|ACID RAIN: CAUSES AND EFFECTS
“Perhaps when all is said and done, it is not really so remarkable that acidification could go unnoticed for years- right up to the end of the 1960’s. In contrast to environmental influences of many other kinds, acidification is a furtive process-in its early days almost unnoticeable. Our senses of smell and taste are not capable of distinguishing between acidified and unaffected lake or well water. The clear limpid water in an acidic forest lake can also, in many cases, lend it a deceptive beauty. And the trees growing in an acidified forest area look just like trees anywhere else, at least as long as the acidification is moderate.”
-Swedish National Environmental Protection Board
TERMS AND DEFINITIONS
Acid deposition: General term used to describe any process by which acids or acid precursors in the are transferred to the earth's atmosphere surface. It supersedes the term 'acid rain'.1
Acid precipitation: Best known mechanism of acid deposition in which rain scavenges acids from the atmosphere and is acidified as a consequence. form of precipitation (rain, snow, sleet, or hail) containing high levels of sulfuric or nitric acids (pH below 5.5–5.6). Produced when sulfur dioxide and various nitrogen oxides combine with atmospheric moisture, acid. 2
Acid precursors: Chemicals which react with normal atmospheric or terrestrial chemicals to form acids. SO2, NO, NO2 and NH3 which ultimately form H2SO4 and HNO3, are the main chemical species of interest in acid deposition. 3
Acid rain: Widely used term which in different contexts appears to be equivalent to 'Acid deposition', 'Acid precipitation' or even atmospheric fallout. Because of its ambiguity it has been replaced in most contexts with the previous two terms.
Adsorption: Adsorption is the ability to incorporate or to take fluid up like a sponge, but to retain and solidify it.
Anoxic: Anoxic conditions are those where there is no oxygen present.
Critical Loads: Estimates of how much pollution the environment can absorb without damage, are called critical loads. 4 The CL value indicates 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 than
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.
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 H+ + HSO3–
H+ + HSO3– + ½ O2 2H+ + SO4 2–
2H+ + SO42– + Ca(OH)2 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
WHAT IS ACID RAIN
Acid rain occurs when the pollutants that come from immobile sources such as smokestacks, power plants, and mobile sources such as cars rise up into the clouds and fall back to earth as contaminated rainfall. The rain becomes acidic because of gases which dissolve in the rain water to form various acids. As the name suggests, acid rain is just rain which is acidic. Rain is naturally slightly acidic because of the carbon dioxide dissolved in it (which comes from human and animals breathing and to a smaller extent from nitrogen compounds that come from the soil and the seas as part of the nitrogen cycle.
Acidic water contains an excess of hydrogen ions. Absolutely pure (distilled) water contains equal numbers of acidic and basic ions (H+ and OH-).8
This gives rain a pH of around 5.0, and in some parts of the world it can be as low as 4.0 (this is typical around volcanoes, where the sulphur dioxide and hydrogen sulphide form sulphuric acid in the rain). Before the Industrial Revolution, the pH of rain was generally between 5 and 6, so the term acid rain is now used to describe rain with a pH below 5.9
Acidic water contains an excess of hydrogen ions. Absolutely pure water contains equal numbers of acidic and basic ions. The pH of pure water is 7,which is a neutral pH. This means that the water is neither acidic nor alkaline. Ordinary unpolluted rainwater has a pH of 5.6.
Acid rain is formed by the reaction of rain water to a combination of various gases. These reactions and the subsequent transformation will be studied here. We will also see where these gases arise from and what their main sources on earth are. Acid rain has some rather disastrous consequences on human, plant and animal life. However, this is not as bad as it seems. We have managed to find some remedies for this situation. What remains to be seen is how successful these will be.
WHY DO WE HAVE ACID RAIN?
- THE SOURCES
Unlike in deposition, anthropogenic source emissions of SO2, NOx and VOC’s, do not vary from season to season.10
The basic components of acid rain are SO2 ,NOx, VOC’s (volatile organic compounds) and several others. Most of the sulphur present in the atmosphere of the Northern Hemisphere is from anthropogenic sources. Coal and lignite power stations contribute to a large amount of this pollution.
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
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
In the photochemical relationship between nitric oxide, ozone, and nitrogen dioxide, the concentration of these chemical species is directly affected by the intensity of sunlight. These chemicals are also known to react photochemically with hydrocarbons and other atmospheric chemicals to form photochemical "smog." 21 NO2 reacts faster with OH to form acid nitrate than does sulphur. In polluted air, NOx can react with organic matter to produce peroxyacetyl nitrate (PAN), a pollutant which can be transported long distances before it is eventually converted to acid nitrate. Reactions between NOx and H2O2 are very slow. HO2 reacts with NO and then nitrous oxide reacts with hydroxyl.
HO2 + NO OH + NO2
NO2 + OH HNO3
The atmospheric degradation of the HFC’s and HCFC’s is initiated by the abstraction of hydroxyl (OH) resulting in the formation of an alkyl radical. This radical reacts with oxygen to yield an alkyl peroxy radical. This reacts with nitric oxide to produce NO2. 22
The rate of this reaction depends on the concentration. During daytime conversion, an important photochemical cycle takes place in which both the production and destruction of ozone occurs. As part of this process, NO2 is converted to nitric acid and water vapour.
The basic reactions go as follows: 23
OH + SO2 HSO3
HSO3 + O2 HSO5
HSO5 + NO [HSO4 + NO2]
HSO4 + NO2 H2SO4 + HNO3
Inputs of trifluoroacetic acid into natural water systems occur through wet and dry deposition, directly from the vapour stage and from runoff from the surrounding watershed.
Dry deposition of gases and particles is dependent on the reactivity of the gases and the size distribution of the particles, as well as to the surface on which the dry deposition occurs. Deposition increases in hilly areas.24 Dry deposition occurs in the following ways: (1) the absorption or adsorption of gases by exposed surfaces such as vegetation, soil, water and manmade structures; (2) gravitational settling of relatively coarse particles; (3) impaction of fine particulate on vegetation and other surfaces.25
Sulphur in the form of acid sulphate and SO2 can also be deposited to the earth in the form of dry deposition. SO2 is adsorbed by soil, leaves and stones and then is oxidized to acid sulphate. The rate of adsorption is proportional to the amount of SO2 in the air. It also depends on the types 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 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 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
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
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.
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 effects 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.
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 inhalation. SO2 and NOx have adverse effects on the human respiratory system and lungs if there is exposure to high levels or high concentrations of these pollutants. Also, as dealt with earlier, contaminants can be leached into the drinking water. These are mainly metals. Metals such as copper and lead are also leached from household cisterns and pipes and directly enter domestic water supplies. Bioaccumulation through the food chain is another problem.
The damage from acid rain to terrestrial wildlife is basically through the food chain. Accumulated heavy materials cause great damage through bio-magnification. This occurs because each successive level of the food chain accumulates more of the pollutant and passes it on to the next level.
A large variety of materials are affected by acid rain. These extend from sandstone and limestone to metals such as zinc, aluminium, copper, plastics, paper, textiles, electrical contacts e.t.c. However, the corrosion processes are not very well understood as yet.32 Limestone and marble are particularly affected. The acid dissolves the calcium carbonate in the stone, and this solution evaporates, forming crystals within the stone. As these crystals grow, they break apart the stone, and the structure crumbles.33
Acid precipitation has an accelerated effect on corrosion by forming a layer of moisture on the metallic surface and by adding hydrogen and sulfate ions. However, rain can also wash away sulfates deposited during dry deposition and can, therefore, retard corrosion.34
The most disastrous effects of acid rain that are visible to the naked eye, are the effects on old monuments and buildings of historical importance. These include the Taj Mahal in India, the Acropolis in Greece and many others around the world.
LONG RANGE TRANSPORTATION OF AIR POLLUTANTS
Polluted air masses containing sulphur, nitrogen, and POP’s (persistent organic pollutants) can travel long distances and affect regions far from the source of the pollution. To take just a few examples, the problems that the U.S. and Canada have had over acid rain is due to the fact that the pollutants that originate in the U.S. move over Canada, due to wind flows and cause acid rain there. Another case is that of India and China. The pollutants emitted by China move southwards with the wind currents. Here they collect over the Indian Ocean located at the southern tip of India. Then due to further wind currents they move over the central part of the Indian subcontinent, where they come down as acid rain in the monsoon season.
Four meteorological variables are particularly significant in the transport and dispersion of air pollution: the mixing height below which air and pollutants mix freely, and the wind, temperature, and moisture within this layer. The wind patterns in the layer below the mixing layer cause confused wind movements.35
The effects of acid rain can never be reversed. The sooner we realize this the better it will be for all concerned. These are some possible mitigation measures.
Acidic substances lead to a low pH. Therefore, in order to reduce the effect of the acidity, the pH should be increased. This can be done by the introduction of alkaline substances. Acidic waters can be remedied by using acid neutralizing chemicals such as limestone [CaO, Ca(OH)2] and lime (CaCO3). This process is known as liming. This is an effective agent for restoring water quality. An alternative suggestion is fertilization of surface waters by adding phosphorous to increase algal productivity and generate acid neutralizing capacity.36
There are however, two limitations on this. Firstly, it can only be applied in certain cases and it can reverse acidification only to a certain degree. Liming has to be repeated several times. This could turn out to be costly in the long run. Metal rich acid water meanwhile will continue to flow into the water body. There is an inherent danger of these becoming redissolved when the acidity rises again.37 Therefore, liming is purely an intermediate measure akin to the uselessness of a painkiller, which merely relieves the problem but does not eliminate it.
Sulphur emissions can be reduced before combustion by physical, chemical or biological coal cleaning; during combustion by sorbent injection; after combustion by flue gas desulphurization; or by the most obvious, though not always possible method of using low-sulphur coals alone or as components of coal blends. Microbial methods to remove 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."44 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.
“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).
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.