Ozone plays an important role in regulating the Earth's climate. About 90% of atmospheric ozone is distributed in the stratosphere, at 10–30 km above the surface of the Earth, with the remainder located in the troposphere. Stratospheric O3 is crucial for life on the Earth because it absorbs incoming solar ultraviolet radiation. Tropospheric O3 is an important GHG because it absorbs infrared radiation from the surface and heats the atmosphere. Because it is a key component of photochemical smog, tropospheric O3 is also an important air pollutant. There are two main sources of O3 in the troposphere; the first is from the reactions of its precursors (CH4, CO, VOCs, NOx, etc.), and the second is the downward transport of O3 from the stratosphere.