Ozone in the Atmosphere

The importance of ozone to life on Earth and to atmospheric chemistry cannot be overstated. Nucleic acids and other macromolecules essential to life absorb strongly in the ultraviolet (UV) and are damaged by UV radiation with wavelengths of less than approximately 300 nm. For proper functioning, such biological macromolecules need to be shielded from the full intensity of solar radiation. Molecular oxygen (O2) absorbs strongly and blocks solar radiation with wavelengths below 230–240 nm from reaching the Earth’s surface. However, oxygen is transparent at wavelengths above approximately 245 nm. Fortunately, absorption of UV radiation of wavelengths of less than 242 nm by molecular oxygen (O2) yields oxygen atoms that add to O2 to form ozone which has a very strong absorption band at 200–300 nm. Even though it is present in only trace amounts in the atmosphere, absorption by ozone effectively blocks harsh solar UV radiation from reaching the Earth’s surface. There is no other molecule in the atmosphere that provides protection from solar UV radiation in the 250–300 nm region. The development of the ozone layer is intimately connected to the development of life on Earth. Oxygen levels in the prebiotic atmosphere were less than 5 ×10−9 of the current level. Photosynthesis after the appearance of life on the planet more than 3.5 billion years ago led to increased oxygen levels in the atmosphere. By approximately 600 million years ago, the O2 concentration had exceeded 10% of the current level, and the corresponding layer of ozone was sufficient to offer an effective UV shield for the migration of life onto land (Wayne, 1991). Life on Earth as we know it would not have developed without the protection offered by the ozone layer, and, equally, the ozone layer would not have developed without life on Earth. In addition to its obviously important physical role in shielding biota from the damaging effects of harsh UV radiation, ozone plays an essential chemical role as a photolytic source for HO radicals.