A gaseous atmospheric constituent that is the triatomic form of oxygen (O3, meaning that each molecule consists of three atoms of oxygen in contrast with the usual two). In the troposphere - at ground level - it is created both naturally and by photochemical reactions involving gases resulting from human activities. In high concentrations, tropospheric ozone can be harmful to a wide range of living organisms. In the stratosphere, ozone is created by the interaction between solar ultraviolet (UV) radiation and molecular oxygen (O2). Stratospheric ozone plays a key role in protecting life on earth from ultraviolet radiation from the sun.