The atmosphere consists mainly of nitrogen and oxygen, with smaller proportions of other gases such as carbon dioxide. The amount of carbon dioxide in the atmosphere is maintained through a balance between processes such as photosynthesis, respiration and combustion. But human activities are polluting the atmosphere.

Photosynthesis by plants is thought to be a key process in the evolution of the Earth’s atmosphere.

Composition of the Earth's atmosphere

You need to know the proportions of the main gases in the atmosphere.

The Earth's atmosphere has remained much the same for the past 200 million years. The pie chart shows the proportions of the main gases in the atmosphere.

It is clear that the main gas is nitrogen. Oxygen - the gas that allows animals and plants to respire, and fuels to burn - is the next most abundant gas. These two gases are both elements and account for about 99% of the gases in the atmosphere. The remaining gases, such as carbon dioxide, water vapour and noble gases such as argon, are found in much smaller proportions.

The level of carbon dioxide in the atmosphere is maintained by several processes, including photosynthesis, respiration and combustion.

Green plants remove carbon dioxide from the atmosphere by photosynthesis. Living organisms - including all plants and animals - release energy from their food using respiration. Respiration and combustion - burning - both release carbon dioxide into the atmosphere.

These processes form a carbon cycle in which the proportion of carbon dioxide in the atmosphere remains about the same. The animation should help you to understand how the cycle works. Note that you do not need to know about decomposition and fossilisation

Carbon dioxide is produced by burning fossil fuels. Increased energy consumption is leading to a rise in the use of fossil fuels, which in turn increases the release of carbon dioxide into the atmosphere.

The rising human population is adding to atmospheric carbon dioxide in other ways, too. When land is cleared for timber and farms, there are fewer trees to remove carbon dioxide from the atmosphere by photosynthesis. If the fallen trees are burned or left to rot, additional carbon dioxide is released into the atmosphere. This is particularly important when forests are cleared to make way for farms. Not only are there then fewer trees to absorb carbon dioxide, but the burning of the trees releases carbon dioxide.

The use of fossil fuels - usually in vehicle engines and power stations - is causing air pollution.

Some of the common air pollutants and how they are formed

pollutanthow it is formed carbon monoxide incomplete combustion of the fuel in car engines oxides of nitrogen, NOx formed from the heat and pressures found in a car engine sulfur dioxide sulfur impurities in the fuel burn

Carbon monoxide is a poisonous gas. Oxides of nitrogen (NOx) react with other pollutants in sunlight to form a photochemicalsmog, which causes breathing difficulties. NOx and sulphur dioxide also formacid rain. This has several effects on the environment, including:

• killing plants and aquatic life
• eroding stonework
• corroding metals

Catalytic converters

Car exhaust systems have catalytic converters. These convert carbon monoxide into carbon dioxide:

carbon monoxide + nitrogen oxide → nitrogen + carbon dioxide

2CO + 2NO → N₂+ 2CO₂

Scientists believe the Earth was formed about 4.5 billion years ago, and that its early atmosphere was probably created from the gases escaping from the Earth’s interior. This early atmosphere probably consisted of mostly carbon dioxide and water vapour, with a smaller proportion of ammonia, NH₃.

Photosynthesis by green plants increased the proportion of oxygen in the atmosphere, until it reached the level seen today - 21 per cent.

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Changes in the atmosphere - higher

How did the proportions of water vapour, carbon dioxide and ammonia in the atmosphere go down, and the proportion of nitrogen go up?

• Water vapour - the proportion went down because, as the Earth cooled down, most of the water vapour condensed and formed the oceans.
• Carbon dioxide - the proportion went down because:
  • it was absorbed by plants for photosynthesis
  • it was locked up in fossil fuels
  • it dissolved in the oceans
• Ammonia - the proportion went down because it reacted with oxygen to produce nitrogen.
• Nitrogen - the proportion went up because it was released by the reaction between ammonia and oxygen. Nitrogen is not very reactive and, once formed, it is not easily removed from the atmosphere again.

Now try a Test Bite - higher