Smog And Acid Rain

Smog

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 :-

1. Sulphurous Smog

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

H₂SO₃+ O→H₂SO₄

H₂SO₄+ NH₃→ (NH₄)₂SO₄

2. Los Angeles Smog

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 , NO_xetc. 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:

1. The presences of ozone, PANs and aldehydes in smog leads to eyes irritations and affects respiratory tracts and throts.
2. The presences of PANs in smog causes extensive agricultural and forestry damage, such as damage to leaves and stomatal tissues.
3. The presences of NO₂in smog causes nose and eye irritation and chronic diseases in lungs and heart.
4. PANs cause dizziness and headache in man.
5. Ozone decolourises the paintings.
6. Smog damages metals, paper, rubber and fabrics.
7. The particulate matters in smog induce lung cancer.
8. Smoke cause early maturity of plants and reduces the rate of photosynthesis.

Control of smog

1. The production of nitrogen oxide (NO_x) and hydrocarbon should be controlled which checks the production of ozone and PANs.
2. The process like incineration, absorption, and condensation should be employed to reduce the different harmful constituents of smog such as oxides of sulphur and nitrogen, hydrogen carbon, carbon-monoxides, dust etc.

Acid Rain

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.

Effects of Acid rain

The damage caused by acid precipitation (acid rain) are of a very diverse nature and can be sammed up as follows :

1. The corrosive action of the acid content in these rains damages buildings, monuments, statues, bridges, fences, wood, cement, concrete structure, railings etc.
   CaCO₃ + H₂SO₄→ CaSO₄ + CO₂ + H₂O
2. Acid rain increases the soil acidity affecting land flora and fauna.
3. Diseases caused by bacteria and pathogens can be spread by acid rain water.
4. Acid rain may cause respiratory and skin diseases.
5. It leaches various metals such as aluminium, zinc, copper, manganese, cadmium, lead etc. from the soil into aquatic bodies. When the concentration of these metals increases beyond the safe limit, it affects development and leads to the death of many aquatic organisms in general and fishes in particular.
6. It affects the drinking water storage sources.
7. It causes acidification of aquatic bodies, which leads to the killing of aquatic plants and animals.The increase in acidity also affects the metabolism, growths and development of aquatic organisms.
8. Plants and animals addicts by the acid rains are easy attacked by pathogens.

Control Measures

Acid rain can only be controlled by managing the sources of the pollution. Some important control measures are given below:

1. The fuels, devoid of sulphur or having low sulphur amounts should be used to minimise the amount of sulphur dioxide gas in the atmosphere.
2. The vehicular exhaust should be minimised by using control values in the outlet of the exhaust pipeline of the automobiles.
3. The aquatic bodies and farmlands should be periodically limed to neutralise the acidity due to acid rain.
4. The leakage of chlorine or it's discharge should be stopped.
5. The scrubbers should be used to reduce the emission of SO₂ during coal burning.
6. SO₂gas released is sprayed with water containing lime, which precipitates SO₂as calcium sulphite ( CaSO₃).
   Ca(OH)₂ + SO₂→ CaSO₃+ H₂O
7. General public awareness should be created regarding the ill effects of environmental pollution and consequences of acid rain.

References

Adhikari, N.R.Plus two practical chemistry.Nepal: Ed-mark Academic Enterprises Pvt. Ltd, 2004.

Jain, S.K.Conceptual Chemistry,.Vol. I and II. S. Chand & Company Ltd., n.d.

Khatri, S.S.Elementary Principles of General Chemistry.TU Kathmandu: Curriculum Development Centre , 1997.

Wagley, Dr. P.Compressive Chemistry.Nepal: WorldWide Publication Pvt. Ltd, 2014.