Air pollution and atmospheric gases

Carbon dioxide in our atmosphere

Carbon dioxide (CO₂) is sometimes described as a pollutant. However, CO₂ exists naturally in our atmosphere, forming part of the carbon cycle which allows essential life processes to occur, such as photosynthesis in plants. CO₂ itself is not a poisonous gas and does not contribute to air pollution. CO₂ is a greenhouse gas, which can have an impact on the environment in other ways. CO₂ can be produced by human activities such as the burning of fossil fuels and many scientists believe that rising levels of these greenhouse gases is causing climate change.

The composition of the earth's atmosphere²

Common air pollutants

Each year a number of pollutants are released into the atmosphere. Some of these occur naturally, such as ash from erupting volcanoes, and smoke from bushfires, but many are released through human activities.

Nitrogen oxides (NOx)

Nitrogen oxides (NOx) are highly reactive gases and come from a mixture of natural sources and manmade combustion processes. Around 80% of NOx released into the atmosphere is from vehicle exhausts¹. NOx contribute to the formation of acid rain and photochemical smog and can cause respiratory problems if at high enough concentrations.

Sulphur dioxide (SO₂)

Sulphur dioxide (SO₂) is a colourless gas produced when sulphur-containing substances are used during industrial processes, such as chemical manufacture and fossil fuel powered electricity generation. Volcanic eruptions can also release large quantities of this gas into the atmosphere. SO₂ sometimes reacts with water vapour in the atmosphere forming sulphuric acid; the main component of acid rain.

Ozone (O₃)

Ozone (O₃) gas exists in the stratosphere and protects us from the Sun’s harmful UV rays. It is also found in the lower atmosphere where it is considered to be a pollutant. This “Ground level ozone” is the primary component of smog and is formed from a chemical reaction between nitrogen oxides (NOx) and organic compounds, released from vehicle exhausts and industrial processes.

Carbon monoxide (CO)

At high enough concentrations, this odourless and colourless gas becomes poisonous to humans, reducing the ability of the blood to carry oxygen around the body. Natural sources of this gas include volcanic eruptions and bushfires. Human sources are primarily motor vehicle exhaust and some industrial activities such as the production of steel.

Particulate matter (PM)

Particulate matter (PM) is a complex mixture of small solid particles and liquid droplets which can cause reduced air visibility and respiratory problems in humans. PM is released into the air from a wide range of combustion processes, both naturally occurring and from human activities.

Emission reduction technologies

Technologies have been developed to remove pollutants from coal-fired power station emissions. These include the use of “scrubbers” which remove SO₂ and NOx from electricity generation exhaust, minimising the air pollution associated with burning coal. An example of this technology is flue gas desulphurisation (FGD), which uses alkaline chemicals such as Calcium carbonate (CaCO₃) to absorb sulphur emissions.

Now similar technologies are being developed to remove the greenhouse gas CO2 from coal-fired power station emissions so that it can be stored underground. This process is known as carbon capture and storage (CCS).

1. ¹ Australian Government, Department of Environment, Water, Heritage and the Arts
2. ² The Encyclopedia of Earth