The big picture

Do you remember being asked to either cover up (e.g. wear a hat and long sleeves) or stay out of the sun to avoid exposure to harmful radiation emitted from the sun? With increased rates of skin cancer and sun induced eye cataracts in many countries we have become more aware of the damage caused by sunlight. Ultra-violet (UV) radiation from the sun has been found to be especially damaging to living cells.

Figure 1. Examples of popular ways to reduce exposure to damaging UV light.

Ozone within the stratospheric layer of the atmosphere absorbs most of the UV radiation reducing the amount that reaches the earth's surface. Without the presence of stratospheric ozone we would not be able to survive.

The build up of ozone in the stratosphere took a long time to occur. Scientists estimate that it was less than 500 million years ago that stratospheric ozone levels were high enough to change conditions on the earth’s surface which allowed plant life to develop outside the oceans.

Figure 2. Stratospheric ozone development and impact on evolution.

Unfortunately chemicals released through human activity have been found to degrade stratospheric ozone. Ozone levels have been decreasing since the 1970s, thereby increasing the amount of harmful radiation reaching the earth’s surface.

This global issue has brought together many different countries that have worked positively together to make a difference. As a consequence of international collaboration the global production of known pollutants that damage stratospheric ozone has fallen significantly.

Stratospheric ozone and its effects

Stratospheric ozone is found within the earth’s atmosphere at around 25 km in altitude. It is responsible for reducing harmful radiation reaching ground level.

Formation of ozone

The amount of ultra violet (UV) radiation that reaches the stratosphere is sufficient to drive the formation of ozone. Ozone is formed by sunlight energy breaking the bonds within diatomic oxygen molecules to form atomic oxygen which in turn reacts with oxygen molecules to form ozone. At the same time ozone molecules react with oxygen atoms to reform oxygen molecules. Without the influence of pollutants the rate of ozone formation is usually the same as the rate of ozone destruction, leading to a constant level of ozone.

Figure 1. Dynamic equilibrium of formation and loss of ozone.

Effects of stratospheric ozone

Ultraviolet (UV) radiation from the sun is categorised according to its wavelength either as UV-A, UV-B or UV-C. Ozone has the ability to absorb some but not all wavelengths:

• UV-A (longest wavelength between 315-400nm) - ozone is unable to absorb this radiation and it passes through to the ground level.
• UV-B (wavelength between 280-315nm) – ozone absorbs most of this radiation but some passes down into the troposphere layer below.
• UV-C (shortest wavelength with highest energy 100-280nm) – ozone and atmosphere is able to absorb all the UV-C radiation, preventing it from reaching the earth’s surface.

Figure 2. Ozone layer absorbs some types of UV-radiation.

UV-B is highly detrimental to humans and other life on earth. Without stratospheric ozone there is increased exposure to UV-B radiation leading to:

• Sunburn and premature aging of the skin (i.e. wrinkled and leathery) which may increase risk of skin cancers.
• Skin cancers e.g.:
  • Non-melanomas such as basal or squamous cancers (both can usually be treated if found early).
  • Malignant melanomas that can spread rapidly and be fatal.

Figure 3. Melanoma – most deadly of skin cancers that develops pigmentation on the skin.

• Cataracts, in which clouding of the lens reduces vision. If untreated it can lead to blindness.
• Immune-suppression in which the ability of the immune system to function is impaired and this can increase risk of infections.
• Health problems in animals e.g. cancers and damage to their eyes.
• Reduction in crop production e.g. studies have demonstrated impaired growth and subsequent lower yields from crops such as rice, soya beans and sorghum.
• Reduced phytoplankton growth which could have impacts throughout the aquatic food web. Human food supply could also be affected through the reduction in available seafood. In addition with less primary production, the uptake of carbon dioxide falls impacting on climate change.
• Reduction in forest productivity which will reduce the amount of carbon dioxide absorbed from the atmosphere further exacerbating climate change.

Many countries around the world have been experiencing an increase in cases of skin cancers associated with exposure to high levels of UV radiation. In response, governments have sponsored public awareness campaigns to encourage people to protect themselves from the sun and stay indoors when UV-B levels are high.

Figure 4. Advert promoting safety in the sun to reduce risk of skin cancer.

Be Aware

Take care not to confuse effects of ozone decline in the stratosphere (i.e. increase in UV radiation reaching the earth) with impacts of ozone in the troposphere.

Theory of Knowledge

Reducing risk of health problems from exposure to the sun relies on changes in human behaviour. Can knowledge in observed patterns of human behaviour help?

Measuring ozone levels

Ozone levels are measured in Dobson Units (DU) which is the depth the ozone molecules would occupy at standard temperature and pressure (STP) of 0°C (273 Kelvin) and 1 Atmosphere.

1DU = 0.01mm of ozone

When ozone concentration falls, the ozone molecules become more dispersed within the same area which is sometimes referred to as ‘ozone thinning’.

Ozone levels are often measured using lasers or a Dobson spectrophotometer which measures the intensity of certain wavelengths. Measurements can be taken from the ground, from aircraft, balloon sondes (weather balloon) or using satellites. Normal levels range from 300 to 500 Dobson Units (that is the equivalent of 3mm to 5mm thickness of ozone). Due to levels of insolation ozone production is highest in the tropical stratosphere and is moved by prevailing atmospheric circulation systems towards the poles.

Threats to stratospheric ozone

Stratospheric ozone is threatened by chemicals that react with the ozone and reduce its concentration. These chemicals are collectively referred to as ozone depleting substances (ODS) and often contain chlorine or bromine. Chlorofluorocarbons (CFCs) are one of the main groups of ODS.

Chlorofluorocarbons

CFCs are a family of chemicals that were discovered in 1930. Examples include CFC-11 trichlorofluoromethane (CCl3F) and CFC-12 dichlorofluoromethane (CCl2F2) sometimes referred to as Freons their trade name.

Figure 1. Chemical structures of CFC-11 and CFC-12.

CFCs are very stable compounds with long life times of between 65 and 110 years (depending on the specific compound). They were initially considered to be non-problematic to the environment due to their high stability. Coupled with the ability to produce them cheaply they soon became widely used as:

• Coolants in refrigerators and air conditioning systems.
• Propellants in aerosol cans.
• Cleaning agents for electrical parts.
• Blowing agents in plastic foam.

Figure 2. Polystyrene foam historically made with CFCs and now made with the temporary replacement HCFC.

Once in the environment CFCs dissipate into the air. CFCs are not soluble and therefore cannot be removed by rain. Over a period of 10-20 years, the CFCs gradually migrate upwards from the troposphere (lower atmosphere 0-10 km above sea-level) into the stratosphere.

Within the stratosphere UV light breaks down the CFC molecule producing chlorine atoms that starts a chain reaction. The chlorine atoms react with ozone breaking it down and reforming chlorine atoms. This unsettles the previous ozone equilibrium. More ozone is destroyed than formed resulting in ozone depletion. Scientists have estimated that one molecule of CFC can destroy about 100,000 molecules of ozone. Eventually the chlorine atoms form hydrogen chloride which diffuses out of the stratosphere into the troposphere where it is washed out by rain.

Other ODS

Additional ozone depleting substances (ODS) include methyl bromide, halons, hydrobromofluorocarbons, carbon tetrachloride and methyl chloroform.

Methyl bromide

• Is used as a soil fumigator to eradicate pests.
• Natural sources include emissions from the ocean and the burning of biomass.
• It releases bromine in the stratosphere that is estimated to be about 50 times more effective than chlorine at destroying ozone.
• Has a life span of about two years.

Figure 3. Methyl bromide is used to kill soil pathogens and pest in soil used to grow strawberries.

Halons

• Similar to CFCs but contain bromine instead of chlorine.
• Used as fire suppressants in fire extinguishers.

Hydrobromonfluorocarbons (HBFCs)

• Similar properties to CFCs and contain either or both bromine and fluorine.
• Used as solvents, cleaning agents and as suppressants in fire extinguishers.

Figure 4. Powerful industrial fire extinguishers historically made using ODS.

Carbon tetrachloride

• Used as a solvent, dry cleaning agent, refrigerant and as a propellant for aerosol cans.

Methyl chloroform

• Used in industrial solvents, degreasing agent, correction fluid, spray adhesive and in aerosols.

Natural emissions of ODS

Natural threats include emissions from volcanoes. For example eruption of Mount Pinatubo in 1991 released sulphate particles and nitrogen oxides which reacted with stratospheric ozone resulting in a decline in ozone levels.

Figure 5. Volcanic eruption releasing emissions into the atmosphere

Changes in stratospheric ozone levels

There is variation in stratospheric ozone levels around the world (spatial differences) and over time. Both seasonal and long term changes in levels have been recorded.

Example of historic ozone depletion

Since the 1970s when measurement of ozone began, a reduction in ozone levels has been recorded around the world. A reduction in levels of between 5 to 15% was recorded during the 1980s. The figure below illustrates the change in ozone levels from 1984 to 1997.

Figure 1. Levels of ozone above Northern America in 1984 and 1997.

Seasonal loss of ozone over the Polar Regions

During the 1980s, satellite images first recorded seasonal changes in the level of ozone over the Antarctic and then a few years later over the Arctic. Ozone levels were found to be at their lowest during the spring to early summer. In the Antarctic this period spanned from September to December and in the arctic from February to June.

Figure 2. Ozone layer in the spring compared to the Summer over the Antarctic.

It has been found that during the dark cold days of winter the wind creates a swirling mass of air called a polar vortex that prevents air from the lower latitudes entering. Cold winter temperatures lead to the formation of polar stratospheric clouds (PSC) within the polar vortex. CFCs and ODS molecules react in the PSC and form chlorine atoms and other ozone depleting chemicals. In the absence of sunlight, they are unable to react with ozone and hence accumulate within the PSC. When the days become lighter, sunlight energy releases the chlorine and other chemicals that cause ozone destruction, rapidly reducing levels of ozone which reflects the seasonal changes observed.

Figure 3. Polar stratospheric clouds over the Antarctic, typically found at an altitude of around 15-25km.

Sunlight gradually breaks up the polar vortex which allows movement of air containing ozone into the area. In the Antarctic, this can temporarily push the ozone depleted mass of air northwards over parts of Australia, New Zealand, South America or South Africa and similarly in the Arctic, ozone depleted air mass can be pushed over parts of Europe, Asia and North America.

Over the summer and autumn within the Polar Regions, sunlight facilitates the production of more ozone and levels subsequently increase.

Theory of Knowledge

Measurements of ozone began in the 1970s - do technological developments always facilitate knowledge and understanding.

Management of stratospheric ozone I

International agreement

The UN was instrumental in establishing binding legislation to encourage countries to reduce damage to the ozone layer. The UN set up the Vienna Convention in 1985 which led to the Montreal Protocol in 1987. It embraces one of the main principals of the UN that we all have a responsibility to protect and manage our shared environment i.e. the ‘global commons’.

Figure 1. The United Nations has played a major role in the success of the Montreal Protocol to date.

Vienna Convention

Vienna Convention for protection of ozone layer 1985 was influential in producing the Montreal Protocol 1987 which sets specific targets on reduction of ODS. The Vienna Convention acts as a framework to protect the ozone layer. Its key objects are:

“…for parties to promote cooperation by means of systematic observations, research and information exchange on the effects of human activities on the ozone layer and to adopt legislative or administrative measures against activities likely to have adverse effects on the ozone layer.” - UNEP

In 2009 it became the first United Nations treaty to be ratified by all 197 nations. It continues to support research and exchange of information that supports the Montreal Protocol.

Figure 2. International agreement – bringing together all countries around the world to tackle a global issue.

International-mindedness

The Vienna Convention was the first UN treaty to be ratified by all 197 world nations. A main focus of the convention is the sharing of responsibility to protect our 'global commons'.

Montreal Protocol

Although the Montreal Protocol on substances that deplete the ozone layer was agreed in 1987, it is regularly revised in light of the information available at the time. Targets set on production and consumption of ODS are legally binding. Deadlines vary from developed countries to developing countries; the later are given more time together with some financial assistance. The Protocol has been ratified by all 197 nations. Examples of targets set:

• Montreal Protocol 1987 set out a timetable to reduce production of chlorofluorocarbon by 50% and freeze production of halons.
• London Amendment 1990 set targets to phase out production and consumption of the main ODS with deadline of 2000 (extended to 2010 for developing countries).
• Copenhagen 1992 moved the deadline to 1996 (except for developing countries).
• Beijing 1999 added more stringent controls on HCFCs and included additional ODS (e.g. bromochloroethane) to be phased out by 2004.

The UN also set up the UNEP OzonAction Programme to assist developing countries to achieve compliance through technical advice and multilateral funds. Developed countries contribute to the funds to help developing countries to switch from using ODS.

It is left to each nation to best decide how they will comply with the targets set in the Montreal Protocol. The standard approach is to incorporate it into national legislation which feeds into policy and action across the country.

Challenges

There are a number of issues that have been identified in dealing with ODS.

Long life span of ODS

Long life span of ODS such as CFCs, which means that they will continue to have an effect for a long term after production and use has stopped.

ODS present in discarded equipment

Old leaky or discarded refrigerates and air conditioners containing CFCs that may leak CFCs into the environment. Although effort is being made to recover and destroy the CFCs, some still remain in old or disused materials.

Figure 3. Old refrigerators and air conditioning units potentially containing CFCs that can escape into the atmosphere.

Lack of alternatives

Cheaper and effective alternatives to ODS may still not be available.

Replacement chemicals also ODS

Chemicals used to replace CFC include hydrochlorofluorocarbons (HCFCs) e.g. chlorodifluoromethane (CHClF2) which are also ODS. However, they have a shorter life span (about 2 to 20 years) than CFCs (up to about 110 years) and cause approximately 2.5% of ozone depletion compare to the same amount of CFCs. Nevertheless, if used in large amounts they could still reduce ozone levels dramatically. Both CFCs and HCFCs are also greenhouse gases. Under the Montreal Protocol, production and use of HCFCs is to be phased out by 2030.

Figure 4. Example of HCFCs, similar in structure to CFCs and widely used as an interim solution to the CFC ban.

Illegal trade

The illegal trade of banned ODS continues. This may be driven by:

• Higher cost of alternatives.
• Cost of altering existing air conditioning systems and refrigerators to use alternative chemicals.
• Less effective replacements.

Lack of policing and enforcement

Appropriate levels of policing and enforcement are necessary to stop illegal trade.

Pollution Management

A range of approaches used to manage ODS are listed in Table 1.

Figure 5. Alternative to using gas blown plastic packaging includes biodegradable pellets made from corn starch and paper packaging.

Management of stratospheric ozone II

Implementation of the Montreal Protocol and its amendments has resulted in significant changes in the production of ODS occurring.

Progress

Following implementation of the Montreal Protocol and its various amendments, production of most of the main ODS has been banned with some exceptions for medical and research use. The following table summarises which ODS have been banned and whether concentrations in the atmosphere are improving.

• Production of chlorofluorocarbons CFCs and related chlorinated hydrocarbons have stopped and are resulting in atmospheric levels stabilising or declining.
• Production of halons has stopped but atmospheric levels continue to rise, possibly due to emissions from old fire extinguishers.
• HCFCs, replacements for CFCs are still produced and therefore atmospheric levels continue to rise. Production of HCFC is to cease by 2030.
• Some countries banned the use of methyl bromide prior to the 2015 deadline contributing to the decline in atmospheric levels recorded. Although, some nations are struggling to find a suitable and cheap alternative and under special exemptions continue currently to use methyl bromide as a fungicide.

Overall there has been a reduction in ODS emissions and although ozone is still declining, the rate of loss has slowed down. Over the last few years, NASA has recorded some improvements in the ozone layer above the Antarctic. Although due to the long life span of CFCs, full recovery is not expected until towards the end of the 21st century.

The following figure illustrates that as the number of countries ratified the Montreal Protocol increased the production of CFCs declined. However the associated levels of ozone was still fluctuating during this period due to the long life span of CFCs and the impact of other ODS.

Figure 1. Cumulative number of countries ratifying the Montreal Protocol, production levels of CFC and the area of the ozone hole.

Value of Montreal Protocol amendments

The figure below demonstrates how important it has been to continually review the situation and use this information to make amendments. Without any protocol stratospheric chlorine levels would have continued to rise. The targets set by the original Montreal Protocol in 1987 would have slowed this down a little. However, our increasing knowledge and understanding of ODS and stratospheric ozone has led to amendments to the Montreal Protocol that will gradually begin to reduce levels of stratospheric ozone.

Figure 2. Effect of Montreal Protocol and amendments on predicted levels of stratospheric chlorine.

The potential of international cooperation

At the first Montreal Protocol meeting 36 nations were involved but by 2015 all 197 nations had ratified the Protocol. The Montreal Protocol is considered to be one of the most successful environmental international agreements in which scientific knowledge has been used to direct change and policies. People from many different backgrounds have come together from scientist, industrialist, researcher, policy and law makers from across the world to work together on a common cause. Hence, the Montreal Protocol is often used as an exemplar of successful international cooperation.

Watch the following video ‘The Antarctic Hole – from discovery to recovery, a scientific journey’ by UNEP which provides a summary of some the key issues of stratospheric ozone depletion: