RESOURCES INDEX | HOMEPAGE | CONTENTS | PDF | HELP | BACK | NEXT | PUPILS' NOTES
 

Ozone

Teachers' Information Sheet

Introduction

Ozone can be found throughout the troposphere and stratosphere, and is an important species in maintaining life on Earth. Ozone is also an effective greenhouse gas. However, ozone has become a household term mainly through the issues of ozone depletion and the ozone hole that has appeared over the Antarctic within the last 25 years.

This lesson aims to introduce the pupils to the topic of ozone depletion and the problems associated with it. As with the previous lessons the format adopted is as a series of questions relating to ozone. The questions asked are:

  • What is ozone?
  • Why is ozone important?
  • What is ground level ozone?
  • What is stratospheric ozone?
  • What is ozone depletion?
  • What is depleting the ozone layer?
  • What is being done to stop ozone depletion?
  • How can you protect yourself against UV radiation?

What is ozone?

This section introduces the students to the concept that ozone is a form of oxygen.

Ozone is a pungent, bluish gas. One molecule of ozone comprises three oxygen atoms. There are three different species of oxygen: atomic oxygen (O), molecular oxygen (O2) and ozone (O3). Of these, molecular oxygen is the most abundant in the atmosphere. Ozone can be formed from, and broken down to, molecular oxygen.

Why is ozone important?

Ozone has two roles in the atmosphere; in the troposphere it is a pollutant and a greenhouse gas, whilst in the stratosphere it is a vital element that allows life on Earth to exist. This section focuses on the unique properties ozone possesses for filtering UV radiation.

In order to understand the importance of ozone in the stratosphere, it is necessary to briefly discuss ultra-violet (UV) radiation, which is one form of radiant energy emitted by the Sun. The Sun emits a range of radiation across the electromagnetic spectrum; ultraviolet is the range between violet light and X-rays. UV radiation is subdivided into 3 categories: UV-A, UV-B and UV-C.

The term ultraviolet light is sometimes used instead of ultraviolet radiation, though it is somewhat misleading as UV is not visible to the human eye. UV radiation affects the Earth's flora and fauna, including the human body. In general terms, the shorter the wavelength of the radiation, the higher its energy, and the more biologically damaging the UV light can be if it reaches the Earth's surface. Ozone is important because it is the only atmospheric species that can absorb UV radiation, preventing it from reaching the Earth's surface.

UV-A radiation is the least damaging form of UV and reaches the Earth in the greatest quantities; it is the form of UV light that is responsible for tanning and the ageing process of the skin.

UV-B is potentially very harmful, as it can result in melanoma and eye cataracts. Most of the Sun's UV-B light is absorbed by the ozone layer. The depletion of the ozone layer will allow more of the UV-B light to reach the surface of the Earth, resulting in more cases of skin cancer and other diseases, damage to cell tissue, and other environmental problems.

UV-C is the most damaging form of UV light. Fortunately it is almost entirely absorbed by the oxygen and ozone in the stratosphere before reaching the surface of the Earth.

What is ground level ozone?

This section focuses on tropospheric ozone and discusses its role as a pollutant and irritant.

As stated earlier, ozone can be found in both the troposphere and the stratosphere. Ground level ozone is formed within the troposphere, and accounts for about 10% of the total amount of ozone in the atmosphere. Ground level ozone is produced through a reaction between sunlight, volatile organic compounds (VOCs) and oxides of nitrogen (NOx).

Ground level ozone, unlike stratospheric ozone, is a pollutant. At ground level, ozone is toxic to living things and can cause eye, nose and respiratory problems in both humans and animals. Ground level ozone can also damage vegetation such as crops or forests, and damage materials like rubber and plastic.

Ground level ozone is also a principle component of photochemical smog. Smog is a mixture of ozone, hydrocarbons, NOx and carbon monoxide (CO). These chemicals mix to form a yellow/brown cloud of smog, which lingers over cities and causes eye and nose irritations. Smog is the major air pollution problem of large industrial cities (such as Los Angeles and Athens). As the formation of smog extends over large distances, high levels of ground level ozone pollution result in both rural and urban areas.

Sunlight + VOCs + NOx ® ozone + smog

Volatile organic compounds (VOCs)
VOCs cover a large range of substances and include hydrocarbons, halocarbons and oxygenates. They all contain carbon, and they are volatile (i.e. they exist primarily as a vapour within the atmosphere). VOCs are an important group of compounds, as they form the basis of several chemical reactions that occur within the atmosphere. There are several types of VOCs within the atmosphere:

Hydrocarbons (alkanes, alkenes and aromatics)
Hydrocarbons are mainly released to the atmosphere by the evaporation of petrol, and through incomplete combustion.

Halocarbons (e.g. trichloroethylene)
Halocarbons are formed through the evaporation of solvents from paint and through industrial degreasing processes.

Oxygenates (alcohols, aldehydes and ketones)
Oxygenates are formed in atmospheric chemical reactions and in vehicle exhausts.

There are also several natural sources of VOCs of which the most important compound is isoprene; this is primarily emitted from conifers, but also by gorse and bracken. Emissions from vehicles are the primary source of urban VOCs.

Oxides of Nitrogen (NOx)
NOx is a collective term used to refer to two species of oxides of nitrogen, nitric oxide (NO) and nitrogen dioxide (NO2). They are formed at high temperatures during combustion processes involving the oxidation of nitrogen in the air and any nitrogenous components of the fuel or material being burned.

The most important sources of NOx gases are the combustion of fossil fuels for power generation and vehicle exhausts. The production of nitrogen oxides from natural sources is higher than man-made emissions. Natural sources of NOx include bacteria, volcanic eruptions and lightning.

What is stratospheric ozone?

This section deals with the issue of stratospheric ozone in greater detail. Sub-sections deal with the various issues involved with stratospheric ozone.

The greatest concentration of ozone is to be found in the stratosphere, about 15-35km above the surface of the Earth. The terms 'stratospheric ozone' and 'ozone layer' are synonymous. Approximately 90% of ozone is produced in the stratosphere. Although the ozone layer extends from 15 to 35 km, if the layer were compressed at normal pressures it would occupy only 4mm in height. The natural ozone levels in the atmosphere allow most of the harmful UV radiation (discussed previously) to be absorbed before it reaches the Earth's surface.

As stated earlier, the ozone layer protects us from these UV rays. Without the ozone layer the UV radiation that would reach the surface of the Earth would be so intense that exposed skin would be burnt within a few seconds.

What is stratospheric ozone depletion?

Ozone is formed in the stratosphere by the action of sunlight on oxygen. This process involves the photo-dissociation of oxygen:

O2 + hu ® O + O (hu = high-energy photon)

O + O2 ® O3

Photo-dissociation also acts to destroy ozone by converting it back to ordinary oxygen:

O3 + hu ® O2 + O

As a result, the amount of ozone in the stratosphere should be in equilibrium: ozone is formed and destroyed at the same rate. In the past few decades ozone has been destroyed faster than it can be made, due to the presence of elements such as chlorine, bromine and fluorine. The following reactions describe how chlorine acts as a catalyst for the destruction of ozone:

Cl + O3 ® ClO + O2

O2 + hu ® O + O

ClO + O ® Cl + O2

The actual reaction mechanisms are much more complex, but the crucial point is that the chlorine atom is 'recycled' and can repeat the process many thousand of times until it is removed from the atmosphere by one of its sinks. Bromine and fluorine act in a similar way.

What is depleting the ozone layer?

The compounds that are known to deplete ozone (chlorine, fluorine and bromine) have extremely low natural concentrations in the stratosphere. However, in the 40 years after the Second World War the release of compounds containing these elements to the atmosphere increased rapidly; some of these compounds stay intact until they reach the stratosphere. The compounds responsible for the presence of ozone depleters in the stratosphere are listed below.

Chlorofluorocarbons (CFCs)
CFCs are anthropogenic compounds containing chlorine, fluorine and carbon. They were first produced in the 1930s when they were developed as refrigerants, and until the Second World War the uses of CFCs were limited. However, after the War and up until 1987 they were widely used as blowing agents in foam rubber, propellants in aerosols, in air conditioning systems and in various other applications (although much less so today). One reason CFCs were so widely used by industry was because they are stable compounds, and it was originally considered that they caused no environmental damage. CFCs, when they are released, are not broken down in the lower atmosphere because of their inert nature, so through time they rise up into the stratosphere. Once they reach the stratosphere they are hit by the incoming UV radiation, releasing a chlorine atom from the compound.

HCFCs
HCFCs were developed as a replacement for the CFCs, which have been phased out under the terms of the Montreal Protocol (see following section). HCFCs are made up of hydrogen as well as chlorine, fluorine and carbon. As a result of the presence of hydrogen within these compounds they are less stable than CFCs and therefore break down in the troposphere. Their shorter atmospheric lifetimes means they are not as damaging as CFCs but they still have an ozone depleting potential (ODP) because of the chlorine they contain.

HFCs
HFCs are an alternative replacement for CFCs; they contain hydrogen, fluorine and carbon, but do not contain chlorine. As a result they have a very low ozone depleting potential (ODP). HFCs are however very powerful greenhouse gases.

Methyl Bromide
Methyl bromide is a colourless, odourless gas that is widely used within the agricultural industry as a pesticide. Its ozone depleting potential has only recently been recognised. It is estimated that methyl bromide could be responsible for between 5-10% of global ozone depletion. Bromine, as a chemical, has a larger ozone depleting potential than chlorine, but it also has a much shorter atmospheric lifetime than CFCs (» 2 years).

Halons
Halons are predominantly used in fire extinguishers as they are fire suppressants. Most of the world's halon supply has therefore yet to be released into the atmosphere. Halons contain bromine and as a result have high ozone depleting potentials.

What is the ozone 'hole'?

The depletion of the ozone layer is not homogeneous across the globe. There has been negligible depletion in the tropics and large-scale periodic depletion over Antarctica. Ozone loss is accelerated over Antarctica because of the local meteorological conditions. At the very low stratospheric temperatures during the winter, polar stratospheric clouds form, providing a catalytic surface for the reaction between chlorine and ozone. The photochemical destruction of ozone begins in spring with the return of sunlight. A circumpolar vortex of wind isolates the ozone depletion over the Antarctic, leading to the formation of the ozone hole. In summer, the circumpolar vortex breaks up and higher levels of ozone from lower latitudes mix with the depleted region above Antarctic, repairing the ozone hole.

This does not lead to a complete absence of ozone, but rather a large reduction in its concentration over a wide area. This is the ozone 'hole'. In recent years the ozone hole has covered an areas of about 23 million square miles (about the size of the USA), with ozone concentrations falling by 60% (against 1970s levels).

What is being done to stop ozone depletion?

The Montreal Protocol on Substances that Deplete the Ozone Layer, was agreed in 1987 and came into force in 1989. The Protocol has been ratified by over 120 countries. The Protocol contains measures that will phase out the substances that are considered to be ozone depleters. The original Protocol agreed to phase out CFCs by the year 1999, but scientific research highlighted the need for quicker action. So in 1992 an agreement was reached to bring forward the phasing out of CFCs to 1995, and to phase out halons by 1994. Measures were also agreed upon for the control of methyl bromide and HCFCs for the first time. These are to be phased out by 2029. Emissions of CFCs and other ozone-depleting chemicals have now fallen dramatically as a consequence of the Montreal Protocol, but because of the long atmospheric lifetime of CFC molecules in the atmosphere, ozone depletion will continue to be a problem for many years.

Consumer pressure has reduced the number of products that contain CFCs. There are a number of steps that can be taken by individual consumers that can help to reduce the amount of CFCs and HCFCs that are emitted into the atmosphere, so preventing further damage to the ozone layer:

  • Use products that contain HFCs rather than HCFCs or CFCs. In the UK, aerosols have been CFC-free since 1989
  • Buy fridges that use butane or propane as refrigerants.
  • Ensure that the CFCs in old fridges are disposed of correctly.
  • Avoid buying goods with foam-blown packaging.

How can you protect yourself from UV radiation?

The increase in levels of UV-B radiation reaching the Earth as a result of a depleted ozone layer will mean a higher risk of skin cancer for human populations. Rises in the number of cases of melanoma (skin cancer) have already occurred in countries such as Australia. There are a number of steps individuals can take to reduce the risk of skin cancer to themselves, and it is important to make people aware of the dangers.

  • Don't stay out in the Sun for too long, especially between the hours of 11 am and 3pm when the Sun is at its most intense.
  • Protect yourself from the Sun, wear a wide-brimmed hat, a long-sleeved T-shirt and trousers.
  • Wear a pair of sunglasses, but ensure they block out UV-B light.
  • Never stay out in the Sun without sun cream; use high-protection-factor sunscreen and reapply it regularly.

Questions and sample solutions

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

Sample solution: Without the ozone layer, the damaging UV radiation would reach ground level unimpeded. This would result in very little vegetation existing above ground level. Humans and animals would develop tumours, cancers and cataracts and would not be able to exist out of doors for any length of time without the risk of sunburn and future cancers. The only forms of life to be relatively unaffected by the increase in UV are deep-sea creatures, as the UV cannot penetrate that deeply into the sea.

2. Sun creams have Sun-Protection-Factors (SPF). What do the numbers on the bottles mean?

Solution: The SPF number on a bottle of sun cream indicates how long a person may stay in the Sun without burning with that particular cream applied to the skin. For example, if a person could stay in the Sun, without any suntan cream on, for 1 hour without burning, the use of a cream with SPF2 would enable them to stay in the Sun for 2 hours. Similarly, if they used a cream with a SPF of 6, then they would be able to stay in the Sun for 6 hours (although this is not really advisable).

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

Some local authorities and large retail electrical stores have schemes to recycle CFCs from fridges.


Glossary

Electro-magnetic spectrum: the full range of electro-magnetic radiation, of which visible light, UV, radio waves etc. are only part.

Ozone Depleting Potential: a measure of the capability of a chemical to destroy ozone. It is measured against CFC-11, which is considered as the standard and has an ozone depleting potential of 1.0.

Photo-dissociation: a chemical reaction that takes place when certain wavelengths of light are administered to the reaction mixture.

Photon: an indivisible quantity of electromagnetic radiation. Its energy content is the product of its frequency (u) and Planck's constant (h).

Stratosphere: the layer of the atmosphere between the troposphere and the mesosphere, i.e. between 10-15km and 50km above the Earth's surface.

Troposphere: the lowest layer of the atmosphere. It extends from ground level to an altitude of 10-15km.