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Ozone

Pupils' Information Sheet

What is ozone?

Ozone is a form of oxygen. Oxygen exists in three different forms in the atmosphere. Ozone is also an effective greenhouse gas (see Lesson One).

Oxygen atoms (O) are the simplest form of oxygen. The atoms exist as separate units; much of the oxygen in the highest part of the atmosphere exists in this form.

Oxygen molecules (O2) are the most common form in which oxygen exists in the troposphere (lower atmosphere), and is the form of oxygen we breathe.

Ozone molecules (O3) are three atoms of oxygen bound together. Ozone is poisonous to breathe.

Why is ozone important?

Without ozone, there would be no life on Earth. Ozone protects us by forming a shield against harmful radiation from the Sun, known as ultraviolet (UV) radiation. Ultraviolet forms a part of the electromagnetic spectrum between violet and X-rays; it is just outside the range of visible light, with a frequency higher than violet (Figure 1). There are three categories of ultraviolet radiation: UV-A, UV-B and UV-C. Although we cannot see UV radiation, it does have an effect on our bodies.

Figure 1: The electromagnetic spectrum

UV-A is the least damaging form of UV radiation and reaches the Earth in the greatest quantities. This is the UV light responsible for the tanning and ageing processes of the skin.

UV-B is potentially harmful, but most of it is absorbed by the ozone in the stratosphere. Prolonged exposure to UV-B light can cause sunburn, and there are worries that damage to the ozone layer may lead to an increase in the incidence of skin cancer.

UV-C is the most damaging form of UV radiation. Fortunately most UV-C is absorbed by the ozone and oxygen in the stratosphere.

Without the ozone layer the harmful UV rays would reach the Earth's surface and cause serious damage to living things. There would be a dramatic increase in the cases of skin cancer and eye cataracts. UV radiation can also make it harder for our bodies to fight off some diseases.

An increase in UV radiation would also affect parts of the food chain. UV is very damaging to plankton in the oceans. Plankton are an important source of food for many of the creatures that live in the sea such as fish and whales. An increase in the amount of UV radiation reaching the Earth's surface is estimated to diminish plankton supplies. If fish do not have enough plankton to eat they may die, and other species may become endangered. This would, in turn, affect the whole food chain.

Ground level ozone

Ozone can be found in both the troposphere and the stratosphere. Ozone formed in the troposphere is known as ground-level ozone. It is an ingredient in smog and is also a damaging pollutant. It is unusual that a substance which is totally undesirable as a constituent of the air we breathe should be so important high up in the atmosphere (in the stratosphere).

Ground level ozone is formed by the action of sunlight on carbon based chemicals known as volatile organic compounds (VOCs) in combination with a group of pollutants called nitrogen oxides (NOx).

Sunlight + VOCs + NOx ® ground level ozone + smog

Ground level ozone is a pollutant that can cause health problems (such as breathing difficulties). Ground level ozone can also damage materials such as rubber or plastic and vegetation. Ground level ozone is an important component of photochemical smog, which is a problem mainly associated with large industrial cities, for example Los Angeles and Athens.

The concentration of ground level ozone has increased rapidly over the past few decades, due to an increase in the number of cars and other vehicles on the road.

VOCs is a term given to solvents, gases and liquid fuels which contain carbon, among other chemicals, and evaporate easily at normal temperatures. They are emitted by vehicles, petrol stations and dry cleaning processes.

Nitrogen oxides are a by-product of combustion from power stations and vehicle exhausts. They are also produced from natural sources such as volcanic eruptions and lightning.

Stratospheric ozone

90% of ozone is contained within the stratosphere; it is more commonly known as the ozone layer. Stratospheric ozone plays an important part in filtering out most of the ultraviolet radiation that reaches the Earth. Ozone in the stratosphere is not a pollutant, and does not contribute to the greenhouse effect. Ozone has very different effects, depending on where in the atmosphere it is. The formation of stratospheric ozone is shown in Figure 2.

Figure 2: The formation of stratospheric ozone

What is ozone depletion?

Ozone depletion occurs when the natural balance between the processes of stratospheric ozone production and destruction is disturbed, resulting in more ozone being destroyed than is being produced.

The main cause of ozone depletion is thought to be a group of compounds containing chlorine and bromine. These 'ozone depleters' act by removing one oxygen atom from the ozone molecule, so converting it into an oxygen molecule. Each atom of chlorine or bromine can destroy many thousands of ozone molecules in this way.

Compounds which contain chlorine do not occur naturally in the upper atmosphere. However over the past 100 years the amount of man-made compounds containing chlorine and bromine has increased, and this has led to a decrease in natural ozone levels.

What is depleting the ozone layer?

There are a number of compounds that have led to an increase in the amount of chlorine and bromine in the atmosphere:

Chlorofluorocarbons (CFCs) do not exist naturally, but are man-made compounds containing chlorine, fluorine and carbon. Production of CFCs began in the 1930s when they were first used in fridges. After the Second World War however, the amount of CFCs being produced increased as they were also used in aerosols, air conditioning units and for various other purposes. When CFCs were first developed they were thought of as safe, inert gases with numerous uses and believed to cause minimal environmental damage.

CFCs, when released from the surface of the Earth, rise slowly into the stratosphere. Once there, they are bombarded by the incoming UV light from the Sun, releasing the chlorine atoms from the parent compound, which can then react with the ozone molecules (Figure 3). Eventually the chlorine atom is removed from the atmosphere by other reactions.

Figure 3: The destruction of stratospheric ozone

Hydrochlorofluorocarbons (HCFCs). These chemicals are made up of hydrogen, chlorine, fluorine and carbon. They were developed as an alternative to CFCs and are now used in place of them in many products. HCFCs do deplete the ozone layer but they have a shorter atmospheric lifetime than CFCs so they don't cause as much damage. However, HCFCs are powerful greenhouse gases.

Hydrofluorocarbons (HFCs) are another replacement for CFCs. They are made up of hydrogen, fluorine and carbon. HFCs are thought to do very little damage to the ozone layer, but they are powerful greenhouse gases.

Methyl Bromide is used primarily by farmers as a pesticide. It has only recently been recognised as an ozone depleter. Bromine, a constituent of methyl bromide, is 75 times as powerful as chlorine at destroying ozone. This means that methyl bromide, molecule for molecule, destroys more ozone than CFCs.

Halons are used in fire extinguishers. They are similar to CFCs but instead of chlorine they contain bromine and they are powerful ozone depleters.

What is the ozone 'hole'?

The ozone layer is being depleted faster in some areas of the world than others. Near the equator, ozone levels have hardly changed. Over the South Pole the concentration of stratospheric ozone drops by 60% every spring, compared to measurements made in the 1970s. This happens due to the special meteorological conditions over Antarctica. The 'hole' is therefore a thinning of the ozone layer, rather than its complete destruction (Figure 4). Very few people live in Antarctica, but the hole is of particular concern to people living nearby, in places like South America or New Zealand.

Figure 4: The Antarctic Ozone Hole (DU = Dobson Units)

What is being done to stop ozone depletion?

The Montreal Protocol is an international agreement that was designed to control the amount of CFCs and other ozone-depleting chemicals being produced, and eventually stop production of such chemicals altogether. It was first drawn up in 1987; since then over 120 countries have signed it. The Protocol agreed that CFCs should be phased out by the year 1995, and that production of HCFCs and methyl bromide should be reduced and eventually eliminated (by 2029). Emissions of CFCs and other ozone-depleting chemicals have now fallen dramatically as a result, but because of the long atmospheric lifetime of CFC molecules in the atmosphere, ozone depletion will continue to be a problem for many years.

What you can do to help protect the ozone layer?

There are also many steps you can take to protect the ozone layer:

  • Buy CFC-free products. (Due partly to consumer pressure most products containing CFCs have now been withdrawn. In the UK, aerosols have been CFC-free since 1989.)
  • If your parents buy a new fridge, get someone to pick up the old one so that they can recycle the CFCs.
  • Ask your local fast food restaurant to stop using foam packaging, although most packaging is CFC free, some still uses HCFCs.
  • Write to your local council and ask them if they recycle the CFCs from fridges.

What can you do to protect yourself?

  • Increased levels of UV-B reaching the Earth through a depleted ozone layer will put people more at risk from developing skin cancer and eye cataracts. There are many things you can do to protect yourself, a few of these are listed below:
  • Don't stay out in the Sun for too long, especially between the hours of 11am and 3pm when the Sun is at its strongest. Many weather forecasts now issue warnings on the days you will be most at risk.
  • Protect yourself when you do go out in the Sun; wear a wide brimmed hat, long sleeved T-shirts, and trousers.
  • Check your sunglasses block out UV light.
  • Put on some high-protection-factor sun cream and make sure you reapply it regularly.
  • After you swim be sure to reapply your sun cream, even though it may say it's waterproof on the label.

Questions/further work

1. Describe how the world might look without the ozone layer to protect it from the damaging rays of the Sun.

2. Sun creams have different Sun-Protection-Factors (SPF). What do the numbers on the various bottles mean (e.g. SPF4, SPF12, SPF25)?

3. Find out if your local council or electrical shops know of any schemes for recycling CFCs.