Smog is the visible gaseous and particular matter concentrating near the earth's surface particularly in the cities due to SO2, CO2, factory and traffic emission and meteorological conditions. When different gases like SO2, NO2, N2O, NO2, unburnt hydrocarbons etc. are released into the atmosphere, they form photochemical smog ( smoke + fog) which become harmful pollutant.
The important constituents of smog are oxides of nitrogen and sulphur, carbon monoxides, hydrogen peroxide, peroxy acetyl nitrate (PAN) , peroxy benzoyl nitrate (PBN) ,peracetic acid hydroperoxide etc. These chemical are released into the air by smoke coming out of industries, from automobile exhaust, fossil burning etc. In addition, hydrocarbons discharged from automobiles react with nascent oxygen or ozone to produces aldehydes, ketones or organic peroxies which are also present in smog.
Photochemical smog is characterized by the brown misty haze which irritates the eyes and lungs,lead to the cracking of rubber and extensive damage to plant life. In plants, prolonged exposures damage chloroplasts, disturb electron transport system and adversely affects a number of enzymes which play a vital role in photosynthesis.
There are various types of photochemical smog. For example :-
It is the mixture of fog , smoke and sulphur dioxide (SO2) gas. This type of smog was marked in London in 1952 where the smog prevailed for fice days killing few thousands. The pollution was caused due to sulphur dioxide and smoke during a temperature inversion. The effects were more severe in morning hours and became deadly after sunrise because of photochemical oxidation. The smog caused bronchitis, pneumonia, respiratory, distress, and eye diseases.
S + O2→ SO2
SO2+ O→ SO3
SO2+ H2O → H2SO3
H2SO4+ NH3→ (NH4)2SO4
Such type of smog was recorded for the first time at Los Angeles in 1944 and there after it is known as Los Angeles smog. The smog contains different times of a day. In the morning, when nitric oxide (NO) is discharged from automobile exhausts build up and react with oxygen to form nitrogen dioxide which is a yellowish brown gas with pungent and chocking odour. Since NO2gas produces a characteristic brown haze, the cities where this type of smog predominates, are called brown air cities. As the sunrises, the UV-rays cause a rapid conversion of NO2to NO and the nascent oxygen atom. The nascent oxygen atoms react with oxygen molecules of air to give ozone (O3). The other highly reactive chemical forms are hydrogen peroxide (H2O2) , hydroxy radicals (OH) etc.
NO2→ NO + O
NO + O2→ NO2+ O
NO2→ PAN + O2
O2+ O→ O3
The molecules like PAN, ozone , NOxetc. are the major components of smog. Nitrogen combines with oxygen to produce a mixture of oxides . In the morning, the concentration of NO is maximum and after the sunshine , NO2becomes higher.
N2+ 2O2→ 2NO2
2NO + O2→ 2NO2
NO + O→ NO
2NO2+ O→ N2O5
O3+ 2NO2→ N2O5+ O2
Effects of smog
The formation of smog is highly destructive and influences both the physiological and metabolic activities of living organisms. Some important effects of smog are given below:
Control of smog
Acid rain is one of the major environmental pollutions of these days. Acid rain may be defined as any precipitation such as rain, fog, mist or snow which is more acidic than the normal i.e. having a lower pH than that of normal rain water. The normal rain water is also slightly acidic having pH approximately 5.6. When the acid content becomes high and pH falls down to 5.4 or even up to 2.0,we call it acid rain.
The term acid rain was first referred by Robert Angus in 1872 and it has became a serious problem in most of the industrialized countries. Acid rain is due to presences of various pollutants like oxides of nitrogen, carbon, sulphur, and halogens radicals or molecules in the atmosphere.This pollutants are discharged into the atmosphere by several natural processes like volcanic eruptions, forest fires, lightning, burning of fossils fuels, decomposition of organic matters etc and human activities like automobile exhausts burning of fuels, thermal power plants, petroleum refinery etc.
The conversion of the pollutant into their corresponding acids are shown in the equations given below:
N2+ O2→ 2NO (nitric oxide)
NO + O3→ NO2+ O2
2NO2+ O3→ N2O3+ 2O2
2NO + O→ N2O5(nitrogen pentoxide)
N2O5+ H2O→ 2HNO3(Nitric Acid)
N2O3+ H2O → 2HNO2(Nitrous Acid)
S + O2 → SO2(sulphur dioxide)
2SO2 + O2→ 2SO3( sulphur trioxide)
SO3+ H2O→ H2SO4(sulphuric acid)
SO2+ H2O→ H2SO3(sulphurous acid)
C + O2→ CO2
CaCO3→ Cao + CO2
CO2 +H2O→ H2CO3 (carbonic acid)
Cl2→ Cl• + Cl•
H2→ H• + H•
H• + Cl• → HCL (hydrochloric acid)
The concentrations of these acids, as well as the quality of water in which the acids are dissolved, determine the pH of rain water. Heavy rains are usually less acidic as there is relatively more water. On the other hand, fogs and mists are more acidic as the acid molecules are dissolved in relatively little water.
The damage caused by acid precipitation (acid rain) are of a very diverse nature and can be sammed up as follows :
Acid rain can only be controlled by managing the sources of the pollution. Some important control measures are given below:
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