Why is atomic oxygen more abundant than nitrogen at an altitude of 200km ?
The overwhelming proportion of oxygen and nitrogen in the atmosphere is as diatomic molecules, molecules with two atoms. Ionizing radiation can dissociate these molecules and produce atomic oxygen and atomic nitrogen, both of which are highly reactive free radicals. The double bond in an oxygen molecule takes much less energy to dissociate than the triple bond in a nitrogen molecule. The ultraviolet light present in the solar spectrum high in the atmosphere is energetic enough to dissociate oxygen molecules, but there are very little of the wavelengths necessary to dissociate nitrogen molecules.
