Tropospheric ozone refers to the ozone that occurs in the lower atmosphere, below 8 miles, or 40,000 feet. At these relatively low altitudes, ozone (O3) is a powerful greenhouse gas, emitted and accumulating over our cities and populated areas. This low level, tropospheric O3,growing rapidly in the northern hemisphere at about one percent a year, should not be confused with the high-altitude 'stratospheric ozone' shield that exists from 10 to 35 miles above the surface of the earth. (In the stratosphere, ozone levels are DECREASING dramatically as a result of reactions with CFC gases.)
Tropospheric ozone is another by-product of fossil fuel combustion. It is produced through a set of complex photochemical reactions involving many hydrocarbons such as methane, butane, and propane. These reactions also require the presence of nitrogen oxides (generic formula NOx). Nitrogen oxides are produced mainly by burning fossil fuels, with automobile use and coal burning creating the most NOx wastes, and therefore the largest quantities of O3. Because tropospheric ozone is highly chemically reactive, it has a short 100 day average lifetime. This means that it does not mix uniformly in the atmosphere, and O3 levels vary widely, however they are known to be higher in the northern hemisphere where there are more cars and coal-fired power plants.
One important way the O3 is removed from the troposphere is by photochemical reactions with the hydroxyl radical (OH) - similar to the way methane is removed. Since man's production of carbon monoxide, CO, reduces the numbers of available OH radicals in the atmosphere, increased CO emissions are responsible for part of the tropospheric O3 build-up. ***
