From the U.S. Geological Survey (USGS), this site describes the different volcanic gases that are released during eruptions and how they affect the atmosphere.
Lecture from the University of Michigan on how Earth's atmosphere evolved into its current form.
From NASA and NOAA, information on the formation of Earth's atmosphere.
PDF document from the American Museum of Natural History on Earth's atmosphere.
Discussion of early Earth's atmosphere from the National Earth Science Teachers Association.
Google - Research database allows searching of the scholarly literature.
An interactive tool from the Smithsonian Institution that allows you to change the amounts of oxygen, carbon dioxide, and ozone in a test atmosphere.