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A gaseous atmospheric constituent in the troposphere that is created by photochemical reactions with gases that result from both natural sources and human activities (photochemical smog). In high concentrations, tropospheric ozone can be harmful to a wide range of living organisms. Tropospheric ozone acts as a greenhouse gas and is not the same as the ozone layer, which is part of the stratosphere.

Ozone Layer

A layer of the atmosphere that begins approximately 15 km above the Earth and thins to an almost negligible amount at about 50 km. The ozone layer shields the Earth from the sun’s harmful ultraviolet radiation. 

Ozone Precursors

Chemical compounds such as carbon monoxide, methane, non-methane hydrocarbons and nitrogen oxides, that, in the presence of solar radiation, react with other chemical compounds to form ozone, mainly in the troposphere