Rain is one of the most essential ingredients for human and animal life. The water provided by rain allows all life on Earth to survive. Although rain is naturally acidic, it is being increasingly acidified by pollution from homes, factories, power stations and cars. The term used to describe this problem is “acid rain”. Acid rain hasn’t just occurred in the last twenty to thirty years. In fact a chemist named Robert Smith found rainfall in Manchester, U.K., to be very acidic. He suggested a link between acid rain and SO2 given off when coal was burnt by local factories. This was over 100 years ago.
Acid rain is caused when pollutants from cars, homes, factories and power stations mix with atmospheric moisture. These pollutants may be carried in clouds for long distances before falling, which means that forests and lakes far away from factories may be damaged by acid rain.
Two of the major ingredients of acid rain are the chemicals sulphur dioxide [SO2] and nitrogen oxides [NOx]. When large quantities of these two particular chemicals come in contact with the atmosphere, they team up with moisture [H2O] to produce strong acids called sulphuric and nitric acid. These two acids which are formed in the atmosphere, are very strong pollutants.
Some air pollution as a matter of fact comes from natural sources, but most is human made. The burning of oil and coal by plants and factories, homes and cars, is the main source of chemicals that cause acid rain. Power stations and factories emit large amounts of sulphur dioxide and also nitrogen oxides, whilst car exhausts contain large amounts of nitrogen oxides. When volcanoes erupt, they emit various gases which have been trapped under the ground, including sulphur dioxide. This can cause air pollution, which can then be made much worse by the addition of human-made emissions. The air in many towns and cities is overfull of harmful pollutants. In the northern hemisphere sulphur emissions are decreasing steadily, due to pollution controls in industry. NOx emissions, however, are not yet in such steady decline.
The acid from rain takes important minerals from the leaves of trees and from the soil. Acid rain also releases toxic metals from the soil which damage the roots of the trees. The trees are weakened, cannot grow properly and are attacked by viruses, fungi and pests. Eventually the trees may die. Direct damage to trees occurs when SO2 blocks the pores on the leaves, through which the trees takes in the air they need to live. Forest floods that are affected by acid rain have high concentrations of metals like aluminium and lead. When animals drink from acidic lakes and dew that have been affected, the metals they take in may gradually poison them.
There is an obvious link between acid rain and damage to human health. People can be harmed by breathing in the chemicals from dry deposition, causing chest illness. Also when acid rain causes the release of metals and chemicals into drinking water, it can damage people’s health. When air pollution is breathed in as people walk along the street, it gets into their lungs. Once in the lungs, it acts like a poison, causing the airways in the respiratory system to become narrower. This lets in less oxygen and breathing becomes difficult. If SO2 is breathed in, it can pass deep into the alveoli, which is where oxygen is passed into the blood. The moisture in the lungs can turn the sulphur dioxide into sulphuric acid, and cause damage to the body. In some polluted cities, such as those in California, smog stations have been set up to monitor the output of polluting exhaust gases.
To reduce acid rain industry’s have to cut down the amount of SO2 being produced when fossil fuels are burnt. Therefore, they can: use coal which contains little sulphur, remove the sulphur which is in the coal, use another type of fuel, or burn the coal in such a way that the sulphur is destroyed. There are many other ways of combating this problem, however, the best way to solve this issue is to prevent emissions of pollutants in the first place. Reducing the amount of NOx emissions caused by car exhaust is an effective solution. Public transport systems need to be improved so that people can travel without having to use their cars. If more people used public transport, it would cut the number of private vehicles on the roads, and would reduce pollution dramatically. Everybody needs to work together to reduce pollutants to make the world a safer and healthier place to live.
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Essay on Acid Rain: Definition, Causes, Adverse Effects and Control!
The term ‘Acid Rain’ was first used by Robert A. Smith in 1872 from his studies of air in Manchester, England. The widespread occurrence of acid rain was recognized only in 1980. Acid rain is a rain or any other form of precipitation that is unusually acidic, i.e., elevated levels of hydrogen ions (low pH).
What we call acid rain is the oxides of sulphur and nitrogen originating from industrial operations and fossil fuel combustion, the major sources of acid forming gases combine with the water in the air. Acid forming gases are oxidized over several days by which time they travel several thousand kilometres.
In the atmosphere these gases are ultimately converted into sulphuric and nitric acids. This acidic mixture then falls as rain, sleet, mist or snow or as solid flakes. Hydrogen chloride emission forms hydrochloric acid. These acids cause acidic rain.
Rain water is turned acidic when its pH falls below 5.6. In fact clean or natural rain water has a pH of 5.6 at 20°C because of formation of carbonic acid due to dissolution of CO2 in water. Parts of India such as North East, Coastal regions of Kerela, Orissa, Bihar, and West Bengal have reported decline in fertility of soil due to reduced pH of soil (increased in acidity). Acid rain is the phenomenon of wet and dry acidic deposition.
Wet deposition of acids occurs when any form of precipitation (rain, snow, and so on) removes acids from the atmosphere and delivers it to the Earth’s surface. This can result from the deposition of acids produced in the raindrops (see aqueous phase chemistry above) or by the precipitation removing the acids either in clouds or below clouds. Wet removal of both gases and aerosols are both of importance for wet deposition.
Acid deposition also occurs via dry deposition in the absence of precipitation. This can be responsible for as much as 20 to 60% of total acid deposition. This occurs when particles and gases stick to the ground, plants or other surfaces.
During the last few decades acid rain occurred within the downwind of areas of major industrial areas in Europe and America. Emissions of sulphur dioxide (SO2) into sulphate or NO2 into nitrate particles and by combining with water vapour into mild sulphuric or nitric acids and return to earth as dew, drizzle, fog, sleet, snow, and rain is the mechanism of acid rain. Acid rain is a rain or any other form of precipitation that is unusually toxic, meaning that it possesses elevated levels of hydrogen ions (low pH). It can have harmful effects on plants, aquatic animals, and infrastructure.
Acid rain is caused by emissions of sulphur dioxide and nitrogen acids, which react with the water, smell molecules in the atmosphere to produce acids. Governments have made efforts since the 1970s to reduce the release of sulphur dioxide into the atmosphere with positive results.
Nitrogen oxides can also be produced naturally by lightning strikes and sulphur dioxide is produced by volcanic eruptions. The chemicals in acid rain can cause paint to peel, corrosion of steel structures such as bridges, and erosion of stone statues.
“Acid u” is a popular term referring to the deposition of wet (rain, snow, sleet, fog, cloud water, and dew) and dry (acidifying particles and gases) acidic components. Distilled water, once carbon dioxide is removed, has a neutral pH of 7. Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are alkaline.
“Clean” or unpolluted rain has an acidic pH, but usually no lower than 5.7, because carbon dioxide and water in the air react together to form carbonic acid, a weak acid. However, unpolluted rain can also contain other chemicals which affect its pH. A common example is nitric acid produced by electric discharge in the atmosphere such as lightning. Carbonic acid is formed by the reaction.
H2O (1) + CO2 (g) → H2CO3 (aq)
Carbonic acid then can ionize in water forming low concentrations of hydronium and carbonate ions:
H2O (1) + H2CO3 (aq) <==> HCO–3 <==> (aq) + H3O+ (aq)
Acid deposition as an environmental issue would include additional acids to H2CO3.
Causes of Acid Rain:
Acid deposition can occur via natural sources like volcanoes but it is mainly caused by the release of sulphur dioxide and nitrogen oxide during fossil fuel combustion. When these gases are discharged into the atmosphere they react with the water, oxygen, and other gases already present there to form sulphuric acid, ammonium nitrate, and nitric acid. These acids then disperse over large areas because of wind patterns and fall back to the ground as acid rain or other forms of precipitation.
The gases responsible for acid deposition are normally a by-product of electric power generation and the burning of coal. As such, it began entering the atmosphere in large amounts during the Industrial Revolution and was first discovered by a Scottish chemist, Robert Angus Smith, in 1852.
In that year, he discovered the relationship between acid rain and atmospheric pollution in Manchester, England. Although it was discovered in the 1800s, acid deposition did not gain significant public attention until the 1960s and the term acid rain was coined in 1972.
Acid rain has been shown to have adverse impacts on forests, freshwaters and soils, killing insect and aquatic life-forms as well as causing damage to buildings and having impacts on human health.
1. Surface waters and aquatic animals:
Both the lower pH and higher aluminium concentrations in surface water that occur as a result of acid rain can cause damage to fish and other aquatic animals. At pH lower than 5 most fish eggs will not hatch and lower pHs can kill adult fish.
As lakes and rivers become more acidic biodiversity is reduced. Acid rain has eliminated insect life and some fish species, including the brook trout in some lakes, streams, and creeks in geographically sensitive areas, such as the Adirondack Mountains of the United States.
However, the extent to which acid rain contributes directly or indirectly via runoff from the catchments to lake and river acidity (i.e., depending on characteristics of the surrounding watershed) is variable.
The United States Environmental Protection Agency’s (EPA) website states: “Of the lakes and streams surveyed, acid rain caused acidity in 75 percent of the acidic lakes and about 50 percent of the acidic streams”.
As the lake becomes more acidic the fish find it more difficult to reproduce successfully. It is not only the acid in the water that kills them, but also poisonous minerals like aluminium that are washed out of the surrounding ground into the water. The birds that eat the fish also begin to suffer as the harmful minerals build up inside.
Soil biology and chemistry can be seriously damaged by acid rain. Some microbes are unable to tolerate changes to low pH and are killed. The enzymes of these microbes are denatured (changed in shape so they no longer function) by the acid. The hydronium ions of acid rain also mobilize toxins such as aluminium, and leach away essential nutrients and minerals such as magnesium.
2 H+ (aq) + Mg2+ (clay) <==> 2H+ (clay) + Mg2+ (aq)
Soil chemistry can be dramatically changed when base cations, such as calcium and magnesium, are leached by acid rain thereby affecting sensitive species, such as sugar maple (Acer saccharum).
Acid rain can damage soil by destroying many vital substances and washing away the nutrients. Soils naturally contain small amounts of poisonous minerals such as mercury and aluminium.
Normally these minerals do not cause serious problems, but when acid rain falls on the ground and the acidity of the soil increases, chemical reactions occur allowing the poisonous minerals to be taken up by the plant roots. The trees and plants are then damaged and any animals eating them will absorb the poisons, which will stay in their bodies.
3. Forests and other vegetation:
The acid takes important minerals away from the leaves and the soil. Without these minerals, trees and plants cannot grow properly. Damaged trees lose their leaves, have stunted growth and damaged bark. This makes it easier for fungi and insects to attack the tree, and as a result the tree may die.
Acid rain not only damages soil but can also affect the trees directly. Pollutants can block or damage the little pores on the leaves through which the plant takes in the air it needs to survive. High altitude forests are especially vulnerable as they are often surrounded by clouds and fog which are more acidic than rain.
Other plants can also be damaged by acid rain, but the effect on food crops is minimized by the application of lime and fertilizers to replace lost nutrients. In cultivated areas, limestone may also be added to increase the ability of the soil to keep the pH stable, but this tactic is largely unusable in the case of wilderness lands. When calcium is leached from the needles of red spruce, these trees become less cold tolerant and exhibit winter injury and even death.
4. Human health effects:
Acid rain does not directly affect human health. The acid in the rainwater is too dilute to have direct adverse effects. However, the particulates responsible for acid rain (sulphur dioxide and nitrogen oxides) do have an adverse effect. Increased amounts of fine particulate matter in the air do contribute to heart and lung problems including asthma and bronchitis.
5. Other adverse effects on Monuments:
Acid rain can also damage buildings and historic monuments and statues, especially those made of rocks, such as limestone and marble that contain large amounts of calcium carbonate.
When sulphur pollutants fall on to buildings made from limestone and sandstone they react with minerals in the stone to form a powdery substance that can be washed away by rain. Acids in the rain react with the calcium compounds in the stones to create gypsum, which then flakes off.
CaCO3 (s) + H2SO4 (aq) <==> CaSO4 (aq) + CO2 (g) + H2O (1)
The effects of this are commonly seen on old gravestones, where acid rain can cause the inscriptions to become completely illegible. Acid rain also increases the corrosion rate of metals, in particular iron, steel, copper and bronze. Famous buildings like the Statue of Liberty in New York, the Taj Mahal in India and St. Paul’s Cathedral in London have all been damaged by this sort of air pollution.
Acid rain can also damage stained glass windows in churches, railway lines and steel bridges. The acid rain slowly eats away them all. Building materials crumble away, metals are corroded.
Control of Acid Rain:
1. Emission of SO2 and NO2 from industries and power plants should be reduced by using pollution control equipment’s such as scrubbers in the smokestacks of factories. These spray a mixture of water and limestone into the polluting gases, recapturing the sulphur.
2. Liming of lakes and soils should be done to correct the adverse effects of acid rain.
3. A coating of protective layer of inert polymer should be given in the interior of water pipes for drinking water.
4. In catalytic converters, the gases are passed over metal coated beads that convert harmful chemicals into less harmful ones.