(Symbol O. ) An element, atomic number 8, atomic weight 16. 0; molecular oxygen, formula O₂, molecular weight 32, is the second most abundant species in the atmosphere, with an abundance of approximately 21% at sea level. The atmospheric abundance of O₂ remains fairly constant up to about 80 km, above which substantial photodissociation to atomic oxygen occurs. Oxygen is a prerequisite to almost all forms of terrestrial life. Oxygen was probably released from minerals such as carbonates resulting in the evolution from a reducing to an oxidizing atmosphere. The general tendency is for reduced emissions from the earth's surface to be oxidized to simpler, oxygen-containing species. Atomic oxygen is formed in the photolysis of molecular oxygen, O₂; ozone, O₃; or nitrogen dioxide, NO₂, in the atmosphere. Below about 40 km, its predominant fate is recombination with molecular oxygen to form ozone. Above that altitude it can participate in other chemical reactions, which may lead to ozone destruction. Both molecular and atomic oxygen have low-lying electronically excited states that are important in the atmosphere. The ¹Δ and ¹Σ states of O₂ are relatively long- lived, and fluorescence from these states contributes to the airglow. The O¹D state of atomic oxygen, formed in ozone photolysis, reacts to form the hydroxyl radical, which is the primary oxidant in the atmosphere.