What is acid rain? Acid rain is the term for pollution caused when sulfur and nitrogen dioxides combine with atmospheric moisture. The term ‘acid rain’ is slightly misleading, and would be more accurate if deemed ‘enhanced acid rain’, as rain occurs acidic naturally. Acidity is measured on what is know as the pH scale. Fourteen is the most basic, seven is the most neutral, and zero is the most acidic. Pure rain has a pH level of 7, which is exactly neutral. The acidity of rain is determined by the pH of pure water in reaction with atmospheric concentrations of carbon dioxide, resulting in carbonic acid. These particles partly dissociate to produce hydrogen ions and bicarbonate ions. A bicarbonate atom is an ion formed by one hydrogen atom, one carbon at atom, and three oxygen atoms, and is very effective in natural waters at neutralizing hydrogen ions and reducing acidity. The dissociation results in the natural acidity of pure rain, which is moderately acidic at a pH of 5.7. Rain less than 5.7 is considered ‘acid rain’, meaning it has reacted with acidic atmospheric gases other than carbon dioxide, such as sulfur dioxide and nitrogen dioxide. Sulfur dioxide is produced by electric utilities, industrial, commercial and residential heating, smelters, diesel engines and marine and rail transport, which creates sulfuric acid in rain. Nitrogen dioxide will also react with the rain, caused largely by transportation (cars, trucks, planes, etc.) and electric utilities, producing nitric acid. There is a certain degree of naturally occurring acidity in rain water.
This acid is from reaction with alkaline chemicals, found in soils, lakes and stream, and can occasionally occur when a volcano erupts as well. Bacterial action in soils and degasing from oceanic plankton also contribute to the acidity found in rain. More than 90% of the sulfur and 95% of the nitrogen emissions which occur in North America are due to the pollution created by humans.1 How Is Acid Rain Formed? Acid rain consists mainly of acids formed in the atmosphere. It consists of the oxides of sulfur, SO2 and SO3, and of nitrogen NO and NO2. Let us examine the major contributor to acid rain, sulfur oxides. Natural sources which emit sulfur dioxide include volcanoes, sea spray, plankton and rotting vegetation. Despite these natural occurrences, the burning of fossil fuels (such as coal and oil) can be largely blamed for the emissions. The chemical reactions begin as energy from sunlight, in the form of photons, hit ozone molecules (O3) to form free oxygen (O2), as well as single reactive oxygen atoms (O). The oxygen atoms react with water molecules (H2O), producing electrically charged, negative hydroxyl radicals (HO).
These hydroxyl radicals are responsible for oxidizing sulfur dioxide and nitrogen dioxide, which produces sulfuric acid and nitric acid. Some particles will settle to the ground (in the form of acid deposition) or vegetation can absorb some of the SO2 gas directly from the atmosphere. When sulfur dioxide comes in contact with the atmosphere, it oxidizes and forms a sulfate ion. It becomes sulfuric acid as it joins with hydrogen atoms in the air and falls down to earth. Oxidation occurs most in clouds, especially in heavily polluted air, where other compounds such as ammonia and ozone help to catalyze the reaction, increasing the amount of sulfur dioxide changing to sulfuric acid. Not all of the sulfur dioxide is converted to sulfuric acid, and it is not uncommon for a substantial amount to float up into the atmosphere, move to another area, and return to earth as sulfur dioxide, unconverted. S (in fossil fuels) + O2 =* SO2 2 SO2 + O2 =* 2 SO3 Much of the sulfur dioxide is converted to sulfur trioxide in the atmosphere SO3 + H2O =* H2SO4 The sulfur trioxide can then dissolve within water to form sulfuric acid Nitric oxide and nitric dioxide are mainly from power plants and exhaust fumes.
Similar to sulfur dioxide, reactions are heavily catalyzed in heavily polluted clouds where iron, manganese, ammonia and hydrogen peroxide are present. Also, the formation of nitric acid can trigger further reactions which release new hydroxyl radicals to generate more sulfuric acid. The following is a typical reaction, which is direct combination of nitrogen and oxygen at the high temperature inside a car engine. N2 + O2 + heat =* 2NO 2NO + O2 =* 2NO2 This nitrogen monoxide immediately reacts with oxygen and forms nitrogen dioxide in the following reaction 3NO2 + H2O =* 2HNO3 (aq) + NO The nitrogen will then dissolve in water in the atmosphere and produce nitric acid There are several other potential contributors to acid rain. These include oxidation by products of alkene-ozone reactions, oxidation by reactions of NxOy species and oxidation by peroxy radicals. Each of these reactions, however prove to be minor contributors and are rather insignificant. How Is Acid Rain Harmful? Environmental Hazards Aquatic Ecosystems Acid rain has an effect on virtually all ecosystems it touches.
Perhaps the most prominent, and equally as troubling is the harmful results it produces when in contact with lakes, streams and ponds. Scientists studying the effects of acid rain went to a lake about 135 km away from the Ontario- Manitoba border called Lake 223. This lake, so far north acid rain did not reach it, was extremely healthy, and was a perfect setting to explore the effects of acid rain on aquatic ecosystems. In 1974, scientists began to add sulfuric acid into the lake. The acid was added very slowly, and it was four years later when they saw a major change. The freshwater shrimp began to die out. Fathead minnows stopped reproducing and began to vanish. As the scientists continued adding acid to Lake 223 in low amounts, large algae mats began to form and crayfish became unhealthy and died. Seven years after the beginning of the experiment, the lake trout stopped reproducing, and most of the fish species, leeches, crawfish and mayflies began to die.
In 1984, the scientists stopped adding the acid. Without the addition of deadly sulfuric acid, the lake slowly began to recover. Some of the fish species began to recover, however some of the scientists estimated it would take one hundred years for the lake to fully recover, even without the addition of any more acid. Fish can still live in a lake with a low acid level, however they will get sick and not grow to proper proportions. Often the fish will not reproduce, and eventually, as the acid level increases, all the fish will die. The acid will also ‘leach’ metals from the bottom of the lake. There are metals contained within the mud and rocks of the lake bottom, however they remain not dangerous as long as they are not released. The acid will draw out these harmful metals and dissolve them in the water, resulting in the deterioration and disappearance of a species. One of these damaging metals is aluminum, which will coat and burn the gills of the fish as it intakes the polluted water.
Some fish found in acidic lakes contain higher levels of mercury in their bodies, which is harmful to humans, resulting in the government telling society to limit the amount of fish they eat from certain lakes and rivers. If the numbers of one species or group of species changes in response to acidification, the ecosystem of the entire body of water is likely to be affected through the predator-prey relationships. Let us examine how acid rain is dangerous to fish. A freshwater fish’s respiration consists of a ‘trade’ of hydrogen ions (H+) in their blood for sodium ions (Na+) from the water around them. If the concentration of hydrogen ions in the water is increased, which is essentially what happens when pH falls, there are (proportionally) fewer sodium ions. Fish are forced to absorb more hydrogen while finding it harder to obtain sodium.
The acidity of their blood increases, while the salt content drops. An experiment involving brown trout showed that at a pH of 5.2 or lower, this process was fatal to this species, and is likely deadly to many other trout species. The following chart shows the steps typical to freshwater fish as the acidity increases. (Fig 1-1) ACIDITY LEVEL (pH) EFFECTS ON AQUATIC LIFE 7 Neutral, H+ and H- are in balance 6.8 Shells of clams and snails become thinner, due to lack of hazardous calcium ions in the water 6.6 The viability of eggs of the fathead minnow is reduced, rain can have and fewer eggs hatch 6.5 Lake trout begin to have difficulty reproducing, clams and snails become scarcer, green algae growth increases 6 Several clam and snail species disappear, several trout species populations decrease, the smooth newt is gone, smallmouth bass, walleyes and spotted salamanders have difficulty reproducing, several mayfly species cease to lay eggs 5.8 Copepods (a critical link of crustaceans in the marine food chain) are gone, crayfish have trouble regrowing exoskeleton after molting 5.7 Several algae species decrease, while filamentous green algae increases, plankton decreases 5.5 Rainbow trout, fathead minnows and smallmouth bass lose considerable population, walleyes, brook trout, roach, lake trout and shiners don’t reproduce, leeches and mayfly larvae vanish.
5.4 Crayfish reproductivity is impaired. 5 Snail and clams are extinct. All but one species of crayfish are extinct, brook trout, walleyes and most bullfrogs are gone, most fish species experience reproduction difficulties, zooplankton population begins to drop, green and green-blue algae mats have largely spread 4.8 Leopard frog numbers decline 4.5 Mayflies and stoneflies vanish, a slowing in growth rate and oxygen uptake of bacteria is notable 4.2 The common toad disappears 4 The oxygen output of Lobelia plants declines 75% 3.5 Virtually all clams, snails, frogs, fish and crayfish vanish 2.5 Only a few species of acid-tolerant midges, bacteria and fungi are alive 2 In practical terms, the lake is sterile Two hundred and twenty lakes in Ontario have been found acidified, meaning their pH is less that 5.1 year round.2 Terrestrial Plant Life It is much more difficult to solve the mystery of forest destruction compared to that of a lake.
This is partially because trees live so much longer than fish do, and acid rain damage in trees may not show up for thirty or forty years. It is also very difficult to replicate forest conditions in a laboratory, such as insects, cold winters, pollution, elevation and abrupt changes in rainfall. Each of these conditions put stress on the trees and can be considered variables. Many scientists are convinced that because of the complexity of a forest ecosystem, it is nearly impossible to prove the death of forests is due to pollution in the form of acid rain, but deduce from many experiments it is a main factor in forest destruction. Deciduous trees are like air filters, and screen particles that pass through the air around them. These particles collect on the leaves of the tree, and studies have shown that when these particles contain acid they can cause damage to the leaves. The leaves are the part of the tree that help make food, hence any damage to the leaves will result in harm to the health of the entire tree. Coniferous trees are vulnerable to the harmful effects of acid rain as well. The tree’s needles are designed to nourish the tree after they fall to the ground. Each needle houses whole colonies of microscopic bacteria and algae that help the tree change nitrogen into food at the roots. Acid rain will often burn away this material, thereby reducing adequate food supply, and weakening the tree’s health.
After the damage has been done to leaves and needles, acid rain harms the trees even more through the soil. Soil has a level of acid. Acid in the soil can do damage to the trees by releasing aluminum, which, once in contact with acid, becomes highly poisonous to forests. The aluminum will enter the tree’s hairlike roots, choking them, and when these become clogged, the upper branches are no longer nourished. Even though there may be plenty of moisture in the soil, the tree can die of thirst. Scientists have discovered that the aluminum content in soil has tripled since the 1960s.3 Acid rain also kills important organisms on the forest floor. The process of decomposition is interrupted as the acid kills many of the bacteria and fungi that live on the forest floor. At a pH level of 4.0, the earthworm dies, further damaging the decomposition process. Without earthworms and bacteria to decompose the debris consisting of animal and bird droppings, twigs and dead leaves, the materials continue to build on the forest floor. When debris builds up, seedlings from the trees are not able to survive, because they can not work their way down to the soil to root. This causes the forest to slowly disappear, as older trees die, and the forest will not be able to rejuvenate itself.
Acid rain is hardest on trees high up in mountains, because it is often covered in mist or fog, literally bathing the trees in an acidic atmosphere. Trees also suffer because of changes in the soil. Acid rains leach metals (draw metals out of mud and rocks) in the soil, and the trees in turn intake these harmful metals through their roots. Figure 1-2 shows the damage that acid rain can to do a forest Human Health It is known that the earth contains many metals that are potentially dangerous to humans, such as lead, mercury, and aluminum. Most of the time these metals are harmless because they are in the soil, bonded to other elements. The problem occurs when acid detaches these metals from the rocks and soils, and can be carried deep into the ground and make their way to underground streams. These streams eventually connect to our water sources. Medical researchers have found these metals can be dangerous, and on rare occasion, is even fatal. Aluminum has been found to kill people who have kidney problems, and can also collect in brain tissue.
Some scientists even suspect that aluminum deposits on the brain cause Alzheimer’s disease. (A disease that results in memory loss, nervous system problems, and death. Acid rain is known to irritate the whole respiratory system, beginning with mucous membranes in the nose and throat, all the way to tissue in the lungs. Consequently, acid rain has an increased effect on people with respiratory problems. The U.S. Council on Environmental Quality estimates health-related problems due to acid precipitation cost the United States $2 billion per year.4 In August 1987, over one hundred people were treated for eye, throat, and mouth irritation when 1.8 metric tonnes of highly toxic sulfur dioxide gas leaked from an Inco plant near Sudbury, Ontario. Even Fig 1-2 This picture shows how a coniferous forest has been virtually destroyed. Acid rain is blamed for the destruction of terrestrial ecosystems around the world. without accidents, the sulfur dioxide regularly emitted from Inco smokestacks has been linked to chronic bronchitis in Inco employees.5 Drinking Water Acid rain damages drinking water in various ways.
Thus far, amounts of metals in drinking water have been minimal, however the fact that metals even leak into the water is troubling to scientists. Since metals remain in the body once ingested, over time, small amounts accumulate into large quantities, and it has yet to be concluded how large an amount will prove to be harmful to humans. Acid rain causes damage by loosening metals off metal water pipes. Modern plumbing uses plastic tubing, but older systems have copper pipes. The copper pipes are held together by a mixture of tin and lead. Lead is known to be extremely dangerous to humans, even in small amounts, and will cause damage to the brain and nervous system. A study that was done in Ontario found that water sitting in plumbing pipes for ten days contained hazardous levels of copper and lead. This discovery could be a widespread danger, since often people will go on vacation and not shut off the plumbing, allowing water to sit and absorb these dangerous metals. Acid rain can also dissolve the reinforcements that occur around large water pipes. In some parts of the United States, asbestos is used to reinforce the cement bases that hold water pipes. Asbestos is not dangerous when bound to the cement, but is highly dangerous when separated, and has been linked to cancer and other serious diseases.
Many health officials worry that loose asbestos will find it’s way to the city’s water when acid rain comes in contact with the cement. Effects On Man Made Structures Scientists are becoming increasingly concerned with acid rain’s destruction of the ‘built environment’. There are objects in our built environment that are irreplaceable. Historic landmarks and statues, old cathedrals and temples, paintings and sculpture – all are part of the built environment and are slowly being damaged. Some of these objects are practical, making life easier, safer or more comfortable. Many factors determine how much damage acid rain will do, including the amount of rain, the location, and direction of wind. All influence the amount of corrosion done. Areas that have a large amount fog or humidity tend to suffer more than dry areas, which is why many steel bridges located over water get rusted and corroded by acid. When metal is decayed, it cannot take the same amount of stress of weight as when it was originally created. Acid rain has been blamed in several collapses of bridges around the world.
Acid rain corrodes the steel track used on railroads, thus the tracks must be constantly checked. Metal in air planes can also be eaten away by acid rain. The United States Air Force spends more that $1 billion every year to repair or replace damaged parts.6 A study done in Sweden showed that metal rusts four times faster in areas that receive a lot of acid rain. This figure is staggering, and yet, metal is not the only material damaged by acid rain. Houses and buildings made of brick and stone are affected as well. Acid rain can dissolve the mortar, which is used in cement to hold bricks together. When the mortar is worn away, the bricks crumble more easily, because they shift and cannot stay intact against the heavy weight of the bricks pressuring from above. The corrosive effects of acid rain are particularly obvious on limestone, because it is composed of calcium carbonate, which is highly reactive with acid rain. Tombstones made of marble (which is metamorphosed or heated limestone) have been badly damaged, while older tombstones made of slate remain intact. Famous buildings such as the Taj Mahal, The United States Capitol building and the Lincoln Memorial in Washington, are all being continually damaged by acid rain.
Statues made of bronze and copper are particularly susceptible to corrosion. These statues turn green naturally, and this covering, called a patina, acts as a protective shield against the elements. Acid rain eats away at the patina, and where the acid dissolves the green covering, it leaves a streaky black coat. This process ruins statues throughout the world. How Does Acid Rain Affect the Economy? Canada/American Relations Canada is particularly susceptible to the effects of acid rain. Its geographical location places it directly in the path of a large amount of U.S. emission, and the granite bedrock of the Canadian Shield has a poor buffering quality. (A buffer is a material that can chemically weaken acid soil and is less harmful to the environment, such as lime or baking soda.) The lack of such a quality renders Eastern Canada highly vulnerable to damage due to United States pollution. Canada suffers more from acid rain than the United States does, even though much of the pollution originates in the United States. Acid rain costs Canadians hundreds of millions of dollars every year.
To try and decrease the large amounts of money the pollution is costing tax payers, Canada has passed laws to force its electrical companies to cut down on harmful emissions. However, no matter what laws are passed in Canada, it is not possible to stop U.S. power plants from sending acid in its direction. Figure 1-3 displays amounts of emissions created by the United States and Canada. The Gavin power plant is an excellent example of how the United States sends tonnes of acid to Canada every year. Every hour, this power plant burns 600 tonnes of coal. The higher the smokestack, the further the dangerous gases will travel, and the Gavin smokestack is 1 103 feet tall.7 Obviously, The Gavin can not be solely blamed for the pollution, but it is power plants such as these that have caused trouble between the two countries.
It is estimated that about 50% of the sulfate deposited in Canada derived from American sources.8 Sixty of the largest plants and thus largest polluters are located in the Ohio Valley, a short distance away from vulnerable Canadian land. In 1980, Canada and the United States signed a Memorandum of Intent, an agreement that both countries would make acid rain control a priority. They both promised to focus on developing ideas to cut down the amount of sulfur dioxide and nitrogen oxide emissions being pumped into the air. In the past, Canada has presented devastatingly large figures to the United States, in an attempt to have them change laws and regulations regarding pollution. Unfortunately, the attempts thus far have been unsuccessful, as the US government requests more testing and studies instead of altering laws. In the recent past, the negotiations between Canada and United States representatives have been hardly reminiscent of efforts put forth by Canadian officials. Many U.S. politicians still qualify acid rain as a ‘minor’ problem, and it is treated as such, according to Raymond Robinson, chairman of the Canadian Environmental Assembly.