Environmental Science 102
Composition of the Atmosphere

What is the Atmosphere?

Atmos- is Greek for:
-sphere is Latin for:
Definition does not apply strictly to 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% CH4 (methane) 0.00017%
Ar (argon) 0.934% Kr (krypton) 0.0001%
CO2 (carbon dioxide) 0.041% N2O (nitrous oxide) 0.00005%
    H2 (hydrogen) 0.00005%
    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


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)
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

Where did all the CO2 go?

Changes in the Composition of the Atmosphere

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


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Carbon Dioxide

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Produced naturally in the atmosphere as O2+ O+combine to form O3
Ozone depletion also occurs due to:

CFC (chlorofluorocarbon) - CF2Cl2 - is 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


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