In this post, I am going to discuss two of the nitrogen oxides that are important in the study of air pollution, namely, nitrogen monoxide (NO) and nitrogen dioxide (NO2). The most abundant oxide is nitrous oxide. This is however chemically rather unreactive and is formed from the natural biological processes in the soil. Nitrogen monoxide first undergoes photochemical reaction. The formed atomic oxygen reacts with another molecule of N2O to give NO. The formed nitric oxide reacts with ozone, thereby causing ozone depletion.
The following equations show how nitrogen and oxygen in the air combine to form nitrogen monoxide:
N2O+O→2NO
The nitrogen monoxide then reacts with more oxygen to become nitrogen dioxide:
NO+O2 → 2NO2
Nitrogen monoxide is formed by the combustion of nitrogen-containing compounds (including fossil fuels). Thus all high temperature processes produce NO, which is then oxidised to NO2 in the ambient air. In the natural world, these reactions occurs in lightning and forest fires. What is nitric oxide used for in the natural world then? Actually, it is an important source of nitrogen for growing plants.
In contrast to nitric oxide, nitrogen dioxide is very reactive and significant species in the atmosphere. The principal reactions among NO,NO2, and HNO3 are indicated below:
In conclusion, nitric oxide and nitrogen dioxide are important constituents of polluted air. These oxides collectively designated as NOx, enter the atmosphere mainly from combustion of fossil fuels in both stationary and mobile sources.
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