C1C

78% - Nitrogen 

21% - Oxygen

0.035% - CO  , noble gases e.g. argon

- Combustion of fossil fuels which increases the level of carbon dioxide and decreases the level of oxygen.

- Respiration by humans and animals which increases the level of carbon dioxide and decreases the level of oxygen.

- Photosynthesis by plants increases the level of oxygen and decreases the level of carbon dioxide.

Phase 1 - Volcanoes gave out steam and CO  

Phase 2 - Green plants evolved and produced oxygen

Phase 3 - Ozone layer allows evolution of complex animals (Ozone layer - O  )

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

carbon monoxide + nitrogen oxide → nitrogen + carbon dioxide

2CO + 2NO → N₂ + 2CO₂

Deforistation - more rainforests are cut down so less photosynthesis.

Increased population - the worlds energy requirements increase

Water vapour went down because:

• as the Earth cooled down, most of the water vapour condensed and formed the oceans.

Carbon dioxide went down because:

• it was absorbed by plants during photosynthesis
• it was locked up in fossil fuels
• it dissolved in the oceans.

Nitrogen went up because:

• it is not very reactive and, once formed, it is not easily removed from the atmosphere again.

Oxygen went up because:

• it was released by plants during photosynthesis