Depletion of Ozone Layer

The ozone layer is important because it absorbs ultraviolet (UV) radiation from the Sun, preventing most of it from reaching the Earth's surface. Radiation in the UV spectrum has wavelengths just shorter than those of visible light. UV radiation with wavelengths between 280 and 315 nanometres (a nanometre is one millionth of a millimetre) is called UV-B, and is damaging to almost all forms of life. By absorbing most UV-B radiation before it can reach the Earth's surface, the ozone layer shields the planet from the radiation's harmful effects. Stratospheric ozone also affects the temperature distribution of the atmosphere, thus playing a role in regulating the Earth's climate.


Why is the ozone layer threatened?

When released to the air, some very stable man-made chemicals containing chlorine and bromine gradually infiltrate all parts of the atmosphere, including the stratosphere. Though they are stable in the lower atmosphere, the chemicals are broken down in the stratosphere by the high levels of solar UV radiation, freeing extremely reactive chlorine or bromine atoms. These take part in a complex series of reactions leading to ozone depletion. A simplified version of the main steps in the ozone depletion process follows:

- Free chlorine or bromine atoms react with ozone to form chlorine or bromine monoxide, stealing one oxygen atom and converting the ozone molecule into oxygen.
- Chlorine or bromine monoxide molecules react with free oxygen atoms, giving up their 'stolen' oxygen atom to form more molecular oxygen and free chlorine or bromine atoms.

The newly freed chlorine or bromine atoms start the process afresh by attacking another ozone molecule. In this way, every one of these atoms can destroy thousands of ozone molecules, which is why very low levels of chlorine and bromine (the concentration of chlorine in the stratosphere in 1985 was 2.5 parts per billion) can break down sufficient ozone to deplete significantly the vast ozone layer.

Which chemicals destroy ozone?

A number of man-made chemicals are capable of destroying stratospheric ozone. They all have two features in common: in the lower atmosphere they are remarkably stable, being largely insoluble in water and resistant to physical and biological breakdown; and they contain chlorine or bromine (elements that are extremely reactive when in a free state) and can therefore attack ozone. For these reasons, ozone-depleting chemicals remain in the air for long periods, and are gradually diffused to all parts of the atmosphere, including the stratosphere. Here they are broken down, by intense high-energy radiation from the Sun, freeing ozone-destroying chlorine or bromine atoms.

Chlorofluorocarbons (CFCs) are the most important ozone-destroying chemicals. CFCs have been used in many ways since they were first synthesized in 1928. Some examples are: as a refrigerant in refrigerators and air conditioners; as a propellant in aerosol spray cans; as a blowing agent.

hydrochlorofluorocarbons (HCFCs) are related to CFCs, and were largely developed as substitutes. Their main uses are as refrigerants and blowing agents. HCFCs are less ozone-destructive than CFCs because their extra hydrogen atom makes them more likely to break down in the lower atmosphere, preventing much of their chlorine from reaching the stratosphere. Nevertheless, the ozone-depletion potential (ODP) of HCFCs is too high to allow their long-term use. Forty different HCFCs are subject to global controls leading to an eventual phase out of their use.

Two other chlorine-containing chemicals have significant ODPs and are subject to global controls: carbon tetrachloride and methyl chloroform (1,1,1-trichloroethane). Both chemicals have been widely employed as solvents, mainly for cleaning metals during engineering and manufacturing operations.

The main bromine-containing chemicals that destroy ozone are called halons. These are bromofluorocarbons (BFCs), the principal use of which has been to extinguish fires. Some halons are potent ozone destroyers-up to ten times more powerful than the most destructive CFCs. Production of three halons ended in developed countries in 1994, and 34 types of halogenated halons (HBFCs) are due to be phased out under the Montreal Protocol.

In recent years, attention has been focused on another bromine-containing chemical with significant potential to destroy ozone-methyl bromide-which is used mainly as an agricultural pesticide. Due to its ozone-depletion potential, the 7th Meeting of the Parties to the Montreal Protocol agreed to phase out methyl bromide by 2010 for developed countries, and a freeze at 2002 for developing countries.

How strong is the evidence that man-made chemicals cause ozone depletion?

The first hypotheses that human activities could damage the ozone layer were published in the early 1970s. For some years afterwards, it remained uncertain whether ozone depletion would actually happen, and if so, whether human activities could be to blame. Initially, some thought that emissions of nitrogen oxides from high-flying supersonic aircraft were the main threat. Others argued that man-made chemicals could make only a tiny difference compared with natural sources of potentially ozone-depleting chemicals, such as volcanoes. Now, though, direct measurement of the stratosphere has proved that chlorine and bromine derived from man-made chemicals are primarily responsible for observed ozone depletion. This conclusion has been further sup-ported by improved scientific understanding of the chemical mechanisms of ozone depletion.

Volcanic eruptions can has-ten the rate of ozone depletion, but their effects are relatively short-lived. In 1991, the eruption of Mount Pinatubo in the Philippines injected some 20 million tonnes of sulphur dioxide into the atmosphere, which contributed to record levels of ozone depletion in 1992 and 1993. In the atmosphere, the sulphur dioxide was rapidly converted into sulphuric acid aerosol, increasing the rate of ozone depletion.

However, stratospheric aerosol concentrations fell to less than a fifth of their peak level in less than two years. By comparison, some CFCs can stay in the atmosphere for more than 100 years; the atmospheric lifetime of CFC-115 is 1700 years.

An international panel of around 295 scientists from 26 countries stands firm in its consensus that ozone depletion is caused by chlorine- and bromine-containing man-made chemicals, mainly CFCs and halons.

How fast is the ozone layer being depleted?

Extensive measurements of the ozone layer by ground-based instruments began in 1957. Since the late 1970s, scientists have taken increasingnumbers of measurements of the ozone layer, using ground-based, balloon- borne and satellite-borne instruments. The measurements have con-firmed that ozone levels are falling almost everywhere in the world. Over the period 1979 to 1994, ozone over the midlatitudes (30°-60°) of both hemispheres has been depleted at an average rate of 4-5 per cent per decade. Ozone levels fell faster in the 1980s than in the 1970s, suggesting that ozone depletion has accelerated.

Where and when is ozone depletion most severe?

Depletion varies with latitude. It is lowest over the equator and increases toward the poles. Over the tropics (20°N-20°S), measurements have shown no significant trend in the total amount of ozone. For the six months after the eruption of Mount Pinatubo, total ozone fell by 3-4 per cent. Over the Arctic, cumulative ozone depletion of up to 20 per cent is thought to have occurred in some altitudes, while ozone loss over the Antarctic has been even greater.

Depletion varies with season. In Northern Hemisphere mid-latitudes over the period 1979-1994, ozone levels fell twice as fast in winter/spring as in summer/autumn. In the Southern became public knowledge in 1985-an event that played an important role in speeding up the international agreement, the Montreal Protocol, to protect the ozone layer. The ozone hole is created due to a combination of special factors found only over Antarctica. Each winter, a 'polar vortex' isolates a large mass of the Antarctic stratosphere. During the winter, no sunlight falls on this air and it becomes extremely cold. The low temperatures encourage the growth of ice clouds, which provide a surface for special chemical reactions. Despite the absence of sunlight, 'inactive' chlorine-containing chemicals are converted into 'active' forms, capable of attacking ozone. When the Sun returns in the spring, this process is speeded up, resulting in very fast destruction of ozone until the polar vortex breaks up, dispersing the air towards the equator.

Recent experiments in the Arctic have shown that some of the mechanisms necessary for extremely rapid ozone depletion are present here too. Fortunately, the polar vortex in the Arctic normally breaks up early in the spring (before sunlight has time to destroy large amounts of ozone) before a full-blown ozone hole can be created.