Seminar topic for happenings of Acid rain, causes and it effect, Essays (university) of Chemistry

Download Seminar topic for happenings of Acid rain, causes and it effect and more Essays (university) Chemistry in PDF only on Docsity! ABSTRACT Acid rain is one of the major environmental threats since 19th century. This paper reviews the 2012 progress report of US EPA (2013) and summarizes the issue in various environmental aspects. Significant reduction in the SO2, NOx emission and deposition of acid have been occurred via the active implementation of Clean Air Interstate Rule (CAIR), Acid Rain Program (ARP) and NOx budget training program(NBP). Cross state air pollution rule and litigation (CSAPR) implemented by US EPA since 2011, reduces the cross boundary movement of effluents between US and Canada. US national composite means of average SO2 annual mean ambient concentration has been declined by 85% in the period between 1980 and 2012. INTRODUCTION “Acid rain” is a broad term used to describe several ways that acids fall out of the atmosphere. A more precise term is acid deposition, which has two parts: wet and dry. Wet deposition refers to acidic rain, fog, and snow. As this acidic water flows over and through the ground, it affects a variety of plants and animals. The strength of the effects depend on many factors, including how acidic the water is, the chemistry and buffering capacity of the soils involved, and the types of fish, trees, and other living things that rely on the water. Dry deposition refers to acidic gases and particles. About half of the acidity in the atmosphere falls back to earth through dry deposition. The wind blows these acidic particles and gases onto buildings, cars, homes, and trees. Dry deposited gases and particles can also be washed from trees and other surfaces by rainstorms. When that happens, the runoff water adds those acids to the acid rain, making the combination more acidic than the falling rain alone. Prevailing winds blow the compounds that cause both wet and dry acid deposition across state and national borders, and sometimes over hundreds of miles. Scientists discovered, and have confirmed, that sulfur dioxide (SO2) and nitrogen oxides (NOx) are the primary causes of acid rain. In the US, About 2/3 of all SO2 and 1/4 of all NOx comes from electric power generation that relies on burning fossil fuels like coal. Acid rain occurs when these gases react in the atmosphere with water, oxygen, and other chemicals to form various acidic compounds. Sunlight increases the rate of most of these reactions. The result is a mild solution of sulfuric acid and nitric acid. the ecosystem’s ability to buffer acidic input. A low value indicates a sensitive ecosystem with low buffer capacity and vice versa. [5] Deposition rate: Rate at which acid species and/or precursors are transferred from the atmosphere to the ground, expressed as unit of material per unit area per unit time. Deposition rates together with critical loads have become the critical parameters in Europe in determining whether an ecosystem is under threat of acidification. Dry deposition: Deposition of acids or acid precursors from the atmosphere onto plant foliage and other solid surfaces by adsorption and direct uptake in the absence of liquid water. The rate at which this occurs depends on the 'deposition velocity'. The size of this coefficient varies according to the surface. Typical values for SO2 deposited on foliage are 0.5-1.0 cm/s. H2SO4: The principal strong acid of anthropogenic origin responsible for the acid in rain. It is either by the oxidation of SO2 to SO3 which then combines with water molecules in the atmosphere to form H2SO4 aerosols or by the dissolution of SO2 in water and its subsequent oxidation. Its formation usually takes between 1 and 14 days in the gas phase. In the aqueous phase, as in cloud or fog, formation can take minutes to hours. HNO3: The second major strong acid contributing to acid of anthropogenic origin. It is an unavoidable product of fossil fuel combustion and is technically more difficult to control. NOx : NO and NO2 are the species of most significance in the formation of acid precipitation due to the formation of HNO3. Nitrate ion commonly present in aerosols is derived from nitric acid. pH: A number which gives a measure of the concentration of hydrogen ions in a solution of water. The smaller the number, the higher the concentration of hydrogen ions. Transportation: Atmospheric transport is the movement of materials, in this case acids and their precursors, in patterns governed by meteorological conditions.6 Wet deposition: It is usually calculated from rainfall data and chemical analyses of rainfall ion composition. Dry deposition is calculated from pollutant deposition velocity and ground-level pollutant concentration (see below). Estimates of both parameters are subject to numerous uncertainties e.g. canopy effects on throughfall composition, in the case of wet deposition, and variations in deposition velocities for different surface types, in the case of dry deposition. Flue Gas Desulfurization: This process begins with either electrostatic precipitation or fabric filters removing fly ash from the combustion gases. The fly ash is then carted away. The flue gases are forced into a slurry of lime and water (also known as slaked lime, or calcium hydroxide, Ca (OH)2) under oxidizing conditions provided by compressed air. The following reactions take place: SO2 + H2O F 0A E H+ + HSO3– H+ + HSO3– + ½ O2 F 0A E2H+ + SO4 2– 2H+ + SO42– + Ca(OH)2 F 0A E CaSO 4.2H2O The acid rain-causing sulfur dioxide (SO2) goes in, the construction product calcium sulfate dihydrate, CaSO4.2H2O (also known as gypsum, plasterboard, or wallboard) – at about 98% purity – comes out. 7 ACID RAIN: CAUSES AND EFFECTS Acid rain: is a rain or any other form of precipitation that is unusually acidic, meaning that it has 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 sulfur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce acids. Some governments have made efforts since the 1970s to reduce the release of sulfur dioxide and nitrogen oxide into the atmosphere with positive The United States emits almost 20 million tons of sulfur dioxide every year, with three-quarters coming from the burning of fossil fuels by electric utilities.[11] Coal burned in most parts of the world is high in sulphur. Transportation, residential combustion, smelters and other industrial processes are the other man-made contributions to SO2 emissions. During smelting, ores containing sulphur are roasted at high temperatures and the sulphur is driven off as SO2. Natural resources such as volcanoes and marsh gases contribute to a small percentage of this concentration through gases such as hydrogen sulphide and dimethyl sulphide which are produced by the action of soil bacteria on rotting vegetation and on inorganic sulphate. When they enter the air, these sulphur compounds are rapidly oxidized to acid sulphate. Virtually all the sulphur deposited in precipitation is in the form of acid sulphate. Typically, less than 5% of the sulphur is dissolved SO2, and this remnant is rapidly oxidized to acid after falling to earth.[12] Smoke stacks are used to emit industrial fumes. It is assumed that the higher the smoke is emitted, the better it is for the atmosphere. However, emissions from tall smoke stacks remain aloft longer and have more time to be oxidized to acid than do emissions from stacks of lesser height since the residence times of pollutants emitted higher into the atmosphere is longer. Hence, sulphur and nitrogen fumes emitted by high smoke stacks are more easily converted to acidic pollutants. The amount of uncontrolled SO2 emissions from a utility or industrial boiler depends on the amount and sulphur content of the fuel burned, the type and operating characteristics of the boiler, and other chemical and physical properties of the fuel.13 There are several natural sources of NOx. These come from denitrification of the soil. 78% of the atmosphere is made up of nitrogen and 80% of this is from anthropogenic sources. Therefore, the other factor that affects the formation of NOx is temperature. The higher the temperature, the greater the formation of NOx. Aircraft fumes as well as fossil fuel combustion from vehicles contribute to the NOx in the atmosphere. This is a direct emission of nitrogen. Fossil fuel combustion is a source of several nitrogen oxides, including N2O and NOx.[14] Another source is coal-fired power plants. Another contribution is via the action of anaerobic bacteria on livestock wastes and commercial inorganic fertilizers. A fourth source is from the burning of grasslands and clearing of forests.15 Natural sources of NOx include forest fires, lightning, oxidation of ammonia and so on. Other pollutants include particulates, hydrocarbons and carbon monoxide. Trifluoroacetic acid is an atmospheric breakdown product of the chlorofluorocarbon replacements HCFC-123, HCFC-124, and HFC-134a. Trifluoroacetic acid partitions into the various aqueous phases that occur throughout the environment. HFC’s and HCFC’s have greater reactivity and therefore lower atmospheric lifetimes than their predecessors, the CFC’s. Because of this heightened reactivity and reduced tropospheric residence time, the HFC’s and HCFC’s are less likely to be transported to the stratosphere where they might mediate the photochemical destruction of ozone.[16] Therefore, the HFC’s and HCFC’s are likely to cause less environmental damage than the CFC’s. Ammonia is another determinator of acid rain. It generally exists as an alkaline vapour with the capacity to neutralize either sulphuric or nitric acid in the atmosphere. It is readily soluble in water and dissolves to form ammonium and hydroxyl ions. Ammonia can react directly with the sulphur in the atmosphere to form ammonium sulphate particles. Most ammonia emissions are released into the atmosphere by natural and biological processes, primarily through the decay and decomposition of organic matter and some through forest fires.[17] THE PROCESS SULPHUR Most airborne acid sulphate appears to be formed in cloud droplets. SO2 dissolves to form HSO3-, which then reacts with hydrogen peroxide (H2O2) to form acid sulphate. H2O2 is the most efficient oxidant in the conversion of dissolved SO2 to H2SO4. This reaction is a product of photochemistry. The lower the pH the faster the reaction proceeds. The oxidation of dissolved SO2 is rapid even at a pH value below 5.18 The oxidation of SO2 to acid sulphate is also catalyzed on the surface of fine particulates present from smoke stacks. The reaction rate is relatively slow. The conversion of SO2 to acid takes only several hours to several days, while NOx conversions take place within hours.[19] Reactions with ozone in solution also are important. Dissolved oxygen in water can slowly oxidize sulfur dioxide, but the reaction is faster if catalyzed by ions of transition metals (such as iron, manganese, and vanadium) or by carbon soot particles. Oxidation of sulphur dioxide by dissolved oxygen in clouds is relatively unimportant compared with oxidation induced by ozone or hydrogen peroxide.20 NITROGEN of materials, the surface area of the materials and the weather, as wet surfaces may remove more sulphur from the atmosphere than dry ones. WET DEPOSITION Wet deposition is a more complex process than dry deposition. In order for wet deposition to occur, the pollutant has to mix with and get attached to the particles of cloud, rain or snow. The pollutant then has to react with the water form. Wet deposition is independent of the surface. OCCULT DEPOSITION Occult or cloud deposition partly resembles dry- and partly wet deposition. It is independent of chemical reactivity, but depends on the structure of the receptor.[26] THE EFFECTS SOILS AND VEGETATION Nitrogen is the growth depleting factor in most ecosystems. Inputs of nitrogen are usually taken up by vegetation and soils. Hence, soils are quite resistant to acidification. After the acid rain enters the soil, it causes nutrients such as calcium and magnesium to be leached from the soil. This deprives the plants of their basic nutrients as well as causes harm to nearby water bodies and to the ground water. Sulphur affects plants in a fatal manner by entering through the plant cell. Sulphur dioxide comes in contact with the chlorophyll of the cell and the other constituents of the cells [particularly water], and is converted there into corrosive sulfuric acid which immediately destroys the tissues in its vicinity. It has been seen that the acidification of the subsoil begins quite soon after the acidification of the topsoil, and that subsoils can become very acidic. The problem is that, although topsoil acidity can be reversed with lime quite quickly, subsoil acidity cannot be corrected until surface soil acidity has been alleviated. Lime does not penetrate to the subsoil while the surface soil is acid. [27] FORESTS Trees in forests have been found to be affected by pollutants in the air. The main causes of this degradation are SO2, NOx, H2SO4 and HNO3. Pollutants can also be absorbed from the soil. This causes the tree to be affected from its roots upward. Infection of the roots is the easiest way to kill a tree. WATER Excess deposition of nitrogen can lead to increased amounts of nitrate which aid in the acidification of lake waters. Acidic deposition affects aquatic life. Acidification may eliminate sensitive algae species and decrease phosphorous and inorganic carbon concentrations.28 It can also cause damage to fish populations. Heavy metals removed from the soil during rains could cause death to aquatic life. Fish absorb polluted water through their gills and this can harmful effect on them such as the amount of oxygen taken up by the blood is reduced and the blood circulation is affected. Lakes in mountainous regions often have crystalline bedrock, with thin soils and sparse vegetation, which together give rise to surface water with low ionic inputs. Such waters are particularly sensitive to inputs of atmospheric pollutants and to changes in climate. They are much less affected by local pollution from agriculture and wastewater and therefore are good indicators of widespread environmental changes. [29] Sulphur dioxide that comes down during wet deposition remains in water bodies for a long time. Microbial reduction of sulphate to sulphide occurs just below the mudwater interface, where anoxic conditions prevail. The reduced sulphur combines with ferrous iron or organic matter to form insoluble sulphides, neutralizing the sulfuric acid. However, as lake levels decline during warming or drought, sulphur stored in upper areas of the littoral zone is reoxidized, causing lakes to reacidify.30 When rain seeps into the ground, it is usually stopped between the soil surface and the groundwater by a layer of soil that has a filtering effect. However, their buffer capacity eventually gets exhausted. This is when the acid begins to get to the water and the pH level falls. HUMAN HEALTH Effects on human health are usually seen through the food chain by bioaccumulation and by water contamination. The chemicals that get deposited in the soil and water are consumed either directly by humans or by way of the food chain. In this way they affect human beings. The acid in the water may corrode copper and lead water pipes contaminating the drinking water. [31] Air pollution does not usually cause adverse reactions immediately. It takes some time for the body to react to the pollutant. This is through Acidity and alkalinity are measured using a pH scale for which 7.0 is neutral. The lower a substance's pH (less than 7), the more acidic it is; the higher a substance's pH (greater than 7), the more alkaline it is. Normal rain has a pH of about 5.6; it is slightly acidic because carbon dioxide (CO2) dissolves into it forming weak carbonic acid. Acid rain usually has a pH between 4.2 and 4.4.[10] Policymakers, research scientists, ecologists, and modelers rely on the National Atmospheric Deposition Program’s (NADP) National Trends Network (NTN) for measurements of wet deposition. The NADP/NTN collects acid rain at more than 250 monitoring sites throughout the US, Canada, Alaska, Hawaii and the US Virgin Islands. Unlike wet deposition, dry deposition is difficult and expensive to measure. Dry deposition estimates for nitrogen and sulfur pollutants are provided by the Clean Air Status and Trends Network (CASTNET). Air concentrations are measured by CASTNET at more than 90 locations.[11] When acid deposition is washed into lakes and streams, it can cause some to turn acidic. The Long-Term Monitoring (LTM) Network measures and monitors surface water chemistry at over 280 sites to provide valuable information on aquatic ecosystem health and how water bodies respond to changes in acid-causing emissions and acid deposition. Combustion of fuels produces sulfur dioxide and nitric oxides. They are converted into sulfuric acid and nitric acid.[12] CHEMICAL PROCESSES Combustion of fuels produces sulfur dioxide and nitric oxides. They are converted into sulfuric acid and nitric acid.[42] Gas phase chemistry In the gas phase sulfur dioxide is oxidized by reaction with the hydroxyl radical via an intermolecular reaction:[5] SO2 + OH· → HOSO2· which is followed by: HOSO2· + O2 → HO2· + SO3 In the presence of water, sulfur trioxide (SO3) is converted rapidly to sulfuric acid: SO3 (g) + H2O (l) → H2SO4 (aq) Nitrogen dioxide reacts with OH to form nitric acid: This shows the process of the air pollution being released into the atmosphere and the areas that will be affected. NO2 + OH· → HNO3 Chemistry in cloud droplets When clouds are present, the loss rate of SO2 is faster than can be explained by gas phase chemistry alone. This is due to reactions in the liquid water droplets. Hydrolysis Sulfur dioxide dissolves in water and then, like carbon dioxide, hydrolyses in a series of equilibrium reactions: SO2 (g) + H2O ⇌ SO2·H2O SO2·H2O ⇌ H+ + HSO3− HSO3− ⇌ H+ + SO32− Oxidation There are a large number of aqueous reactions that oxidize sulfur from S(IV) to S(VI), leading to the formation of sulfuric acid. The most important oxidation reactions are with ozone, hydrogen peroxide and oxygen (reactions with oxygen are catalyzed by iron and manganese in the cloud droplets).[13] Technical solutions Many coal-firing power stations use flue-gas desulfurization (FGD) to remove sulfur-containing gases from their stack gases. For a typical coal- fired power station, FGD will remove 95% or more of the SO2 in the flue gases. An example of FGD is the wet scrubber which is commonly used. A wet scrubber is basically a reaction tower equipped with a fan that extracts hot smoke stack gases from a power plant into the tower. Lime or limestone in slurry form is also injected into the tower to mix with the stack gases and combine with the sulfur dioxide present. The calcium carbonate of the limestone produces pH-neutral calcium sulfate that is physically removed from the scrubber. That is, the scrubber turns sulfur pollution into industrial sulfates.[14] In some areas the sulfates are sold to chemical companies as gypsum when the purity of calcium sulfate is high. In others, they are placed in landfill. The effects of acid rain can last for generations, as the effects of pH level change can stimulate the continued leaching of undesirable chemicals into otherwise pristine water sources, killing off vulnerable insect and fish species and blocking efforts to restore native life.[15] Fluidized bed combustion also reduces the amount of sulfur emitted by power production. Vehicle emissions control reduces emissions of nitrogen oxides from motor vehicles. International treaties International treaties on the long-range transport of atmospheric pollutants have been agreed for example, the 1985 Helsinki Protocol on the Reduction of Sulphur Emissions under the Convention on Long-Range Transboundary sulphur are also being used. The main process involving coal microbial degradation is the removal of inorganic sulphidic minerals.38 Switching to low sulphur coal may be a good alternative, but it also has some drawbacks. Western low-sulfur coal has a lower bituminous value than most high-sulfur coals. Power plants therefore have to burn more of it to generate the same amount of electricity. Thus, they produce more carbon dioxide and contribute more to the problem of global warming. Low-sulphur coal also tends to have more mercury and other trace metals than do other coals. Therefore, the switch to low-sulfur coal has likely worsened the North American toxic air pollution problem.39 In coal fired power plants, sulphur emissions are removed with a "scrubber", where a limestone slurry is injected into the flue gas to react with the SO2. The resulting gypsum slurry can eventually be used in other industrial processes. The main problem with scrubbers is that they are expensive, and they decrease the overall operating efficiency of a power plant. The decreased efficiency results in increased emissions of carbon dioxide, a major greenhouse gas.40 With respect to agricultural fertilizers, most of them increase the risk of acidification. However, using calcium sulphate as the source of sulphur will cause no acidification. Ammonium sulphate, on the other hand, increases the rate of acidification.41 Similarly, with nitrogen, producing it at lower temperatures and at shorter durations of combustion, less NOx is produced. There are also ways of reducing the nitrogen emissions from motor vehicles by fitting them with 3-way catalytic converters, which filter out nitrogen oxides. This can be further aided by the introduction of lead free gasoline. Switching to alternative sources of energy may be the best alternative we have left. Nuclear energy, geothermal energy, hydro electricity, wind power are all forms of energy that have been found to be more efficient than the burning of coals. As far as nuclear energy is concerned, however, it is a matter of replacing one evil with another. The following are some of the practices that are on China’s agenda to improve the quality of rain. All new coal- fired power plants are required to install particulate control devices, such as electrostatic precipitators and fabric filters, which can remove more than 99%of particulate emissions. There is also a ban on the digging of new mines that contain high-sulphur coal. Many large cities have begun to switch from gasoline to cleaner fuels, such as liquefied natural gas and liquefied petroleum gas, for taxicabs and urban mass-transportation fleets. To raise public awareness of air pollution and its impact, nearly 60 cities in China publish reports on air quality at least once a week.42 The discovery of the decade has been emissions trading. Emissions trading has been revolutionary in the sense that it has facilitated a rather stark change in the philosophy of air-pollution control policy in the United States. Traditionally the government was responsible for defining environmental goals, for dictating the best control technologies for meeting those goals, and for monitoring and enforcing compliance with its mandates. This proved to be an excessively challenging responsibility because of the sheer number of substances, sources and possible control strategies. Although setting standards and monitoring and enforcing compliance remain government responsibilities, under emissions-trading strategies, emitters have the opportunity to use their own ingenuity to determine the best way to comply with the goals. Introducing this flexibility has resulted in substantially lower compliance costs, higher levels of compliance and more innovative ways to control pollution (including promotion of pollution prevention rather than more traditional end-of-pipe reduction technologies).43 Some members of the EU, such as Sweden, want a minimum limit set on emissions, not a maximum. They feel that if a nation wants too enforce a more stringent limit, it should be permitted to without any threat of being considered against fair competition. "The directive should set only a minimum requirement, allowing countries to go further if they so wish," argues Per Elvingson of the Swedish Society for Nature Conservation. "The primary aim of the directive is not to maintain free trade but to protect the environment." [17] This is a fact that the rest of the world had yet to recognize. So far, there are some individuals and groups who initiate pollution prevention and reduction plans and there are other groups who implement it. It is for the implementation group to realize that reduction and prevention has to occur not just because refusal to do so will result in penal action, but because it is what is required for the well being of the environment.[18] “Acid Rain,” 1995 Information Please ™ Almanac, Annual 1995. “Mountain Lakes; Sensitivity to Acid Deposition and Global Climate Change,” Ambio, Vol. 27, No. 4, June, (1998). Chenggang (Charles), Wang, “China’s Environment in the Balance,” World and I, Vol. 4, No. 10, October, (1999). Deng, Yiwei, et al, “Factors affecting the levels of hydrogen peroxide in rainwater,” Atmospheric Environment, Vol. 33, (1999). Goulding, K.W.T., L.Blake, “Land use, liming and the mobilization of potentially toxic metals,” Agriculture, Ecosystems and Environment, Vol. 67, (1998). Kroeze, Caroline, “Potential for mitigation of emissions of nitrous oxide from the Netherlands (1980-2015),” Ambio, Vol. 27, No.2, March , (1998). Munton, Don, “Dispelling the myths of the acid rain story,” Environment, Vol. 40, No. 6, July-August,(1998). Raloff, Janet, “When Nitrate reigns: air pollution can damage forests more than trees reveal,” Science News, Vol. 147, No. 6, Feb, (1995). Rubiera, F, et al, “Biodesulfurization of Coals of Different Rank: Effect on Combustion Behavior,” Environmental Science and Technology, Vol. 33, No. 3, (1999). Tickell, Oliver, “Burning oil fuels acid rain,” Geographic Magazine, Vol. 68, No. 2, Feb., (1996). Tietenberg, Tom, “Acid Rain and Environment Degradation: The Economics of Emission Trading,” American Scientist, Vol.86, No. 1, Jan- Feb, (1989). Wujcik, Chad E., et al, “Trifluoroacetic acid levels in 1994-1996 fog, rain, snow and surface waters from California and Nevada,” Chemosphere, Vol.36, No.6, (1998). BOOKS Bennet, David A, “The Acidic Deposition Phenomenon and its Effects: Critical Assessment Document,” Washington: Office of Acid Deposition, Environmental Monitoring, and Quality Assurance, August, (1985). Brunnee, Jutta, “Acid Rain and Ozone Layer Depletion: International Law and Regulation,” U.S.A.: Trans. National Publications Inc., (1988). Environmental Resources Limited for Commission for the European Communities, “Acid Rain,” New York: Unipub, (1983). GCA Corporation for U.S. Department of Energy, “Acid rain information book,” May, (1983).,2nd edition. Gould, Ray, “Going Sour: Science and Politics of Acid Rain,” (1985). Boston: Bikhauser. Hultberg, Hans, Richard Skeffington, “ Experimental reversal of acid rain effects,” New York: John Wiley and sons, (1998). Irving, M. Patricia, “Acidic deposition: State of science and technology,” September, (1991). Miller, G. Tyler, “Living in the environment,” (1999). California: Brooks/ Cole Publishing Co. 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