What is NOx? Environmental Impacts NOx Sources Reduction Efforts

NOx is actually a generic term for a group of gases called nitrogen oxides. Nitrogen oxides contain one molecule of nitrogen and a varying number of oxygen molecules. Some of these gases are colorless and odorless, but one of them, nitrogen dioxide (NO₂) combined with particulate, can appear as a reddish-brown layer of smog over urban areas.

How NOx Forms
Air is mostly nitrogen molecules (green in the diagram) and oxygen molecules (purple in the diagram). When heated to around 3000 degrees Fahrenheit, the molecules break apart and oxygen atoms link with the nitrogen atoms to form NOx, an air pollutant.

Although NOx can occur naturally in the environment, most NOx is formed from manmade sources when fuel is burned at high temperatures. More that half of all NOx emissions come from motor vehicles, but other contributing sources include industrial processes, power generation (utilities), and residential fuel combustion. (See the following diagram for the most recent annual information available on NOx emissions - 2003).