The Scottish Government

O What is the ozone layer?

O Why is the ozone layer important?

O What evidence do we have for ozone depletion?

O How is stratospheric ozone destroyed?

O Why does the ozone hole appear over the Antarctic in the spring?

O Effect of volcanoes on ozone
O What is ground level ozone?
O Will the ozone recover?

High in the atmosphere a layer of ozone protects life on earth from the damaging ultra violet rays of the sun. Ozone, which is composed of three oxygen atoms bound together, is a naturally occurring substance and is present at very low concentrations throughout the atmosphere. The concentration is highest in the stratosphere, which is the atmospheric layer between approximately 10-15 to 50km above the Earth's surface.

The ozone layer filters out much of the potentially harmful ultraviolet radiation from the sun that enters the atmosphere. (Radiation at wavelengths between 240 and 320 nanometers is absorbed by ozone.) Any depletion of the ozone layer results in an increase in UV radiation reaching the Earth's surface, which can have a detrimental impact on our environment and health.



Most living species have some natural tolerance to UV radiation, but it is now well known that even moderate exposure carries a risk of harmful effects. In humans, enhanced long-term exposure is likely to increase levels of skin damage including sunburn and skin cancers and of eye abnormalities such as snow blindness and cataracts. UV radiation can also suppress the immune system with potentially serious consequences for the spread of infectious diseases.



Animals, plant and marine life may experience similar effects, and there are consequences for biodiversity and food supply. For example, harmful effects on aquatic life tend to occur just below the surface of the water where eggs and larvae of many species are found, and where plants forming the foundation of the food chain are most abundant. Many crops, such as wheat, barley, rice and soybean, are vulnerable and crop yield and quality can both be affected.



Enhanced UV radiation may degrade natural and synthetic polymers, such as wood and plastics, used by the construction, transport and agriculture industries and limit the usefulness of these materials in some applications. UV radiation has an impact on the chemical composition to poor air quality, which is already a serious problem in some urban areas.

Scientists from the British Antarctic Survey reported the first unequivocal evidence of damage to the ozone layer in 1985. They observed that during each spring, ozone was almost completely destroyed over large parts of the Antarctic.



This phenomenon of severe ozone depletion, often referred to as the ozone hole attracted a great deal of public and scientific interest, and since then there have been considerable efforts to measure and understand changes in ozone concentrations around the world. Ozone levels have been falling gradually across much of the world since the 1970s. The largest losses are over the poles, but losses of 3-6% (compared to pre-1980s levels) were observed during 1997-2001 in mid-latitudes in both hemispheres; over the UK there have been losses of about 6% since 1980. Conversely there has been little change in ozone levels over the tropics.



The greatest ozone depletion has occurred in the Antarctic region, where the ozone hole has continued to develop each spring since the early 1980s and has gradually increased in depth and area. In 2001, the ozone hole covered a maximum area of 26 million square kilometres (over twice the size of Europe) during the southern hemisphere spring. Ozone levels over the Antarctic during September and October were 40-50% below pre-1980s levels during recent years, and there were short-term localised losses of up to 70%. The Antarctic ozone hole has persisted into the summer during recent years, which increases the impact of ozone loss on UV radiation exposure. It is not yet possible to tell whether the Antarctic ozone hole has maximised; it did not grow so rapidly during the 1990s as during the 1980s, but has nonetheless continued to grow.



There is now clear evidence for similar periods of major ozone loss in the Arctic region although these are less intense and consistent than those in the Antarctic. Different meteorological conditions mean that the ozone depletion is less severe than that observed in the Antarctic, but temporary losses of 30% have been observed during the last decade. In March 1996, Arctic air, which had been depleted in ozone, passed over the UK resulting in record low ozone levels being observed. Although determining any trend in changing UV levels is difficult, as there is no doubt that occurrences of high UV radiation are strongly correlated with periods of low ozone. For example, in 1992, radiation levels doubled over southern South America as the ozone hole rotated to cover the tip of the continent.

Ozone is continually formed and destroyed in the stratosphere in a natural cycle that is a result of chemical reactions involving sunlight and oxygen. The amount of ozone in the atmosphere varies naturally with latitude and altitude, from day-to-day and with the seasons. Most ozone is formed in the tropics where levels of solar radiation are high, but is then swept around the globe by winds so that the ozone layer is thicker towards the poles. This natural cycle of ozone formation and destruction is a balanced system with no net changes in ozone. However, some chemicals can accelerate the part of the cycle that destroys ozone, which results in a net destruction.



In 1974, scientists suggested that the rapidly increasing use of some human-made chemicals, such as chlorofluorocarbons (CFCs), interfered with the natural ozone cycle in the stratosphere. The subsequent discovery of the ozone hole by scientists at the British Antarctic Survey gave added weight to the theory, and more recent scientific has proved that CFCs and similar compounds cause severe stratospheric ozone depletion. The ozone-depleting substances, which include CFCs, carbon tetrachloride, 1,1,1 trichlorethane, halons, methyl bromide and hydrochlorofuorocarbons (HCFCs) all contain halogens, such as chlorine and bromine. They were traditionally used in a wide variety of applications, as refrigerants and foam blowing agents, in fire fighting equipment, as solvents and aerosol propellants. They are very useful chemicals as they are stable, which means that they do not react easily with other substances. It is exactly this property that has made them able to damage the ozone layer.



Ozone-depleting chemicals do not react with other substances in the lower atmosphere, where they could form other, less harmful substances, but instead have a long atmospheric lifetime (essentially the time a substance can 'live' in the atmosphere without being destroyed or removed in some way). This long lifetime enables them to disperse upwards into the stratosphere, where a combination of reactions and meteorological conditions eventually break them down and release atomic chlorine and bromine. The chlorine and bromine atoms released directly into the ozone layer subsequently catalyse the destruction of ozone through a cycle of chemical reactions. A catalytic cycle is one in which a molecule enables a reaction to take place, or alters the outcome of the cycle, without being affected itself. The catalytic cycle is therefore particularly harmful for the ozone layer, as chlorine and bromine atoms are not destroyed by the reactions, and are free to participate in the cycle over and over again; each chlorine atom can destroy up to 100,000 ozone molecules before it is removed from the stratosphere. The natural cycle of formation and destruction of stratospheric ozone continues, but the additional rapid destruction of ozone through this process results in a net ozone depletion.

Human-made chemicals in the atmosphere are broken down in the stratosphere by a combination of chemical reactions and meteorological factors. CFCs and other ozone depleting substances are broken down into reservoir species by sunlight. These reservoir species still contain chlorine or bromine molecules but do not directly affect ozone. The reservoir spcies tend to get pushed towards the poles by global winds. During the Antarctic winter, the air is particularly cold and the temperature drops below -80°C, enabling polar stratospheric clouds to form. These provide a surface on which the reservoir species can react, and release the chlorine and bromine molecules. Sunlight can break down the molecules into atoms, which then take part in the catalytic cycles that destroy ozone. The reservoir species only react on the surface of polar stratospheric clouds. This results in large concentrations of molecular chlorine forming in the atmosphere above the Antarctic during winter. When the sun rises above the horizon in the spring, the molecules are quickly broken down into active atoms, which take place in the catalytic cycles, destroying ozone and forming the "hole" in the ozone layer.



The exact location and size of the ozone hole varies as it will depend on meteorological conditions (such as temperature, which determines whether polar stratospheric clouds will form). In recent years, the area has extended over the entire Antarctic continent and surrounding ocean, and occasionally includes the southern tip of South America. Maximum depletion is usually observed in late September/early October, after which the hole starts to recover as the Antarctic stratosphere becomes too warm for polar stratospheric clouds to exist, and there is mixing with ozone-rich air from surrounding regions.



Significant ozone depletion now occurs over the Arctic, particularly in the spring. However, meteorological conditions there are often very different to those in the Antarctic and ozone depletion tends to be less severe. In the Arctic, average winter temperatures are higher so polar stratospheric clouds are less likely to form. Nevertheless, there is now compelling evidence for on-going depletion of the ozone layer over the Arctic and surrounding mid-latitude regions. Over 350 scientists and researchers from the United States, European Union, Canada, Iceland, Japan, Norway, Poland, Russia, and Switzerland took part in a Nasa-led study during the winter of 2002/2003 to measure ozone and other atmospheric gases over the Arctic. The scientists used aircraft, large and small balloons, ground based instruments and satellites to provide the data, which can subsequently be used for research and analysis. This extensive monitoring programme will help develop our understanding of ozone loss processes in the area.

Natural events such as volcanic eruptions can also influence the ozone layer. The eruption produces clouds of aerosol particles. Although volcanic aerosol does not react directly with ozone, the particles provide a surface on which the destructive reaction with other ozone-depleting substances can take place. Following the eruption of Mount Pinatubo in 1991, ozone levels were depressed for 2-3 years.

In the stratosphere, the ozone layer shields us from the harmful effects of the sun, but nearer the ground, as a component of photochemical smog, ozone is a damaging pollutant. Ground level ozone in high concentrations can damage certain materials, crops and trees, and can be harmful to human health. For example, individuals may experience respiratory problems while taking vigorous exercise out of doors. Ground level ozone is produced by the action of sunlight on a mixture of pollutants, which are mainly emitted from motor vehicles, power stations and industry. Ozone levels in the UK tend to be highest in rural and upland areas and lowest in busy city streets where ozone may actually be destroyed by components of motor vehicle exhaust.



Action to reduce ground level ozone is being taken internationally and the UK is contributing fully to these efforts. For example, there are controls on the emission of volatile organic compounds (VOCs) from the storage and transport of petrol, and new vehicles are subject to stringent emission standards.



The issue of ground level ozone is dealt with by the Air Quality Team and more information can be found on the Air Quality Website.

Yes, but it is difficult to predict exactly when the recovery might begin and how fast it will occur. Scientists predict that, assuming full compliance with the Montreal Protocol, ozone depletion will reach maximum levels during the next few years and then the loss will gradually decline until the ozone layer returns to pre-ozone hole levels during the second half of this century. Emissions of some ozone-depleting substances are decreasing steadily as production levels become lower in response to the Montreal Protocol, and there is an associated decrease in the atmospheric abundance of some CFCs (notably CFC 11 and CFC 113), carbon tetrachloride, and methyl chloroform. However, the abundance of most of the halons continues to increase and concentrations of HCFCs and HFCs are rising, since they are being phased out. The long lifetimes of these substances also enable those emitted in earlier years to continue to ascend into the stratosphere. Observations have shown that the abundance of chlorine in the atmosphere is probably at or near a peak now, and the abundance of bromine is still increasing, so the ozone layer is still in a very vulnerable state and continued compliance with the Protocol is essential.

ã Crown, The Ozone Layer, Defra (2003)