The Origin of the Atmosphere

The early atmosphere of the earth contained mainly carbon dioxide.
It was probably formed by volcanic activity.  (Volcanoes spewed carbon
dioxide, water vapor, nitrogen.)  Earth's core cooled a bit, volcanic
activity slowed down, water vapor condensed  clouds, rivers, lakes
form.  Primitive plants produce oxygen during photosynthesis. Less
carbon dixide, more oxygen meant the earth could cool down.
see a graph of the current and early atmosphere of the earth here

The Current Atmosphere (Today)

Today's atmosphere is made up of mainly nitrogen (78%) and
oxygen (21%)  There are 3 other gases which are very important
even though their concentration is low:
carbon dioxide - needed by plants, needed to keep air temperature
    comfortable by absorbing outgoing solar radiation
Ozone - protects organisms from harmful ultraviolet radiation
Water vapor - plants and rely on water to sustain life

Problems in today's atmosphere

Smog - pollution formed from the burining of fossil fuels (gasoline, coal)
             The pollution may combine with moisture to form a haze or the
             pollution may react with sunlight to form a haze.  Smog causes
             health problems - headaches, respiratory problems.

Greenhouse effect - When heat is trapped in the earth's atmosphere by
             carbon dioxide.  Without the greenhouse effect the earth would
             freeze, but today there is more carbon dioxide in the atmosphere
             The atmosphere absorbs more heat and people are concerned
             concerned about global warming.  This could change life on earth.

Ozone depletion - ozone in the stratosphere protects organisms from
             harmful ultraviolet rays of the sun.  Some ultraviolet radiation is
             necessary, but too much can cause skin cancer, cataracts, ...
             An ozone hole was discovered.  It is thought to be caused by the
             release of chloroflorocarbons (aerosol cans, air conditioners,
             styrofoam, ...)

People Can Help

The Clean Air Act - sets standards for cars, factories, ... about the amounts
             of pollutants that can be released into the air.

Ozone - 112 nations have united and have agreed to stop using chloroflorocarbons.

You can help - Reduce, Reuse, Recycle; conserve energy; use ozone safe
             products.