Atmospheric ozone may have evolved several hundred million years earlier than was previously believed according to Joel S. Levine, NASA/Langley Research Center, Paul B. Hays, University of Michigan, and James C. G. Walker, National Astronomy and Ionosphere Center.
Their new ozone evolution chronology, presented in a paper given at the spring AGU meeting, is based on a photochemical model developed to investigate the evolution of ozone as photosynthetic activity increased molecular oxygen from a trace gas to its present level. The ozone evolution model, which considers the effects of varying oxygen levels as well as the effects of nitrogen and hydrogen chemistry and vertical eddy transport, simplifies the relationship between oxygen level and ozone content. Oxygen regulates ozone formation via direct chemistry while also limiting the depth of penetration of solar ultraviolet radiation into the atmosphere. Lower levels of oxygen permit the important deeper penetration of solar ultraviolet radiation into the atmosphere. Thus the formation of stratospheric ozone is initiated by the formation of oxygen atoms via the photodissociation of the oxygen molecule by solar ultraviolet radiation. The newly formed oxygen atom then combines with an oxygen molecule in the presence of any third molecule to form ozone.