The air around us is made up of nitrogen (78%), oxygen (20.9%), carbon dioxide (0.03%), argon (0.9%) and other gases (0.17%). These other gases are made up of substances that can cause health problems, including:
- Nitrogen dioxide (NO2): nitrogen dioxide is created when nitric oxide reacts with oxygen in the air. The highest NO2 levels occur where there is heavy traffic e.g. from cars and other vehicles.
- Ozone (O3): ozone protects us from harmful UV radiation at high altitudes. However, at ground-level ozone is a harmful pollutant, which is created when sunlight reacts with other pollutants.
- Sulphur dioxide (SO2): sulphur dioxide is produced when a material, or fuel, containing sulphur is burned. It is a gas associated with industrial areas, in particular industrial processes such as the burning of fuels (coal and oil). The plumes of volcanic eruptions contain naturally occurring sulphur dioxide emissions.
- Carbon monoxide (CO): carbon monoxide is a colourless, odourless poisonous gas produced by the incomplete, or inefficient, combustion of fuel.
- Methane (CH4): methane is a colourless gas which is produced in stomachs when bacteria breaks down food. It is also released into the atmosphere when waste that we throw away decomposes.
- Particulate Matter (PM): particulate matter is made up of a wide range of particles, some of which occur naturally, while others are man-made. Naturally occurring particulates include sand and sea salt, while the man-made variety includes those emitted from vehicle exhausts.
Air pollution is the contamination of air by harmful substances such as those listed here. Air quality is a measure of how clean or dirty the air is.
Sources of pollution and causes of poor air quality
Air pollution can be formed through both natural and man-made processes.
Some of the natural sources of air pollution are organic compounds from plants, sea salt, suspended soils and dusts (e.g. from the Sahara desert). Other natural sources, such as soot and PM are released during volcanic eruptions, forest fires, and other such catastrophes. These natural disasters can produce lots of pollutant gases and smoke, which can increase background pollution levels for years - even in areas far away from the original source.
Transport - roads and rails: Vehicles like cars, vans, buses and lorries run on petrol or diesel. When these fuels are burnt in the engine, pollutants are given out from the exhaust of the vehicles. Near busy roads the main pollutants are: nitrogen oxides, carbon monoxide and PM. Trains cause less pollution than the same individual journey made by car. However, trains still pollute the environment. Electric trains do not have any emissions, but they use electricity that is generated at power stations, and which cause pollutants such as nitrogen oxides, sulphur dioxide and PM to be released into the atmosphere.
Agriculture: Animals such as cows and sheep emit a large amount of methane through belching and flatulence.
Industry and power generation: During the Industrial Revolution in the 1800s lots of factories, such as cotton factories, were built in the UK near to the large towns and cities. Today the main industrial hubs tend to be away from cities. Nitrogen dioxide and sulphur dioxide are the main pollutants associated with these industrial processes. To generate electricity, fuels such as coal, gas or oil are burned at power stations, and when these fuels are burned they release nitrogen oxides, sulphur dioxide and PM as well as greenhouse gases, such as carbon dioxide, which contribute to climate change.
Waste: In the UK, the main sources of CH4 emissions are waste management (mostly landfill) and agriculture.
Weather conditions can worsen or improve air quality. Strong winds can rapidly transport pollutants hundreds of kilometres from international sources (e.g. the Sahara desert, other countries’ power plants), whereas during weak winds associated with high-pressure systems (anticyclones), pollutants can accumulate around local sources. Rainfall can clean the atmosphere or pollute the environment depending on the harmful substances in the air.