a band which is 25 km thick, it is not very dense. If we were to bring all the stratospheric ozone down to ground level it would form a layer which is only 3 mm thick. The ozone layer is fragile and it is being threatened. In recent years it has thinned. Human activities have damaged it to the extent that it has developed periodic tears above the poles. Some gases are particularly harmful to high-level ozone: chlorofluorocarbons (CFCs) for short and nitrous oxide are under the most suspicion. CFCs are gases used in a wide variety of process. They were invented in 1932 and were used because they were cheap, nonflammable and stable.
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CFCs are released into the air when a product containing them is destroyed. When CFCs reach the upper atmosphere, the chlorine they contain breaks down the fragile layer of ozone.
Each CFC molecule is very stable and can stay in the atmosphere for up to 130 years. During that time it may destroy up to 100,000 ozone molecules. Since their discovery in the 1930s there have been a build up of CFCs in the atmosphere; recently, there has been a mirror-image fall in the high-altitude ozone levels.
Because of their long life, even if we stopped using CFCs today they would still be eating the ozone layer well into next century. CFCs are also important greenhouse gases.
Some everyday materials are the main contributors to CFCs in the atmosphere. CFCs are sometimes used as a propellant in aerosols. They can cause up to 31% of atmospheric CFCs. CFCs also escape during manufacture of rigid foam, which is used for insulation and packaging. It can cause up to 50% of atmospheric CFCs. In fridges CFCs are used in insulation and coolant. This can cause up to 7% of atmospheric ozone.
Nitrous oxide – otherwise known by its other name – laughing gas – mainly comes from natural vegetation, but increasing amounts are produced by cars and power stations burning fossil fuels, and from nitrogen-based artificial fertilizers. Like the CFCs, nitrous oxide is long-lived – remaining in the atmosphere for up to 150 years. It slowly moves up from the ground into the stratosphere where it helps break down ozone.
The depletion of the ozone layer has become a global problem. A thinning ozone layer will allow more ultraviolet radiation to reach the earth’s surface, which can have a direct effect on our health. It can damage our immune systems, make us prone to infectious diseases, cause skin cancers and damage our eyes. American scientists have estimated that for every 1% decrease in the concentration of high-level ozone there would be a 5% increase in the number of skin cancers and an extra 25,000 eye cataract victims in the United States only!
A thinning ozone layer can damage small organisms, that live in the sea – collectively known as plankton. Phytoplankton is considered to one of the most important source of oxygen in our atmosphere. Ultraviolet radiation can also damage the eggs and larvae of fish and crustaceans. It will change food chains in the sea and will cause the dramatic reduction of fish stock in the sea and the amount of sea animals and seabirds.
Land plants will be also badly affected and food chains in terrestrial habitats will be broken by high levels of UV. Damage to plankton and land plants may affect the carbon dioxide cycle in the atmosphere, which could worsen the Greenhouse Effect.
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