NITROGEN OXIDES (NOx)
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.