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Nitrogen Oxides: What is NOx?

Nitrogen Oxides (mainly NO and NO2), or NOx, is the generic term for a group of highly reactive gases, which contain nitrogen and oxygen in various amounts and chemical configurations. Most of the nitrogen oxides are colorless and odorless. However, one very common pollutant, nitrogen dioxide (NO2) along with other particles in the air can often be seen as a reddish-brown layer of smog over many cities or heavily populated areas.

How is NOx Created?

Nitrogen oxides form when fuels are burned at high temperatures, as in a combustion process. The primary sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources like home boilers that burn fuels.

Causes for Concern about NOx

When NOx reacts with the oxygen in the air, the result is ground-level ozone. Ground-level ozone has very negative effects on the respiratory system, such as causing lung cancer, and on agricultural production. NOx also reacts to form nitrate particles, and acid aerosols, which all cause respiratory problems. Nitric acid, formed when NOx reacts with water, can cause acid rain and the deterioration of the quality of water. Acidic gases along with airborne particles cause visibility impairment and lower air quality.

Types of NOx

There are generally three primary sources of NOx: thermal NOx, fuel NOx, and prompt NOx. Although all of these are formed through combustion processes, they all differ slightly.


Thermal NOx
Thermal NOx is formed at very high temperatures, usually above 2200°F, and is a result of the oxidation of the diatomic nitrogen found in combustion air. Thermal NOx is the most produced form of NOx created during combustion. It is a function of the temperature and the residence time of the nitrogen at that temperature; the higher the temperature of the flame the higher the formation of thermal NOx.
Fuel NOx
Fuel NOx is formed when the nitrogen in fuels combines with the excess oxygen in the air. Fuel NOx is a major problem in the burning of oil and coal as it can make up as much as 50% of total emissions when combusting oil and as much as 80% of total emissions when combusting coal.
Prompt NOx
Prompt NOx is formed in the earliest stage of combustion. Prompt NOx is made by the reaction of atmospheric nitrogen with radicals in the air. The levels of prompt NOx are generally very low, so it is usually only of interest for the most exacting emission targets.