Pollution of the Air We Breathe

A very tiny amount of atmospheric ozone comes from natural sources. Most of it is a product of human activity. In the troposphere, ozone formation is mostly a consequence of pollution from motor vehicle exhaust. Atmospheric nitrogen (N ) at ordinary atmospheric temperatures is essentially inert. When gasoline burns in an internal combustion engine, however, oxides of nitrogen are produced. The products are called NO, where x usually 

Both gases are toxic. Other gases in the atmosphere will also oxidize NO to NO, so the buildup of NOj is fairly rapid, although remember that all of these gases are still present in the atmosphere at concentrations of only parts per million. 

When visible sunlight strikes NO molecules, the energy in the sunlight dissociates the molecules into NO molecules and O atoms, as shown  

Chemists call this type of reaction a photochemical reaction because light causes the reaction to occur. Oxygen atoms contain an unpaired electron (sometimes shown by a dot in the symbol, or 0-), making them very, very reactive. Chemists call species that contain one or more unpaired electrons, free radicals. Although the concentration of free radical oxygen atoms is only on the order of parts of million, the concentration of molecules is 

000 ppm (21 percent of the atmosphere). It is true that there are four times as many N molecules as molecules, but remember that N is largely nonreactive at the temperature 

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