What is NOx?

NOx is a term used to describe nitrogen oxides. NOx is a reactive gaseous compound that is one of the criteria air pollutants regulated by the USEPA Clean Air Act. The two primary oxides of nitrogen involved in air pollution are NO and NO2. Nitric oxide (NO) is colorless and essentially odorless. Nitrogen dioxide (NO2) is a reddish-brown gas with a pungent smell. NOx (NO & NO2) can irritate the lungs, cause bronchitis and pneumonia, and lower resistance to respiratory infections. NOx is also a precursor to the formation of ground level ozone (smog) and a contributor to acid rain.

NOx emissions are produced during the combustion of fuels. The primary man-made sources of atmospheric NOx include; area mobile sources (cars and trucks), off-road sources (construction and agricultural machines, trains, etc.) and stationary sources (power plants, industrial boilers & heaters, etc,).

In a combustion process, three types of NOx can be formed:

• Thermal NOx: From high temperature combustion in the presence of free oxygen.
• Fuel NOx: From the nitrogen bound in the fuel when it is oxidized during combustion.
• Prompt NOx: From the combustion flame and the ambient nitrogen in the ambient air.

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