nitric oxide (NO), with minor amounts of nitrogen dioxide (NO₂). NOx is formed whenever fuel is burned at high temperatures in air, from nitrogen in the air as well as in the fuel. Motor vehicles and stationary combustion sources (furnaces and boilers) are the primary man-made sources, although automotive emission controls are reducing the automobile’s contribution. Natural emissions of NOx arise from bacterial action in the soil. NOx can react with hydrocarbons to produce smog.

Also see catalytic converter, emissions (automotive), emissions (stationary source), pollutants, hydrocarbon emissions, nitration, volatile organic compound.