Ozone's role in the enhancement of the greenhouse effect has been difficult to determine. Moreover, concentrations of ozone gas are found in two different regions of the Earth's atmosphere. This stratospheric ozone provides an important service to life on the Earth as it absorbs harmful ultraviolet radiation. In recent years, levels of stratospheric ozone have been decreasing due to the buildup of human created chlorofluorocarbons in the atmosphere. Since the late 1970s, scientists have noticed the development of severe holes in the ozone layer over Antarctica.
Ozone is also highly concentrated at the Earth's surface in and around cities.
Most of this ozone is created as a by product of human created photochemical smog . This buildup of ozone is toxic to organisms living at the Earth's surface. Average composition of the atmosphere up to an altitude of 25 km.