Parameter Tree:
EARTH SCIENCE > ATMOSPHERE > ATMOSPHERIC CHEMISTRY > OXYGEN COMPOUNDS > MOLECULAR OXYGEN

Definition:
Oxygen (O2) is found on Earth as a gas and constitutes about 20.8% of the
air we breathe. Elemental molecular oxygen consists of two oxygen atoms
bonded together. A photochemical reaction of oxygen is (ultimately)
responsible for the production of ozone in the stratosphere. Oxygen
concentrations found in ice core samples have been used to determine past
atmospheric levels of oxygen and helped in determining past climates.
[Nature; v351; 217-219; 1991.] [Nature; v365; 143-147; 1993.]


Reference:
Atmospheric and Air Chemistry Glossary from the Sam Houston State University.
http://www.shsu.edu/~chemistry/Glossary/glos.html