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The headline indicator focussed on one form of air pollution: fine particles. Other substances released into the air can be harmful to both people and the environment. Some substances pollute directly and are known as primary pollutants. Others (so-called secondary pollutants) react with the atmosphere, or each other, to produce pollution. This section begins by looking at sulfur dioxide (a primary pollutant) and then considers ozone and photochemical smog (formed from secondary pollution by oxides of nitrogen).
SULFUR DIOXIDE EMISSIONS
Sulfur dioxide (SO2) is emitted by the burning of coal and during industrial processes such as wood pulping and paper manufacturing. It is also emitted by vehicles. It irritates the eyes, nose and throat, and people with impaired lungs or hearts and asthmatics are particularly at risk of developing health problems.(SEE FOOTNOTE 1)
Most of Australia is now unaffected by sulfur dioxide pollution. And in 1999, prompted by a 30% reduction in SO2 emissions during the late 1990s, there were only a few localities of concern.(SEE FOOTNOTE 2)
Maximum one hour averages of SO2 in regional centres such as Mount Isa and Kalgoorlie have tended to decline since the late 1980s. Among the regional centres shown in the graph, concentrations breached guidelines in Mount Isa in 1999. But concentrations had fallen dramatically in Kalgoorlie in recent years (because of improved mineral extraction and processing), while there were no exceedences in recent years in the Illawarra and Gladstone areas, where coal-fired power generation occurs.(SEE FOOTNOTE 2)
Highest one hour averages of SO2, selected regional centres(a)
OZONE AND PHOTOCHEMICAL SMOG
Ozone is formed when oxides of nitrogen react with sunlight in the atmosphere. It is a colourless gas and a natural part of the upper atmosphere, where it filters ultraviolet radiation from the sun. But increased concentrations in the lower atmosphere can irritate eyes and kill vegetation.
In parallel with ozone formation, nitrogen dioxide reacts with substances in the atmosphere like water vapour to form acid aerosol nitrates. These mix with ozone to form smog. As sunlight is an important factor in the formation of ozone (and hence smog), smog is more likely on sunny days in cities. A recently published study linked ozone and nitrogen dioxide pollution with increases in daily death rates in Melbourne.(SEE FOOTNOTE 3)
Ozone concentrations, therefore, provide an estimate of smog. During the 1990s there was no real decline in the number of days when maximum hourly ozone concentrations (averaged over four hours) exceeded guidelines in our five largest capital cities, and hence no decline in smog.(SEE FOOTNOTE 2) Between 1997 and 1999, four-hourly ozone guidelines were broken on 52 days in Sydney. Over the same three years, there were ten days or fewer in Brisbane, Melbourne and Perth when guidelines were broken, and no days in Adelaide in 1998 or 1999 (no data are available for 1997).
Number of days when ozone concentrations exceed guidelines(a), selected capital cities(b)
OZONE DEPLETING EMISSIONS
As indicated above, ozone near the Earth's surface can be a harmful pollutant, but in the upper atmosphere (the stratosphere) it absorbs most of the harmful ultraviolet (UV) radiation in the sun's rays. When excessive UV radiation reaches the Earth's surface it can cause health problems to people and other organisms, including damage to the eyes, skin and immune system. It can also affect crop yields and marine plankton (which might have flow-on effects to many marine ecosystems). Radiation can degrade plastics, wood, paper, cotton and wool.
Certain substances trigger the destruction of ozone. Human activity has been responsible for increasing the concentrations of these substances in the upper atmosphere: the main ozone depleting emissions are chlorofluorocarbons (CFCs) used in refrigeration, foam plastics and aerosol products.
As a result of these emissions, between 2% and 4% of ozone over Australia has been lost each decade since the 1950s, and we are now exposed to greater levels of UV radiation than in the past. There was international recognition of the problem in the mid-1980s when most countries signed the Montreal Protocol governing the global consumption of ozone depleting substances.
Since then the accumulation of ozone depleting substances in the atmosphere has slowed. Concentrations peaked in the mid 1990s and are now declining. But the substances already in the atmosphere continue to destroy ozone, and because of these time lags the depletion of ozone over Australia and Antarctica may not yet have peaked, although ozone may not decline much further.(SEE FOOTNOTE 2) The largest losses have been observed over Antarctica (more than 60% of natural levels)(SEE FOOTNOTE 2) particularly in spring, losses that have led to the so-called Antarctic ozone 'hole', an area of the stratosphere within which ozone concentrations are well below levels at which they were at the beginning of the twentieth century.
Some scientists expect that complete ozone recovery may be achieved by about 2050, although it may be delayed by as much as 50 years by climate change. Greenhouse gases trap heat in the lower atmosphere, thereby keeping the stratosphere cooler. At very low temperatures, certain stratospheric clouds form above the poles, and in spring they react with ozone depleting substances which then destroy ozone.
Estimates of Australia's total consumption of ozone depleting substances, weighted according to the ozone depleting potential of each, are presented in the graph. Consumption in 1991 was over 8,000 ozone depleting tonnes (ODTs: an aggregated scale of measurement which allows one to add together quantities of different gases and weights them according to the amount of ozone each could potentially deplete). In 2000, it had fallen, in response to international restrictions, to 550 ODTs, entirely composed of methyl bromide and hydrochlorofluorocarbons (HCFCs).
Australia stopped production of CFCs during the 1990s, and we are ahead of the Montreal Protocol's schedule in reducing our use of HCFCs, which are minor ozone depleting substances that are used as interim replacements for CFCs.
Consumption of ozone depleting substances
1 National Pollutant Inventory Database. URL: http://www.NPI.gov.au last viewed 14 March 2002.
2 State of the Environment Committee 2002, Australia - State of the Environment Report 2001, CSIRO Publishing, Melbourne.
3 Victorian Environment Protection Authority 2000, Melbourne Mortality Study URL: http://www.epa.vic.gov.au/envquality/air/mortalitystudy/mortstudy.htm last viewed 27 November 2000.