Composition of the Atmosphere
PHYSICAL GEOGRAPHY 101

What is the Atmosphere?

Definition does not apply strictly to the Earth
Jupiter, Mars, and Venus for example all have atmospheres

Atmos- is Greek for:
-sphere is Latin for:
Air - the mixture of gases that surround the Earth

Composition of the Earth's Atmosphere

Major Components			Minor Components
N2 (nitrogen)	78.084%		Ne (neon)	0.0018%
O2 (oxygen)	20.946%		He (helium)	0.0005%
H2O (water)	0-5%		Kr (krypton)	0.0003%
Ar (argon)	0.934%		CH4 (methane)	0.00017%
CO2 (carbon dioxide)	0.034%		H2 (hydrogen)	0.00006%
			N2O (nitrous oxide)	0.00003%
			Xe (xenon)	0.000009%
			O3 (ozone)	0.000004%
Total Mass	5.136x1026 g		particles (dust and soot)	0.000001%
Total Mass	5.7x1016 tons		CFC (chlorofluorocarbons)	0.00000001%

Evolution and Origin of the Earth's Atmosphere

Origin

Linked with the origin of the Earth
The present atmosphere is not the result of acculation of solar nebula gases
The process of differentiation formed the atmosphere
More dense material sank toward the Earth's center, while less dense material rose towards the surface.
This less dense material includes gases. This is known as outgassing
The original Earth atmosphere is believed to have been composed of:

H₂O (water)
H₂(hydrogen)
CO₂ (carbon dioxide)
N₂ (nitrogen)
and other trace gases.

Outgassing of these materials may have largely occurred at volcanoes (this process is still observed today)

Where did the Oxygen come from?

Photosynthesis is the process in plants in which the plant takes in CO₂from the atmosphere and releases O₂ as a waste product.
Photosynthesis evolved in blue-green algae approx. 3.5 billion years ago - began releasing small amounts of O₂ into the oceans and atmosphere
At approx. 1.5 billion years ago, organisms evolve that utilize oxygen - suggesting that there was approx. 1% atmospheric oxygen present.
By 550 million years ago approx. 3% atmospheric oxygen present.
By 450 million years ago approx. 10% atmospheric oxygen present.
During the Pennsylvanian Period, 320-286 million years ago, oxygen levels may have exceeded present levels

Changes in the Composition of the Atmosphere

What are some of the factors which change the amounts of the various components?

Oxygen

Produced by:
Removed by:

Carbon Dioxide

Produced by:
Removed by:

Water

Produced by:
Removed by:

Methane

Produced by:
Removed by:

Ozone

Produced naturally in the atmosphere as O₂+ O⁺combine to form O₃
Ozone depletion also occurs naturally
Nitrous oxide, N₂O, is produced in the soil by bacteria
Nitrous oxide is broken down into Nitric oxide, NO, by solar radiation
Nitric oxide + ozone, O₃, results in oxygen, O₂, and nitrogen dioxide, NO₂
Ultraviolet radiation will also breakdown ozone.
Oxygen, O₂, can combine with ozone, O₃, to form two oxygen, 2O₂

Ozone depletion is also caused by chemical reaction from industrial waste.
CFC (chlorofluorocarbon) - CF₂Cl₂ - is now believed to be a major cause of ozone depletion.
This molecule is broken down by ultraviolet radiation - resulting in the freeing of chlorine, Cl.

The following reaction show how chlorine can destroy ozone.
Cl^- + O₃ combine to form ClO + O₂
ClO + O⁺ combine to form Cl^- + O₂
The chlorine is then free to react with another ozone molecule
Eventually the chlorine is removed from this cycle by combining with other elements, but not before it can destroy an average of 100,000 ozone molecules.

Particles

Produced by:
Removed by:

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