Scientists currently believe that the earth's initial atmosphere contained only hydrogen and helium during its formation 4.5 billion years ago.  How, then, did the atmosphere become what it is today?

Volcanic eruptions are believed to have started the process that eventually resulted in today's atmosphere.  Shield volcanoes emit gasses into the atmosphere when they erupt.  This phenomenon is known as outgassing.  After  numerous such eruptions in Prehistoric times, the atmosphere's composition became more diversified.  The following shows the gaseous emissions from a shield volcano and their percent by volume.  Note that these only apply to the emissions themselves, not to the atmosphere as a whole.

These figures are quite different from the Composition today.  Hit the "Back" button on your page browser or click here to review the Current Composition of the atmosphere.  Notice the abundance of water vapor and carbon dioxide being emitted.  But we still do not see an oxygen source.  Scientists believe the photosynthesis and photodissociation processes helped to achieve today's oxygen levels.

The Rise of Oxygen. With increasing amounts of water vapor being emitted from eruptions, the vapor likely condensed and formed clouds.  These in turn precipitated and formed bodies of water.

With water vapor in the atmosphere and liquid water on the earth's surface, the formation of oxygen became possible.   Oxygen likely formed in two ways:  The photodissociation of water and the photosynthesis reaction.  The photodissociation of water requires ultraviolet radiation from the sun.  Here is the chemical equation for the photodissociation of water:

                    2H₂O -> 2H₂ + O₂  (Wallace and Hobbs 9).

_{Note how one molecule of oxygen is formed for every two molecules of water that reacts.}

The photosynthesis reaction  H₂O + CO₂ -> {CH₂O} + O₂  (Wallace and Hobbs 9) was carried out through biological means.  It is thought that the first single-celled organisms (appearing about 600 million years ago) lived in an anaerobic environment 5-10 meters underwater.  An anaerobic environment lacks free oxygen.   This optimal depth protected the new life from the sun's ultraviolet radiation, yet enough visible light filtered down for photosynthesis.  With an oxygen source established, oxygen began to diffuse into the atmosphere.   This lead to the formation of ozone (O₃) in the higher levels of the atmosphere.  Because ozone absorbs ultraviolet radiation, it protected the evolving organisms and allowed them to photosynthesize closer to the water surface.  Eventually, there was enough protection from the UV radiation to allow life on land to survive.  Life finally emerged onto land about 400 million years ago (Wallace and Hobbs 8).

Now that oxygen has been accounted for, how have the other constituents' levels come to be?

    N₂: The nitrogen level gradually increased over time and is now in balance with biological processes.
    A:  Argon is an inert gas.  It is released from the radioactive decay of crustal material.
    CO₂: One might expect higher carbon dioxide levels based on the volcanic emissions.  However, much of it has been locked up in the lithosphere, making its volumetric contribution small.   However, carbon dioxide is being closely monitored because of its significance as a Greenhouse gas.

And what happened to the sulfur dioxide and water vapor that was outgassed?

    SO₂: Sulfur dioxide adheres to surfaces.  Therefore, it was quickly removed from the atmosphere as it adhered to aerosols and other surfaces.
    H₂O_v: The water collected into oceans, ice, rivers, lakes, ice, underground water, and the atmosphere.  Much of the water is thought to have leaked through the seams in the earth's crust (Wallace and Hobbs 6).