Modern industrial society produces a large number of gases and particles which can pollute outdoor air. In the past, one of the major sources has been the burning of coal in power stations, other industries and homes. Fortunately Australian coal and oil has a much lower sulphur content and this sort of pollution has never been a major problem in our capital cities. Nowadays the main problem is motor vehicle exhaust.
Petrol engines produce oxides of nitrogen and unburnt volatile organic compounds.
Under the influence of sunlight, these substances combine with oxygen in the air to produce ozone, the main gas in smog. This ozone near the ground should not be confused with the ozone layer high in the atmosphere which protects us from harmful ultraviolet rays. Another gas, nitrogen dioxide, is responsible for the brown colour of smog. Many Australian cities experience smoggy days during the summer. Fine particles in the air cause the hazy winter days seen in our cities.