A class of polluting emissions produced by the consumption of fossil fuels, especially by automobiles and coal-fired generating stations. Nitrogen oxides are converted to acids, usually highly corrosive nitric acid, when exposed to water molecules in the atmosphere, and this eventually falls to earth as acid rain. Nitric acid isn't as corrosive as sulfuric acid, but it is emitted in larger quantities and is much more difficult to control. These emissions are especially high in vehicles with poorly-tuned engines or faulty emission control systems.

Most people know that the earth's atmosphere is comprised mainly of nitrogen and oxygen, with about nine times more nitrogen than oxygen. But a clear distinction needs to be made between atmospheric nitrogen and oxygen, and the nitrogen oxides emitted from combustion. Atmospheric nitrogen and oxygen occur as non-reactive N2 and O2 molecules, and these two molecules rarely combine to form acids. In combustion, nitrogen and oxygen combine relatively easily to form a single molecule, and this acidic molecule is particularly reactive with water vapor.

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