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Nitrogen Oxides: NO2


Nitrogen oxides result both from natural processes and anthropogenic sources: 96% of anthropogenic sources are combustion processes. NO is produced as a by-product of combustion processes with an abundance of oxigen: high temperatures and excess opxigen will lead the nitrogen available in the air feeding the combustion process to oxidise at temperature above 1600 K:

N2 + O2  -> 2 NO

The primary result of the oxidation is NO (about 90% in an industrial furnace). This NO is continuously oxidised with either oxygen O2 or ozone O3 to NO2 under normal atmospheric conditions.

Natural sources of NO2 are bactieral activity in the soil, releaseing nitrous oxide into the atmosphere. In the upper troposphere and stratosphere, atomix oxygen reacts with nitrous oxide to nitrc oxide (NO):

N2O + O -> 2NO

The dissociation of ozone supplies the atomic oxygen. The nitric oxyde reacts again with ozone to form nitrogen dioxide:

NO + O3 -> NO2 + O2

Ultimately, NO2 is converted to negatively charged NO2 (nitrit) or NO3 (nitrate) in particulate form, which is washed out by precipitation (dissolution in water forming nitric acid HNO3) and contributes to acid rain.


The gas is redish brown at high concentrations and gives a yellowish tint at lower concentrations.

Exposure to NO2 concentrations above 5 ppm for minutes or more results in irritaion of the respiratory tract and coughing. Contiuned exposure can lead to an accumulation of fluids in the lungs (pulmonary endema). AT 5 ppm it has a pungent, sweetish odor. The average NO2 concentration in tobacco smoke is about 5 ppm. Slight increases in respiratory diseases and decrease in pulmonary function have been observed at concentration starting from 0.10 ppm.

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