Unpolluted air Composition
The Earth is surrounded by a mixture of gases called the atmosphere. The main gases in the atmosphere, and their approximate percentages in dry air, are shown below.
Gases in the atmosphere
| gas | percentage in dry air |
|---|---|
| nitrogen | 78 per cent |
| oxygen | 21 per cent |
| argon | 1 per cen |
The atmosphere also contains a small but important amount of carbon dioxide, approximately 0.04 per cent, and tiny amounts of a few other gases.
Activity. Draw a pie chart showing the relative amounts of the three main gases in the air.
If you need help to do that click here
Humidity
In addition to these, air also contains water vapour. The percentage of water vapour varies from one location to another, and according to weather conditions. This is why the percentages of the other gases are usually quoted for dry air.
Humidity is the amount of water vapor present in the air. Water vapor is the gaseous state of water and is invisible to the human eye.[1] Humidity indicates the likelihood of precipitation, dew, or fog.
Earth's atmosphere composition has changed over time
The Earth’s atmosphere has changed over billions of years, but for the past 200 million years it has been much as it is today.
Story of the atmosphere
The composition of the present day atmosphere is very different from that of the early Earth. Evidence can be interpreted in different ways leading to a number of explanations as to how the atmosphere has changed.
The atmosphere of the early Earth was largely made up of carbon dioxide and water vapour, probably coming from volcanoes. When the Earth cooled the water vapour condensed to form the oceans.
Carbon dioxide dissolved in the oceans and began to form sedimentary rocks. A lot of carbon dioxide was trapped underground and eventually formed fossil fuels.
Indirect evidence
The early atmosphere cannot be measured directly so scientists look for indirect evidence. This includes the chemical make-up of rocks, examining air bubbles in ice cores and looking at fossils.
Some fossils suggest early organisms underwent the process of photosynthesis, using the carbon dioxide and releasing oxygen. This meant carbon dioxide levels in the atmosphere went down and oxygen levels went up.
