NOx is the general term used for NO and NO2, or nitrogen oxides.  Three primary sources of NOx:  thermal NOx, fuel NOx, and prompt NOx are produced during combustion, especially at high temperatures. In applications such as boiler and kiln, high peak flame temperatures caused by rapid fuel-air mixing in excess available oxygen promote the formation of NOx emissions.

A significant form of air pollution, called photochemical smog, as well as tropospheric ozone, is formed when NOx and volatile organic compounds (VOCs) react in the presence of sunlight.

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