SUSTAINABILITY MATTERS FACT SHEET 7: THE HOLE IN THE OZONE LAYER

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Sherman Richard
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1 SUSTAINABILITY MATTERS FACT SHEET 7: THE HOLE IN THE OZONE LAYER What is the ozone layer? Ozone is an allotrope of oxygen, which means it is a pure element, but has a different chemical structure to that of the normal oxygen that we rely on to live. Ozone is O 3: a highly reactive, but naturally occurring substance, that is a harmful pollutant at surface level, but an essential component of the upper atmosphere, the stratosphere. The stratosphere starts about 15 km above the earth s surface and extends to about 50 km, and the majority of ozone in the earth s atmosphere is found here, as shown in Figure 1. The concentration of ozone in the stratosphere is very low: 12 molecules in every million are ozone! Ozone concentrations vary naturally depending on latitude, with the highest concentrations in the higher latitudes and the lowest around the Equator. Ozone measurements are made in various ways, including instrumentation carried on balloons and aircraft as well as from satellite- and surface-based remote sensing instruments. Why is it important? The relatively high level of ozone in the stratosphere has given rise to the term ozone layer, and provides protection from solar radiation which would otherwise hit the earth s surface and cause harm to various life forms, including humans. This particularly applies to ultraviolet radiation, which can cause genetic damage leading to cancer on prolonged exposure. There are three types of ultraviolet radiation UV-A, UV-B and UV-C, a distinction based on wavelength. UV-A is the higher wavelength lower energy part of FIGURE 1 Atmospheric ozone (from DW Fahey, Twenty Questions and Answers About the Ozone Layer

The observations showed unusually low total overhead ozone during the late winter/early spring months of September, October, and November.

2 the spectrum and is not absorbed by ozone. The more dangerous, higher energy UV-B and UV-C are absorbed in the ozone layer. First signs of trouble The first decreases in ozone levels were observed in the early 1980s over research stations located on the Antarctic continent. The observations showed unusually low total overhead ozone during the late winter/early spring months of September, October, and November. Total ozone was lower in these months compared with previous observations made as early as The early reports came from the British Antarctic Survey and the Japan Meteorological Agency. The results became more widely known in the international community after they were published in the journal Nature in 1985 by three scientists from the British Antarctic Survey. Soon after, satellite measurements confirmed the spring ozone depletion and further showed that in each late winter/ spring season starting in the early 1980s, the depletion extended over a large region centred near the South Pole. The term ozone hole came about from satellite images of total ozone that showed very low values encircling the Antarctic continent each spring, as shown in Figure 2. The cause The discovery of the cause of this ozone depletion was not long in coming, since scientists had already noted the increasing levels of halogen-containing species, such as ClO and Cl itself, which were capable of reacting with large numbers of ozone molecules. The source of these halogen species were the synthetic industrial gases, chlorofluorocarbons (CFCs) and halons, which had numerous applications, including refrigeration, air conditioning, foam manufacture, aerosol sprays, and fire extinguishers. These CFCs had been developed by Thomas Midgeley, a chemical engineer, working for General Motors and Du Pont Chemicals, in the 1930s as a wonder chemical. He also developed the petrol additive, tetraethyl lead! Chlorofluorocarbons are one- or twocarbon alkanes, where all hydrogens are replaced by either fluorine or chlorine atoms. The two most common are CCl 2F 2 and CCl 3F. Halons also include bromine, a common one being CBrF 2 CBrF2. The chemistry behind the ozone hole Ozone is created in the stratosphere by the reaction of normal oxygen molecules with the powerful ultraviolet radiation from the sun, as shown in Figure 3. FIGURE 3 Stratospheric ozone production (Fahey) FIGURE 2 Ozone depletion over the South Pole (Fahey) Ozone molecules, being naturally reactive, are always being destroyed by reaction. This is a natural process, and is balanced by continuing formation. However, the Sustainability Matters 7. The Hole in the Ozone Layer Page 2 of 5

3 sudden arrival of additional species capable of reaction with the ozone (ie the CFCs) meant a negative disruption to this balance. The CFCs and halons are naturally stable compounds because they don t have any C-H bonds. This means that they have long lifetimes (up to 100 years). This gives them plenty of time to slowly diffuse into the upper atmosphere where the intense solar radiation is sufficient to break them down, and to release the ozone-depleting halogen atoms. Then the trouble starts, because the Cl & Br atoms can cause the breakdown of hundreds of ozone molecules each by acting as a catalyst (see Figure 4). FIGURE 4 Reaction sequence for the catalytic decomposition of ozone by Cl (Fahey) Ozone Level (nb) J F M A M J J A S O N D Month 1992 FIGURE 5 Variation in ozone over the Antarctic 1992 Sustainability Matters 7. The Hole in the Ozone Layer Page 3 of 5

4 Unexpected behaviour Two curious aspects emerged early in the study of ozone depletion. The major occurrence of the depletion occurred: in spring time, and gradually repaired itself (more or less) through the remainder of the year over the South Pole, and much less so over the North Pole or anywhere else Why should this be? The Poles were after all where the highest concentration of ozone is, and there is nothing about spring time that causes high CFC emissions. The answer comes from the intensely low temperature over Antarctica in winter far colder than over the Arctic and the isolation of the stratosphere in winter over the South Pole. This leads to the formation of polar stratospheric clouds (PSCs), which contain large expanses of ice crystals, on the surfaces of which the breakdown of the CFC compounds takes place. When springtime comes, the PSCs disperse and the halogens are released to cause their havoc. As the year progresses into summer and autumn, the ozone layer begins to repair itself. The chlorine atoms react with other species, and are not released or replaced. However, when winter comes, the clouds reform, the cycle begins again. The solution Fairly obviously, the way to stop this happening was to stop CFCs reaching the upper atmosphere. The only way to achieve this was to reduce their use or replace them entirely. In 1985, an international conference was held in Vienna (Austria) which began the process of phasing out the use of CFCs. In 1987, the Montreal Protocol on Substances that Deplete the Ozone Layer was finalised in September The Protocol has been signed by over 160 countries, including Australia. The Montreal Protocol is one of the most successful international agreements yet drawn up in any area. It set out a mandatory timetable for the phase out of ozone depleting substances. This timetable has been under constant revision, with phase out dates accelerated in accordance with scientific understanding and technological advances and more chemicals added to the list of controlled substances. Progress Table 1 lists the change in consumption of halons and CFCs in a number of countries between 1986 (before Montreal) and 1994 (after these substances were completely banned). The success is obvious. TABLE 1 CFC consumption (tonnes) Country Argentina Australia Brazil Canada China European Union India Indonesia Japan Malaysia Mexico Philippines Poland Russia South Africa South Korea Thailand Ukraine United States Venezuela Recent scientific evidence indicates that the Protocol controls are starting to achieve the expected results, with a slowing in the rate of ozone-depleting substances entering the atmosphere (see Figure 6). Assuming all countries continue to meet the Protocol phase out timetable, it is expected that upper atmosphere ozone depletion will stabilise by about the year 2000, and recover by about 2040 (see Figure 6). Sustainability Matters 7. The Hole in the Ozone Layer Page 4 of 5

5 FIGURE 6 CFC level in the lower atmosphere (left) and in the stratosphere (right) ( Sustainability Matters 7. The Hole in the Ozone Layer Page 5 of 5

STRATOSPHERIC OZONE DEPLETION. Adapted from K. Sturges at MBHS

STRATOSPHERIC OZONE DEPLETION. Adapted from K. Sturges at MBHS STRATOSPHERIC OZONE DEPLETION Adapted from K. Sturges at MBHS Ozone Layer Ozone is Good up high Stratosphere Bad nearby Troposphere Solar Radiation - range of electromagnetic waves UV shortest we see if