Oxides of Nitrogen - as Pollutants

A number of oxides of nitrogen such as NO, N2O, N2O3 are introduced into the atmosphere due to natural sources and due to human activities. Out of these only two oxides , I.e NO and NO2 are considered as Pollutants. Nitric oxides is colorless and odourless gas whereas nitrogen dioxide is reddish Brown gas having pungent smell and is suffocating in nature.

Sources of oxides of nitrogen production :-
(a) Natural sources :- Natural bacterial action is the only natural source which discharges oxides of nitrogen , mainly in the form of NO, into the atmosphere in huge amounts. Besides , lightning discharge also results in the combination of N2 and O2 to form NO.

(b) Man-Made Sources :-

* Combustion of fossil fuel both in motor vehicle and stationary sources or powe plants produces so high temperature at which oxidation of the atmospheric nitrogen takes place forming NO and NO2.

The first reaction takes place at a temperature of 1200-1750*C. The reverse reaction is prevented due to rapid cooling of NO. The second reaction takes places about 1100*C.

* Chemical industries like sulphuric and nitric acid industries produce oxides of nitrogen as by - products which are discharge into the air. 

Sinks of Oxides of nitrogen pollutant :- The average residences time of NO is 4 days while that of NO2 is 3 days. During this time, they are converted into nitric acid through the reactions in which ozone plays an important role.

Harmful effects of Oxides of nitrogen pollution :-
* Neither NO nor NO2 is a health hazards at the present pollution levels. Out of the two , NO2 is more dangerous as it is a toxic gas and affects a respiratory system and damages the lungs. It's concentration at 100ppm may be fatal for animals and human beings. Though NO has about 1500 times greater affinity than CO for hemoglobin but fortunately, it is unable to enter the blood stream from the atmosphere. But it's presence in large amount may result in the increase of NO2 by oxidation and thus become harmful.
* Some sensitive Orr ornamental plants are affected by the presence of Oxides of nitrogen in the ambient air even at low concentration. Their photosynthetic activities decreases  and they start shedding their leaves and fruits.
* In the presence of Oxides of nitrogen , textiles like cotton , rayon etc. start fading.
* The acid rain causes the pH of the soil to decreased to 4 or 5 and thus reduced the fertility of the soil.
* The most serious hazards of nitrogen oxides is in the formation of an unpleasant mixture of gases and particulates that make up photochemical smog.

Control of oxides of nitrogen :-
* Just as in case of CO , the catalytic converters are used in the automobiles exhaust which is in the first stage convert the oxides of nitrogen to free N2 or to a small amount of NH3.

* The flue gases coming from the power plants or industrial units and containing NO2 and SO2 are free from these gases by scrubbing them with sulphuric acid.