Emissions of oxides of nitrogen, commonly referred collectively as NOx, are regulated because of their adverse effects on health and the environment. They play an important role in acid rain, the formation of harmful ozone and photochemical smog in the lower atmosphere and the depletion of the beneficial ozone in the upper atmosphere.
The most environmentally important oxides of nitrogen are:
NO, NO2, and, more recently, N2O.
N2O is a recent concern because it is a "greenhouse" gas which contributes to global warming and because it can aid in destroying the upper atmosphere ozone layer which protects us from ultraviolet radiation. Fortunately, very little N2O is emitted from the flame of a typical burner.
Over 90% of the NOx from a typical flame is in the form of NO and the remainder is NO2. However, since NO is eventually converted to NO2 in the atmosphere, most regulations treat all of the NOx as NO2.
NOx emissions from combustion sources are due to the oxidation of atmospheric N2 and the oxidation of nitrogen chemically bound in fuel molecules. Mechanisms for forming NO include the fuel NOx, prompt NOx and thermal NOx mechanisms. Some of these mechanisms are well understood, while others are still under investigation.
Although NO and NO2 molecules last only a matter of days in the atmosphere, N2O is a very stable species that can last 100 to 200 years in the lower atmosphere. Because of its long life span some N2O eventually reaches the upper atmosphere where it decomposes under ultra violet light and it's reaction products efficiently remove ozone from the upper atmosphere.
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