What is nitrogen oxides?

 Oxides of nitrogen are a mixture of gases that are composed of nitrogen and oxygen. At high-temperature oxygen and nitrogen undergo an endothermic reaction producing various oxides of nitrogen but oxygen and nitrogen cannot react at low temperature. The general term for nitrogen oxide is denoted NOx. The term NOx is common means that molecule containing one nitrogen and one or more oxygen atoms, but actually, it is a mixture of NO (nitric oxide) and NO2. Different types of oxides of nitrogen are present in this environment they have a valence state from 1 to 5 for example :

• Nitrous oxide (N2O)
• Nitric oxide (NO)
• Dinitrogen dioxide(N2O2)
• Dinitrogen trioxide (N2O3)
• Nitrogen dioxide (NO2)
• Dinitrogen tetroxide(N2O4)
• Dinitrogen pentoxide(N2O5)

All NOx gases are harmful to human health and the environment. The environmental pollutants under the oxides of nitrogen include nitrous oxide(N2O), nitric oxide (NO) and nitrogen dioxide(NO2).nitric oxide(NO) and nitrogen dioxide (NO2) form smog, acid rain and ground-level ozone and this cause air pollution. These oxides formed by different ways such as combustion of fossil fuels (i.e. coal, diesel, petrol etc.), acid manufacture, explosive industry and acid pickling plant and major source are vehicular emission and powerplants. But It’s not formed by artificial ways only, because some natural process such lightning and Biogenic process such as the microbial process in the soil also causes the production of oxides of nitrogen. This particular portion is a part of unit 7, p-block elements and carries a total weightage of 7 periods and 6 to 7 marks.

Formation of NOx

NOx produced by lightning per year is 8.6 million tonnes, but NOx emission resulting in fossil fuel and combustion are 28.5millon. Agricultural fertilization and use of nitrogen-fixing plants from atmospheric NOx by nitrogen fixation by microorganisms. Nitrogen fertilizer added to the soil excess ammonia and nitrate not used by plants can be converted to Nitric Oxide(NO) by microorganisms in the soil. Sometimes ammonia transformed into nitrate by the process of nitrification. Nitrate reduced to nitrite then nitric oxide(NO) then nitrous oxide(N₂O) finally nitrogen through this process NOx is emitted to the atmosphere. The reverse process of nitrification (denitrification). At the time of lightning, the rapid healthy and cooling of gases produces nitric oxides(NO), which quickly react with more oxygen to create nitrogen dioxide(NO₂). In cities NOx formed mainly by the combustion of fossil fuels, During the fossil fuel, combustion process nitrogen contained in the air and fuel interacts with oxygen to form nitrogen oxide(NO), dinitrogen oxide(N₂O), nitrogen dioxide(NO₂), and other nitrogen oxides are formed in less amount.

Nitrogen oxide emission from coal combustion can occur in 3 sources 

1) Thermal NOx: This NOx is formed when nitrogen and oxygen in the combustion air combine with one another at a high temperature, NOx formation increased above 2,800 degrees Fahrenheit flame temperature. 

2) Fuel NOx: Reaction of nitrogen bound in the fuel with oxygen in the combustion air, it is a problem of gaseous fuels. 

3) Prompt NOx: This type of NOx formed by the rapid reaction of atmospheric nitrogen with hydrocarbon radicle. This NOx is formation is less compared to the other two types of NOx.

Effects of NOx

1) Acid rain: Nitrous oxides are one of the reasons for acid rain, These oxides when released into the air undergo further oxidation and are finally absorbed in water vapours to form a mist. When clouds become saturated, the acidic mists precipitate as acid rain. Acid rain destroys the Fertility of the soil and also causes damage to the leaves of trees and plants, It also increases fish mortality, It affect the paintworks, and cause yellowing of white fabrics. Acid rain is more dangerous to concrete this destroys its alkaline structure consisting of stone aggregates and cement. 

Stable molecule N2 and O2 convert into a significant amount of NO, it occurs at high temperature. 

NOx in lightning can become oxidized to produce nitric acid (HNO3) this is precipitated as acid rain. 

O3+NO2 -> NO3 + O2 

(ozone+nitrogen dioxide->nitrogen trioxide+ oxygen ) 

NO3+NO2 -> N2O5 

(nitrogen trioxide+ nitrogen dioxide->dinitrogen pentoxide) 

N2O5+H2O -> 2HNO3 

(dinitrogen pentoxide + water-> nitric acid) 

2) Smog: Smog is a mixture of fog and smoke. Natural fog contains mainly nitrogen oxides coming from domestic fires, industrial furnace, power stations, and internal combustion furnace, power stations and internal combustion engine using petrol or diesel as fuel .one of the important smog is photochemical smog, it is formed by the sunlight react with and volatile organic compound in the atmosphere. Photochemical smog mainly occurs in summer. The major reaction taking place is the decomposition of NO2 in the presence of Ultraviolet light. The reaction produces oxidized hydrocarbons i.e. peroxyl acetyl nitrate (PAN), formaldehyde, ozone aldehyde, acrolein etc. These oxidised hydrocarbons and ozone form photochemical smog in the presence of humidity. This smog causes coughing, irritation to the eyes, lungs, nose, throat. It also causes damage to plants. 

3) Depletion of ozone layer: ozone layer acts as a filter, it absorbs harmful ultraviolet radiations coming from the sun. The concentration of ozone is determined by the ratio of nitric oxide(NO) and nitrogen dioxide(NO2), in the daytime the concentration of nitric oxide(NO) and nitrogen dioxide(NO2) together at that time ozone are steady state it is called “photostationary state”. 

Nitrous oxide(N2O)+ultraviolet radiation -> N2+O 

N2O+O -> nitric oxide(NO)+NO 

NO+Ozone(O3) -> nitrogen dioxide (NO2)+O2 

This relation between ozone and NOx is called the “Leighton relationship “. 

Nitric oxide emission from high flying supersonic aircraft also contributes to ozone depletion

NO+ClO -> Cl + NO2 

(nitric oxide+clorine monoxide -> chlorine +nitrogen dioxide) 

Cl+O3->ClO+O2 

(chlorine ozone->chlorine monoxide oxygen ) 

NO+O3->NO2+O2 

(nitric oxide+ozone->nitrogen dioxide+oxygen ) 

4) Others : 

• Nitric oxide (NO)are not serious but it cannot readily dissolve in water so it cannot be easily absorbed by lung tissues. In the presence of sunlight nitric oxide can react with oxygen to form nitrogen dioxide. NO cause some health problems it causes hypotension. It causes headache in migraine it damages brain cells that lead to neurodegenerative diseases like Parkinsons, Alzheimer, Huntington disease and amyotrophic lateral sclerosis.
• Nitrogen dioxide (NO2) is a reddish-brown toxic gas with an unpleasant smell, it causes respiratory illness, increased level of NO2 causes irritation of the eyes and congestion. It also decreased the function of the lungs. A high level of nitrogen dioxide causes asthma and also breathing a high level of nitrogen dioxide can cause rapid burning, swelling of tissues in the throat and respiratory tract it reduced the oxygenation of body tissues, sometimes it causes death. In northern hemisphere hotspots, the presence of NO2 is very high level, at the winter season the peoples in that region used combustion powerplant for home heating so NO2 stays in the air longer in that time, due to this reason nitrogen dioxide is less visible at the time of winter.NO2 is harmful to the vegetation because it is toxic in short term concentration, it reduces plant growth.NO2 reduce the pH of soil and water. Another effect of NO2 is it fade and discolour furnishings and fabrics it also reduces visibility.
• Nitrous oxide(N2O) is Inhaled once, it combined with oxygen it emits a sensation with the body that helps feel relaxed and comfortable so it is also known as laughing gas. It effectively manages pain and anxiety so it is used during the time of dental treatment .it has no odour and colour.N2O is a greenhouse gas, it stays in the atmosphere a long time compared to carbon dioxide it enhances the greenhouse effect. The atmospheric N2O concentration has been growing due to human activities. It is more harmful to the climate than carbon dioxide i.e. it is 300 times harmful than carbon dioxide. The use of nitrogen-based fertilizer and burning fuels and wood are the main source of N2O. Compared to other greenhouse gases nitrous oxide absorbs radiation and traps heat in the atmosphere it exists for an average of 114 years. N2O stimulate seed germination it affects plant life. Nitrous oxide may react with ozone to become a secondary pollutant peroxyacetyl nitrate (PAN) which is an eye irritant.