What is air made of, and how do human activities pollute it and change the climate?
State the composition of clean air, name common air pollutants and their sources and effects, and describe the greenhouse effect
A focused N(A)-Level answer on air. The composition of clean air, common pollutants and where they come from, their effects, and how the greenhouse effect warms the planet.
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What this dot point is asking
SEAB wants you to state what clean air is made of, to name common air pollutants with their sources and harmful effects, and to describe the greenhouse effect. The central idea is that burning fuels adds gases to the air that damage health, the environment and the climate.
The answer
The composition of clean air
Clean dry air is a mixture of gases, mostly:
- nitrogen, about ,
- oxygen, about ,
- small amounts of other gases, including carbon dioxide (a small but important fraction) and the noble gas argon.
Common air pollutants
Burning fuels, especially fossil fuels, releases pollutants:
- carbon dioxide: from burning any fuel. It is a greenhouse gas and adds to global warming.
- carbon monoxide: from incomplete burning (not enough oxygen). It is a poisonous gas that reduces the oxygen the blood can carry.
- sulfur dioxide: from burning fuels that contain sulfur. It dissolves in rain to make acid rain.
- oxides of nitrogen: formed in hot engines. They also cause acid rain and breathing problems.
Effects of pollution
- acid rain (from sulfur dioxide and nitrogen oxides) damages trees, kills fish in lakes, and wears away buildings and statues,
- carbon monoxide is toxic and can be deadly in enclosed spaces,
- particulates (tiny solid bits of soot) and other pollutants harm the lungs.
The greenhouse effect
Some gases in the atmosphere, called greenhouse gases (mainly carbon dioxide and methane), trap heat. They let the Sun's energy through to warm the Earth, but they absorb some of the heat that the warm Earth radiates back out, stopping it escaping to space. This keeps the planet warm enough to live on. Burning fossil fuels adds extra carbon dioxide, strengthening the effect and causing global warming and climate change.
Examples in context
Example 1. Why car exhausts are fitted with catalytic converters. A catalytic converter changes harmful gases in exhaust, such as carbon monoxide and nitrogen oxides, into less harmful ones like carbon dioxide and nitrogen. This reduces the toxic and acid-rain pollutants a car releases.
Example 2. The danger of a blocked gas heater. A faulty or poorly ventilated heater burns gas with too little oxygen, producing carbon monoxide. Because it is colourless and odourless and stops the blood carrying oxygen, carbon monoxide is dangerous indoors, which is why homes use carbon monoxide alarms.
Try this
- Cue. State the approximate percentage of oxygen in clean air. About .
- Cue. Name the pollutant that causes acid rain and its source. Sulfur dioxide, from burning fuels containing sulfur.
- Cue. Explain why carbon monoxide is dangerous. It is toxic and reduces the amount of oxygen the blood can carry.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marks(a) State the two main gases in clean dry air and their approximate percentages. (b) Name one pollutant gas produced when fuels are burned and state one harmful effect.Show worked answer →
(a) Clean dry air is about nitrogen and about oxygen.
(b) Any one of: carbon dioxide (adds to the greenhouse effect and global warming); sulfur dioxide (causes acid rain); carbon monoxide (a toxic gas that reduces oxygen carried by the blood).
What markers reward: roughly nitrogen and oxygen, and one correct pollutant with a matching harmful effect.
Original3 marks(a) Name the gas mainly responsible for the increased greenhouse effect from burning fossil fuels. (b) Describe briefly how the greenhouse effect warms the Earth.Show worked answer →
(a) Carbon dioxide is the main greenhouse gas from burning fossil fuels.
(b) Greenhouse gases let the Sun's energy reach the Earth but trap some of the heat that the warm Earth gives off, stopping it escaping to space. This keeps the surface warmer.
What markers reward: carbon dioxide named, and the idea that greenhouse gases trap heat radiated by the Earth, keeping it warmer.
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