Acid rain

Abstract

Since the beginning of time, humans have learned to make use of many things in nature such as fire and electricity. From the early times through the Industrial Revolution to the Space Age, humans have produced inventions that use many of the earth’s varied energy resources to make living easier. In many cases the energy comes from burning fossil fuels such as coal, oil and natural gas. Some of the inventions that make our lives easier are also causing pollution. Pollution is the release of harmful substances into the environment. One form of pollution is acid rain. Acid rain can damage plants, animals, soil, water, building materials, and people. Scientists have discovered that burning fossil fuels creates acid rain through air pollution. People burn fossil fuels such as coal and oil to make electricity. The electricity heats and lights buildings and runs appliances such as televisions and video recorders. Fossil fuels power our cars, buses, and airplanes. The air pollution created when these fuels burn does not stay in the air forever. It can return to the earth as acid rain. And when it does, it may weaken the plant and animal life it contacts. Acid rain is only one form of pollution that results from burning fossil fuels. It is one of particular interest, however, because it can be transported over long distances. Scientists, engineers, and researchers are learning how to measure the amount and effects of pollution in the air, forests, water, and soil. They are inventing ways to reduce the amount of pollution that enters the environment and prevent new damage in the future.

Definition

Acid rain also known as acid precipitation or acid deposition, acid rain is precipitation containing harmful amounts of nitric and sulfuric acids formed primarily by sulfur dioxide and nitrogen oxides released into the atmosphere when fossil fuels are burned. It can be wet precipitation such as rain, snow, or fog or dry precipitation which is absorbed gaseous and particulate matter, aerosol particles or dust. Acid rain has a pH below 5.6. Normal rain has a pH of about 5.6, which is slightly acidic.

Causes of acid rain

Acid rain occurs as a result of a complex series of chemical, atmospheric and environmental reactions that begin with air pollution emitted from utility companies, industries, motor vehicles and the burning of fossil fuels such as coal and oil. Fossil fuels are derived from the prehistoric production of organic matter. As a result of these processes, sulfur dioxides and nitrogen oxides are released into the air where they combine with other pollutants, oxygen and water vapor to form sulfuric and nitric acid compounds. These acids are what cause rain to become overly acidic. The acids can also travel for hundreds of miles in the air before being deposited to the earth as acid precipitation. Acid rain results primarily from the emission of two major pollutants: sulfur dioxide (SO₂) and nitrogen oxide (NO_X). Sulfur dioxide is produced mainly from the burning of coal and oil. Nitrogen oxide is specifically associated with high temperature combustion processes. The majority of nitrogen oxide emissions is comes from electric utilities and motor vehicles.

Effects of acid rain

Major concerns associated with acid rain include what effects it may have on humans and how it may disturb the natural balance of the environment. Acid rain can affects humans, as well as how it effect the ecology of lakes, rivers, and streams. Acid rain may affects forests, crops and even buildings. Believe it or not, acid rain is so powerful that if can actually eat away at man-made structures such as cement statues and buildings.

Human Health

Rain which contains high levels of acid, looks exactly the same as clean rain. In fact, if someone is walking down the street and it begins to rain, even though the rain may contain acid in it, they will not even be able to detect it. As a matter of fact, not only would they not be able to detect acid in rain water, but even if they swam in a lake with excessive amounts of acidity, they would probably not even know it. Humans are most susceptible to the air pollution which causes acid rain to occur in the first place. As discussed earlier, sulfur dioxide and nitrogen oxide are the chemical compounds which when released into the air react with other chemicals, pollutants, oxygen and even water vapour to cause acid rain. It is these two chemical compounds which have the potential to irritate and damage lungs.

Other effects acid rain has upon humans relates to visibility. If the air contains large amounts

of pollutants, human visibility is severely limited. When sulfur dioxide emissions are released into the air, they form small sulfate particles or aerosols which reduce visibility by scattering light. Nitrogen oxide emissions also form aerosols in the atmosphere that reduce visibility.

Ecology of Lakes, Rivers and Streams

As discussed earlier, a normal pH for a body of water ranges from 6 to 9. Some water bodies are naturally high in acid as a result of pre-existing environmental conditions. Water bodies with an average pH become acidic when surrounding soils lose the ability to buffer themselves from the acid rain in order to neutralize the effects of the acid. Water bodies which become contaminated with acid rain also experience a decline in the variety of aquatic life which they are able to support. As water bodies become more acidic, the diversity of organisms which can survive within them gradually decreases. Different types of plants and animals are able to tolerate different levels of acidity in the water. In general, the 7-6 young of most animal species are more sensitive to the effects of acid rain than the adults. Some types of aquatic life, such as frogs, are able to tolerate high levels of acidity in the water in which they live. However, when the frogs depend upon certain insects for their food supply and the insects are unable to tolerate the high level of acidity, the insect population could

become extinct from the stream. As a result, the frog may also end up dying due to lack of food.

Lakes, rivers and streams provide important feeding and breeding habitats for numerous varieties of fish, insects and other aquatic organisms. These organisms which include both plant and animal life and the associated environments in which they live are referred to as an ecosystem. The ecosystem consists of a very delicate balance in which all of the organisms are dependent upon each other in order to survive. Therefore, if one organism is unable to survive in an acidic environment, it’s extinction can have drastic effects upon the entire ecosystem.

Effects on Forests

Scientists and foresters believe that acid rain is one of the major causes of reduced growth of trees in the world. Other possible causes are insects, diseases and drought in addition to other air pollutants. While scientists suspect acid rain as one of the major culprits, they are currently unsure of the exact affects acid rain has on forests and are also unsure at what rates these affects may occur. Research has shown that some areas which are subject to acid rain show a great deal more evidence of damage, while others which receive the same amount of acid rain are hardly affected at all. Scientists are fairly sure that acid rain does not directly kill trees, but instead weakens the trees gradually and also makes them more susceptible to disease.

Acid rain affects forests when there is dense fog. Since fog is a type of precipitation, it too can

contain excessive levels of acid. When there are periods of dense fog, the forests are smothered by the fog and essentially surrounded by acid. The acid in the fog wears away the waxy coating present on the tree leaves, resulting in brown spots. When the leaves become damaged they are unable to undergo photosynthesis, thus inhibiting the trees ability to produce food.

This can also severely weaken the trees ability to ward off diseases and insect attacks. Once the trees have been weakened, they are also more likely to suffer damage caused by cold weather. When acid rain filters down to the forest floor and into the soil, the soil may or may not have the ability to neutralize some or all of the acid which is present in the water. This ability of the soil to neutralize the acid is referred to as the soil’s buffering capacity. A buffer is a substance which has a large capacity to absorb or release acid at a given pH. If acid is added to a system containing a buffer, the additional acid will be absorbed by the buffer and the pH will remain relatively constant. The soil acts as a buffer and resists changes in the pH of the 7-7 water. Without its buffering capacity, the soil would experience rapid fluctuations in pH. Scientists believe that the acid in the rain acts to dissolve important nutrients present in soil and then washes them away before existing vegetation has the opportunity to feed off of them in order to grow.

Acid rain does not immediately kill trees once it comes in contact with them, but instead the

acid weakens the trees by damaging the leaves which are then unable to undergo photosynthesis. As a result, the tree gradually weakens and eventually dies. Fortunately, farmers are able to apply fertilizers, such as limestone, which act as effective buffers neutralizing the soil’s acidity.

Effects on Man-Made Materials.

Evidence has shown that pollutants in the air, such as acid compounds, can actually eat away at stone, metal and paint. Thus, not only is our natural environment at risk from acid rain, but so are our buildings, statues, roads and bridges. Man made materials are expected to undergo a certain amount of deterioration over given periods of time. However, when the materials are exposed to acid rain, the deterioration happens much quicker. Repairing the damage to buildings, houses and roads can cost billions of dollars. Unfortunately, damage done to unique sculptures and statues can not be so easily repaired or replaced.

Mechanism

Acid rain is defined as any form of wet precipitation - fog, dew, snow, hail or rain - which has a pH less than 5.6. Pure water has a pH of 7.0 (neutral); however, natural, unpolluted rainwater actually has a pH of about 5.6 (acidic). Rainwater is naturally slightly acidic. The acidity of rainwater comes from the natural presence of three substances such as carbon dioxide CO₂, nitric oxide NO, and sulfur dioxide SO₂ can be founded at the lower atmosphere. CO₂ is produced naturally by the decomposition of organic material and is the primary source of acidity in unpolluted rainwater. CO₂ is produced artificially through combustion of fossil fuels. NO is produced naturally by electrical discharge. NO is produced artificially from internal combustion engines and power plants. SO₂ is produced naturally by volcanic eruptions. SO₂ is produced artificially by combustion of fossil fuels. When power stations burn coal, they release sulfur dioxide (SO₂) and nitrogen oxides (“NOx” gases such as NO₂) into the atmosphere. These chemicals sometimes fall back to the ground (dry deposition). On other occasions they combine with water vapour in the air and form dilute acids in the rainwater (wet deposition). CO₂ reacts with water to form carbonic acid (Equation 1). Carbonic acid then dissociates to give the hydrogen ion (H+) and the hydrogen carbonate ion (HCO₃^-) (Equation 2). The ability of H₂CO₃ to deliver H+ is what classifies this molecule as an acid, thus lowering the pH of a solution.

Equation 1 : CO₂ (g) + H₂O (l) -> H₂CO₃ (aq)

Equation 2 : H₂CO₃ (aq) -> H⁺ (aq) + HCO₃^- (aq)

Nitric oxide (NO) is formed during lightning storms by the reaction of nitrogen and oxygen, two common atmospheric gases (Equation 3). In air, NO is oxidized to nitrogen dioxide (NO2) (Equation 4), which in turn reacts with water to give nitric acid (HNO3) (Equation 5). This acid dissociates in water to yield hydrogen ions and nitrate ions (Equation 6), lowering the pH of the solution

Equation 3 : N₂ (g) + O₂ (g) -> 2NO (g) (lightning)

Equation 4 : 2NO (g) + O₂ (g)-> 2NO₂ (g)

Equation 5 : 3NO₂ (g) + H2O (l)-> 2HNO₃ (aq) + NO (g)

Equation 6 : HNO₃ (aq)-> H⁺ (aq) + NO₃^- (aq)

About one-fourth of the acidity of rain is accounted for by nitric acid (HNO₃). What about the other 75% of the acidity of rain? Most is accounted for by the presence of sulfuric acid (H₂SO₄) in rainwater. Although sulfuric acid may be produced naturally in small quantities from biological decay and volcanic activity, it is produced almost entirely by human activity, especially the combustion of sulfur-containing fossil fuels in power plants. When these fossil fuels are burned, the sulfur contained in them reacts with oxygen from the air to form sulfur dioxide (SO₂). Combustion of fossil fuels accounts for approximately 80% of the total atmospheric SO₂ in the United States. Sulfur dioxide, like the oxides of carbon and nitrogen, reacts with water to form sulfuric acid (Equation 7-8).

Equation 7 : SO₂ (g) + O₂ (g)-> SO₃ (g)

Equation 8 : SO₃ (g) + H₂O (l) -> H₂SO₄ (aq)

Sulfuric acid is a strong acid, so it readily dissociates in water, to give an H⁺ ion and an HSO₄^- ion (Equation 9). The HSO₄^- ion may further dissociate to give H⁺ and SO₄^2- (Equation 10). Thus, the presence of H₂SO₄ causes the concentration of H⁺ ions to increase dramatically, and so the pH of the rainwater drops to harmful levels.

Equation 9 : H₂SO₄ (aq) -> H⁺ (aq) + HSO₄ ^- (aq)

Equation 10 : HSO₄ ^- (aq) -> H⁺ (aq) + SO₄^2- (aq)

Controlling of Acid Rain

There a few type of control that can be do to controlling of acid rain. The main aim to control acid rain is to reduce the contains of nitrogen oxide and sulphur dioxide in the air.

Vehicles

A motor vehicle produces air pollutants when fuel is burnt to give mechanical power. In a totally efficient combustion process, hydrocarbons and oxygen will react to form carbon dioxide and water. However, the combustion process is never perfect; some of the hydrocarbon fuel is only partially burnt forming carbon monoxide and water, whilst some of the hydrocarbons are not combusted at all. These can, and often are, emitted from the exhaust as unburned hydrocarbons. During the combustion process the temperature can reach 2500°C. At these temperatures nitrogen and oxygen from the air in the combustion chamber react to form nitrogen oxides. Vehicle pollution can be significantly reduced by fitting a catalytic converter to the exhaust system. This is a relatively low cost method of pollution control (around £350) which has little effect on vehicle performance and fuel consumption.

The most widely used catalytic converter consists of a cylindrical ceramic body with a honeycomb structure, chemically treated and coated with platinum group metals. The honeycomb structure enables a high surface area (equivalent to three football pitches) to be incorporated within a relatively small space. This is critical to the durability and reliability of performance. The catalyst is usually incorporated into the car exhaust system. There are three basic types of catalyst, an oxidation catalyst which controls the emission of hydrocarbons and carbon monoxide by oxidising the pollutants to water and carbon dioxide, a three way catalyst which provides efficient removal of nitrogen oxides, carbon monoxide and hydrocarbons, and a diesel catalyst which controls the emissions of hydrocarbons (the characteristic diesel smell) and carbon monoxide. The catalyst will also substantially reduce smoke emissions.

Industry Emissions Control

· Pre-combustion and during combustion technology

Examples of pre-combustion technology include coal scrubbing and oil desulphurisation. Another removal process is to change the design of the boiler and to install pressurised fluidised bed combusters (FBC) which removes sulphur from coal during combustion. Another process which removes sulphur dioxide from coal during combustion is the Integrated Gasification Combined Cycle. Coal is gassified under pressure with a mixture of air and steam which results in the formation of gas which can then be burned to produce electricity.

· Post-combustion technology

One of the post-combustion controls is Flue Gas Desulphurisation (FGD). In FGD processes, waste gases are scrubbed with a chemical absorbent such as limestone to remove sulphur dioxide. There are many different FGD processes, the main ones being the limestone-gypsum process and the Wellman-Lord regenerative process. The limestone-gypsum FGD is installed at Drax in Yorkshire, the largest UK power station. This technology involves mixing limestone and water with the flue gases to produce a slurry which absorbs the sulphur dioxide. The slurry is then oxidised to calcium sulphate (gypsum) which can then be used in the building trade.

However, whilst the benefits of FGD are known, the costs of installing FGD are very high. To install a 2GW power station with FGD costs around £300 million.

Legislative controls

Legislative controls is one way to controls the emissions of a sources from the sources such as the factory of vehicles. This is because some of factory or vehicles will produce more SO₂ or NO_x than that being allowed.

With legislative control, the limit of emission the pollutant such as SO₂ can be reduced. For example, the certain model of cars is allowed to emitted 8ppm per hour. If more than controls, the users can be sued by the government because the users is polluting the environment.

Conclusion.

As a conclusion, acid rain must be prevented from getting worse year by year. There a few ways to reduce acid rain from occur such as catalytic converter is really useful to convert danger gaseous to natural gas. Acid rain is really dangerous to human health, forest, and aquatic living things. Therefore, prevention is really need to reduce acid rain.

References:

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3. Gould, Roy. 1985. Going Sour: Science and Politics of Acid Rain. Cambridge, MA: Birkhauser Boston, Inc.

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