The composition of the earth's atmosphere is gradually changing over time
due to the natural processes that occur on earth, as well as the many anthropogenic processes that have
been introduced by humans. Natural biological processes such as the
respiration of animals, including humans, removes oxygen from the
atmosphere and in turn produces carbon dioxide (CO2). The
photosynthesis of plants uses the carbon dioxide to produce oxygen which
is released back into the atmosphere. Other natural processes include
geologic events such as volcanic eruptions. Not only do these violent
eruptions eject massive amounts of particle debris into the atmosphere,
but they also spew out large amounts of carbon dioxide and water vapor.
While much of the atmosphere's composition is maintained by natural
processes occurring on and within the earth, how human activities interact
with the atmosphere has profound implications upon the quality and
continuance of life on earth. Although the percent volume of man-made
substances is only a minute fraction of the total volume of the
atmosphere, this amount is significant because it is this portion of the
atmosphere that comprises most of what we term as pollutants.
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