Acid Rain And Smog

Acid Rain

Acid rain is one of the major environmental pollution of thee days.Acid rain may be defined as any precipitaion 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 approximately 5.6.When the acid content becomes high and pH falls down to 5.4 or even upto 2.0,we call it acid rain.

The term acid rain was first referred by Bobert Angus in 1872 and it has become a serious problem in most of the industrialized countries.Acid rain is due to the presence of various pollutants like oxides of nitrogen,carbon,sulphur and halogen radicals or molecules in the atmosphere.These pollutants are discharged into the atmosphere by several natural processes like volcanic eruptions,forest fires,lightening,burning of fossil fuels,decomposition of organic matters,etc.and human activities like automobile exhausts burning of fuels,thermal power plants,petroleum refinery,industries,etc.Coal contains 25 of sulphur and therefore burning of coal result in emission of SO₂ in atmosphere which is major contributing factor for acid rain.NO_x is another factor for acid rain.The NO_x and SO_x entering the atmosphere are converted into HNO₃ and H₂SO₄ respectively.

The conversion of the pollutants into their corresponding acids are shown in the equations are given below:

$$N_2+O_2 \rightarrow 2NO$$

$$NO+O_3 \rightarrow NO_2+O_2$$

$$2NO_2+O_3 \rightarrow N_2O_3+2O_2$$

$$2NO_2+O \rightarrow N_2O_5$$

$$N_2O_5+H_2O \rightarrow 2HNO_3$$

$$N_2O_3+H_2O \rightarrow 2HNO_3$$

$$S+O_2 \rightarrow SO_2$$

$$2SO_2+O_2 \rightarrow 2SO_3$$

$$SO_3+H_2O \rightarrow H_2SO_4$$

$$SO_2+H_2O \rightarrow H_2SO_3$$

$$C+O_2 \rightarrow CO_2$$

$$CaCO_3 \xrightarrow[]{Δ} CaO+CO_2$$

$$CO_2+H_2O \rightarrow H_2CO_3$$

$$Cl_2 \xrightarrow[]{hν} Cl^•+Cl^•$$

$$H_2 \xrightarrow[]{hν} H^•+H^•$$

$$H^•+Cl^• \rightarrow HCl$$

The concentrations of these acids as well as as the quantity 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.

HNO₃ generated in the above reaction is removed as a precipitate nitrates after reaction with the bases(NH₃,particulate lime)

$$SO_2+\(\frac{1}{2}\)O_2+H_2O \xrightarrow[Soot particles]{(HC,NO_x)} H_2SO_4$$

The presence of hydrocarbons and NO_x catalyses the oxidation rate of the reaction.In water droplets,ions such as Mn(II),Fe(II),Ni(II) and Cu(II) catalyse the oxidation reaction.Soot particles are also known to be strongly involved in catalysing the oxidation of SO₂.

HNO₃ and H₂SO₄ combine with HCl from HCl emission(both by natural and anthropogenic sources) to generate acidic precipitation which is widely known as acid rain.

Effects of Acid Rain

The damages caused by the acid rain are of diverse nature and can be summed up as follows:

1.The corrosive action of the acid content in these rains damages buidings,monuments,statues,bridges,fences,wood,cement,concrete structures,railings,etc.

$$CaCO_3+H_2SO_4 \rightarrow CaSO_4+CO_2+H_2O$$

2.Acid rain increases the soil acidity affecting land flora and fauna.

3.Diseaes 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,coppper,manganese,cadmium,lead,etc. from yhe soil to the 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.

It causes acidification of the aquatic bodies,whhich leads to the killing of aquatic plants and animals.The increase in acidity also affects the metabolism,growth and development of aquatic organisms.

8.Plants and animals affected by acid rain are easily attacked by pathogens.

Control Measures

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

1.The fuels,devoid of sulphur or having low sulphur amount should be used to minimize the amount of sulphurdioxide gas in the atmosphere.

2.The vehicular exhaust should be minimized 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 neutralize the acidity due to acid rain.

4.The leakage of chlorine or its discharge should be stopped.

5.The scrubbers should be used to reduce the emission of SO₂ during coal burning.

6.SO₂ gas released is sprayed with water containing lime,which precipitaes SO₂ as calcium sulphite(CaCO₃).

$$Ca(OH)_2+SO_2 \rightarrow CaSO_3+H_2O$$

7.General public awareness should be created regarding the ill effects of environmental pollution and consequences of acid rain.

Smog

The term smog derives from the combination of the words smoke and fog.Smog mean man made air pollution in urban areas.There are mainly two types of smog which have been encountered,namely,classical or london type or sulphurous smog and photochemical or Los Angeles-type smog.

Photochemical Smog

Smog is the visible gaseous and particulate matter concentrating near the earth's surfaces particularly in cities due to SO₂,CO₂,factory and traffic emissions and meteorological conditions.When different gases like SO₂,NO₂,NO,N₂O,unburnt hydrocarbons,etc. are released into the atmosphere,they form photochemical smog which become harmful pollutant.

The important constituents of smog are oxides of nitrogen and sulphur,carbon monoxide,hydrogen peroxide,peroxy acetyl nitrate(PAN),peroxy benzoyl nitrate(PBN),peracetic acid,hydroperoxide,etc.These chemicals are released into the atmosphere by smoke coming out of industries,from automobile exhaust,fossil burning,etc.In addition,hydrocarbon discharged from automobiles react with nascent oxygen or ozone to produce aldehydes,ketones or organic peroxies which are also present i smog.

Photochemical smog is characterized by brown misty haze which irritate thr 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 andd adversely affect a number of enzymes which play a vital role in photosynthesis.

Such type of smog was reported for the first time in Los Angeles in 1844 and there after it is known as Los Angeles smog.The smog contains different compounds in 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 nitrogendioxide which is a yellowish brown gas with pungent and choking odour.Since,NO₂ gases produce a characteristic brown haze,the cities where this type of smog predominates,are called brown air cities.As the sun rises the UV-rays cause a rapid conversion of NO₂ to NO and 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.

$$NO_2 \xrightarrow[]{sunlight} NO+O$$

$$NO+O_2 \rightarrow NO_2+O$$

$$NO_2 \rightarrow PAN+O_2$$

$$O_2+O \rightarrow O_3$$

The molecules like PAN,ozone,NO_x,etc. are the major consequences of smog.Nitrogen combines with oxygen to produce a mixture of oxides.In the morning,concentration of NO is maximum and after the sunshine,NO₂ becomes higher.

$$N_2+2O_2 \rightarrow 2NO_2$$

$$2NO+O_2 \xrightarrow[]{sunlight} 2NO_2$$

$$NO_2 \xrightarrow[]{UV} 2NO+O$$

$$NO+O \rightarrow NO_2$$

$$2NO_2+O \rightarrow N_2O_5$$

$$O_3+2NO_2 \rightarrow N_2O_5+O_2$$

Sulphorous Smog

It is a mixture of fog,smoke and sulphur dioxide gas.This type of smog was marked in London in 11952 where the smog prevailed for five days killing few thousands.The pollution was caused due to sulphur dioxide and smoke during the temperature inversion.The effects were more severe in morning hours and became deadly after sunrise because of photochemical oxidation.The smog caused pneumonia,bronchitis,resoiratory distress and eye diseases.

$$S+O_2 \xrightarrow[]{hν} SO_2$$

$$SO_2+O \rightarrow SO_3$$

$$SO_2+H_2O \rightarrow H_2SO_3$$

$$H_2SO_3+O \rightarrow H_2SO_4$$

$$H_2SO_4+NH_3 \rightarrow (NH_4)_2SO_4$$

References

Poudel, Bhoj Raj Poudel and Dr.Megh Raj. A Text Book Of Inorganic Chemistry. Kathmandu: National Book Centre, 2012.

Sons, John Wiley and. J.D Lee Concise Inorganic Chemistry. 2007.