Ozone, a triatomic form of oxygen (O ₃ ), is a gaseous component of the atmosphere. In the troposphere, it is formed by photochemical reactions of gases that arise both from natural sources and from human activity (photochemical smog). In high concentrations, tropospheric ozone can be harmful to a wide range of living organisms. Tropospheric ozone acts as a greenhouse gas. In the stratosphere, ozone is formed by the interaction between solar ultraviolet radiation and molecular oxygen (O ₂ ). Stratospheric ozone plays a critical role in the stratospheric radiation balance. Depletion of stratospheric ozone due to chemical reactions that may be enhanced by climate change leads to an increased ground-level flux of ultraviolet (UV-) B radiation.