Nitrogen oxide is an essential ingredient of photochemical smog that is produced during the high temperatures associated with combustion of vehicle’s engines.
 

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Initial reaction of nitrogen dioxide with sunlight

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The oxygen atom generated from the initial reaction reacts with atmospheric, diatomic oxygen, to form ozone. This is not the good, protective ozone of the stratosphere, this is the polluting ozone of the lithosphere, which traps heat and contributes to thermal inversion.

 

This simplified equation represents the key ingredients and products of photochemical smog. Hydrocarbons (including VOC’s), carbon monoxide, and nitrogen oxides from vehicle exhausts are irradiated by sunlight in the presence of oxygen gas. The resulting reactions produce a potentially dangerous mixture that include other nitrogen oxides, ozone, and irritating organic compounds, as well as carbon dioxide and water vapor.