Acid rain is rain that is more acidic than normal. Acid rain is a complicated problem. Caused by air pollution, acid rain’s spread and damage involves weather, chemistry, soil, and the life cycles of plants and animals on the land and from acid rain in the water. Scientists have discovered that air pollution from the burning of fossil fuels is the major cause of acid rain. Power plants and factories burn coal and oil. Power plants use that coal and oil to produce the electricity we need to heat and light our homes and to run our electric appliances. We also burn natural gas, coal, and oil to heat our homes. The smoke and fumes from burning fossil fuels rise into the atmosphere and combine with the moisture in the air to form acid rain. The main chemicals in air pollution that create acid rain are sulfur dioxide and nitrogen oxides.
Acid rain usually forms high in the clouds where sulfur dioxide and nitrogen oxides react with water, oxygen, and oxidants. This forms a mild solution of sulfuric acid and nitric acid. Sunlight increases the rate of most of these reactions. Rainwater, snow, fog, and other forms of precipitation containing those mild solutions of sulfuric and nitric acids fall to the earth as acid rain. Water moves through every living plant and animal, streams, lakes, and oceans in the hydrologic cycle. In that cycle, water evaporates from the land and sea into the atmosphere. Water in the atmosphere then condenses to form clouds. Clouds release the water back to the earth as rain, snow, or fog. When water droplets form and fall to the earth they pick up particles and chemicals that float in the air. Even clean, unpolluted air has some particles such as dust or pollen. Clean air also contains naturally occurring gases such as carbon dioxide.
The interaction between the water droplets and the carbon dioxide in the atmosphere, and to a lesser extent, from chlorine which is derived from the salt in the sea, gives rain an average pH of about 5.6, making even clean rain slightly acidic. Other natural sources of acids and bases in the atmosphere may lower or raise the pH of unpolluted rain. However, when rain contains pollutants, especially sulfur dioxide and nitrogen oxides, the rain water can become very acidic. Acid rain does not account for all of the acidity that falls back to earth from pollutants. About half the acidity in the atmosphere falls back to the earth through dry deposition as gases and dry particles. The wind blows these acidic particles and gases onto buildings, cars, homes and trees. In some instances, these gases and particles can eat away the things on which they settle. Dry deposited gases and particles are sometimes 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. The combination of acid rain plus dry deposited acid is called acid deposition. The chemical reactions that change air pollution to acid rain can take from several hours to several days. Years ago, when smokestacks were only a few stories high, pollution from smokestacks usually stayed near the ground and settled on land nearby. This caused unhealthy conditions for plants and animals near the smokestacks. To reduce this pollution, the government passed a law permitting the construction of very tall smokestacks. At that time, people thought that if the pollution were sent high into the air it would no longer be a problem. Scientists now know that this is incorrect.
Sending pollution high into the sky increases the time that the pollution stays in the air. The longer the pollution is in the air, the greater are the chances that the pollutants will form acid rain. In addition, the wind can carry these pollutants for hundreds of miles before they become joined with water droplets to form acid rain. For that reason, acid rain can also be a problem in areas far from the polluting smokestacks. Dry deposition is usually more abundant near the cities and industrial areas where the pollutants are released. There are also natural sources of acids such as volcanoes, natural geysers and hot springs. Nature has developed ways of recycling these acids by absorbing and breaking them down. These natural acids contribute to only a small portion of the acidic rainfall in the world today. In small amounts, these acids actually help dissolve nutrients and minerals from the soil so that trees and other plants can use them for food.
The large amounts of acids produced by human activities overload this natural acidity. Acid rain is poorly understood and we will probably spend billions on research ending up with two opposing groups of experts. One of the biggest money contributers to our congressmen is a lobby group dedicated to defeating or diluting any acid rain controls. “Acid Rain,” or more precisely acid precipitation, is the word used to describe rainfall that has a pH level of less than 5.6. This form of air pollution is currently a subject of great controversy because of it’s worldwide environmental damages. For the last ten years, this phenomenon has brought destruction to thousands of lakes and streams in the United States, Canada, and parts of Europe. Acid rain is formed when oxides of nitrogen and sulfite combine with moisture in the atmosphere to make nitric and sulfuric acids. The two primary sources of acid rain are sulfur dioxide, and oxides of nitrogen.
Sulfur dioxide is a colourless, prudent gas released as a by-product of combusted fossil fuels containing sulfur. A variety of industrial processes, such as the production of iron and steel, utility factories, and crude oil processing produce this gas. In iron and steel production, the smelting of metal sulfate ore, produces pure metal. This causes the release of sulfur dioxide. Metals such as zinc, nickel, and copper are commonly obtained by this process. Sulfur dioxide can also be emitted into the atmosphere by natural disasters. Ten percent of all sulfur dioxide emission comes from volcanoes, sea spray, plankton, and rotting vegetation. Overall, 69.4 percent of sulfur dioxide is produced by industrial combustion. Only 3.7 percent is caused by transportation The other chemical that is also chiefly responsible for the make-up of acid rain is nitrogen oxide.
Oxides of nitrogen is a term used to describe any compound of nitrogen with any amount of oxygen atoms. Nitrogen monoxide and nitrogen dioxide are all oxides of nitrogen. These gases are by-products of firing processes of extreme high temperatures (automobiles, utility plants), and in chemical industries (fertilizer production). Natural processes such as bacterial action in soil, forest fires, volcanic action, and lightning make up five percent of nitrogen oxide emission. Transportation makes up 43 percent, and 32 percent belongs to industrial combustion. Nitrogen oxide is a dangerous gas by itself. As mentioned before, any precipitation with a pH level less than 5.6 is considered to be acid rainfall.
The difference between regular precipitation and acid precipitation is the pH level. A pH scale is used to determine if a specific solution is acidic or basic. Any number below seven is considered to be acidic. Any number above seven is considered to be basic. The scale is color coordinated with the pH level. Most pH scales use a range from zero to fourteen. Seven is the neutral point (pure water). A pH from 6.5 to 8, is considered the safe zone. Between these numbers, organisms are in very little or no harm. Not only does the acidity of acid precipitation depend on emission levels, but also on the chemical mixtures in which sulfur dioxide and nitrogen oxides interact in the atmosphere.
Sulfur dioxide and nitrogen oxides go through several complex steps of chemical reactions before they become the acids found in acid rain. The steps are broken down into two phases, gas phase and aqueous phase. There are various potential reactions that can contribute to the oxidation of sulfur dioxide in the atmosphere each having varying degrees of success. The most common process is when sulfur dioxide reacts with moisture found in the atmosphere. When this happens, sulfate dioxide immediately oxidizes to form a sulfite ion. Afterwards, it becomes sulfuric acid when it joins with hydrogen atoms in the air. A common reaction for sulfur dioxide to becomes sulfuric acid is by oxidation by ozone. This reaction occurs at a preferable rate and sometimes is the main contributor to the oxidation of sulfuric acid.
There are other insignificant reactions that contribute to acid rain, but contribute to little to mention. These reactions unfortunately prove to be insignificant for various reasons. These reactions mentioned above, are gas phase reactions. Sulfur dioxide oxidation is most common in clouds and especially in heavily polluted air where compounds such as ammonia and ozone are in abundance. These catalysts help convert more sulfur dioxide into sulfuric acid. But not all of the sulfur dioxide is converted to sulfuric acid. In fact, a substantial amount can float up into the atmosphere, transport to another area and return to earth unconverted. Like sulfur dioxide, nitrogen oxides rise into the atmosphere and are oxidized in clouds to form nitric or nitrous acid. These reactions are catalyzed in heavily polluted clouds where traces of iron, manganese, ammonia, and hydrogen peroxide are present.
Nitrogen oxides rise into the atmosphere mainly from automobile exhaust. In the atmosphere it reacts with water to form nitric or nitrous acid. Over the years, scientists have noticed that some forests have been growing more and more slowly without reason. Trees do not grow as fast as they did before. Leaves and pines needles turn brown and fall off when they are supposed to be green. Eventually, after several years of collecting and recording information on the chemistry and biology of the forest, researchers have concluded that this was the work of acid rain. A rainstorm occurs in a forest. The summer spring washes the leaves of the branches and fall to the forest floor below. Some of the water is absorbed into the soil. Water run-off enters nearby streams, rivers, or lakes. That soil may have neutralized some or all of the acidity in the acid rainwater. This ability of neutralization is called buffering capacity.
Without buffering capacity, soil pH would change rapidly. Midwestern states like Nebraska and Indiana have soil that is well buffered. Nonetheless, mountainous northwest areas such as the Adirondack mountains are less able to buffer acid. High pH levels in the soil help accelerate soil weathering and remove nutrients. It also makes some toxic elements, for example aluminum, more soluble. High aluminum concentrations in soil can prevent plants from using the nutrients in the soil. Acid rain does not kill trees immediately or directly. Instead, it is more likely to weaken the tree by destroying its leaves, thus limiting the nutrients available to it. Or, acid rain can seep into the ground, poisoning the trees with toxic substances that are slowly absorbed through the roots.
When acid rain falls, the acidic rainwater dissolves the nutrients and helpful minerals from the soil. These minerals are then washed away before trees and other plants can use them to grow. Not only does acid rain strip away the nutrients from the plants, they help release toxic substance such as aluminum into the soil. This occurs because these metals are bound to the soil under normal conditions, but the additional dissolving action of hydrogen ions causes rocks and small bound soil particles to break down. When acid rain is frequent, leaves tend to lose their protective waxy coating, When leaves lose their coating, the plant itself is open to any possible disease. By damaging the leaves, the plant can not produce enough food energy for it to remain healthy.
Once the plant is weak, it can become more vulnerable to disease, insects, and cold weather which may ultimately kill it. Acid rain does not only effect organisms on land, but also effect organisms in aquatic biomes. Most lakes and streams have a pH level between six and eight. Some lakes are naturally acidic even without the effects of acid rain. There are several routes through which acid rain can enter the lakes. Some chemical substances exist as dry particles in the atmosphere, while others enter directly into the lake in a form of precipitation. Acid rain that has fallen on land can be drained through sewage systems leading to lakes. Another way acids can enter the lake is by spring acid shock.
When acid snow melts in the spring, the acids in the snow seeps into the ground. Some run-off the ground and into streams and lakes. Spring is a vulnerable time for many species since this is the time for reproduction. The sudden change in pH level is dangerous because the acid can cause serious deformities in their young. Generally, the young of most species are more sensitive older animals of the same apeices. But not all species can tolerate the same amount of acid. For example, frogs may tolerate relatively high levels of acidity, while snails are more sensitive to pH changes. Acid molecules can cause mucus to form in the gills of fish, preventing the fish to absorb oxygen well. Also, a low pH level will throw off the balance of salt in the fish’s tissue. Calcium levels of some fish cannot be maintained due to the changes in pH level. This causes a problem in reproduction: the eggs are too brittle or weak.
Lacking calcium causes weak spines and deformities in bones. Sometimes when acid rainfall runs off the land, it carries fertilizers with it. Fertilizer helps stimulate the growth of algae because of the amount of nitrogen in it. However, because of the increase in the death of fish the decomposition takes up even more oxygen. This takes away from surviving fish. In other terms, acid rain does not help aquatic ecosystems in anyway. Acid rain does not only damage the natural ecosystems, but also man-made materials and structures. Marble, limestone, and sandstone can easily be dissolved by acid rain. Metals, paints, textiles, and ceramic can easily be corroded. Acid rain can also downgrade leather and rubber. Man-made materials slowly deteriorate even when exposed to unpolluted rain, but acid rain helps speed up the process.
Acid rain causes carvings, monuments, and statues carved in stones to lose their features. In limestone, acidic water reacts with calcium to form calcium sulfate. For iron, the acidic water produces an additional proton giving iron a positive charge. When iron reacts with more oxygen it forms iron oxide (rust). The repairs on building and monuments can be quite costly. In 1990, the United States spent thirty-five billion dollars on paint damage. In 1985, the Cologne Cathedral cost the Germans approximately twenty million dollars in repairs. The Roman monuments cost the Romans about two hundred million dollars from acid rain repairs. Most importantly, acid rain can affect health of a human being. It can harm us through the atmosphere or through the soil from which our food is grown and eaten from.
Acid rain causes toxic metals to break loose from their natural chemical compounds. Toxic metals themselves are dangerous, but if they are combined with other elements, they are harmless. The toxic metals might be absorbed by the drinking water, crops, or animals that a human may consume. These foods that are consumed could cause nerve damage to children, severe brain damage or even death. Scientists believe that aluminum is somehow related to Alzheimer’s disease. One of the serious side effects of acid rain on humans is respiratory problems. The sulfur dioxide and nitrogen oxide emission gives risk to respiratory problems such as dry coughs, asthma, headaches, and eye, nose, and throat irritation. Polluted rainfall is especially harmful to those who already suffer from asthma or those who have a hard time breathing. But even healthy people can have their lungs damaged by acid air pollutants.
Acid rain can aggravate a person’s ability to breathe and could even lead to death. In 1991, the United States and Canada signed an air quality agreement. Ever since that time, both countries have taken actions to reduce sulfur dioxide emission. The United States agree to reduce their annual sulfur dioxide emission by about ten million tons by the year 2000. A year before the agreement, the Clean Air Pact Amendment tried to reduce nitrogen oxide by two million tons. This program focused on the source that emits nitrogen oxide, automobiles and coal-fired electric utility boilers. Since most nitrogen oxide emissions are from cars, catalytic converters must be install on cars to reduce this emission. The catalytic converter is mounted on the exhaust pipe, forcing all the exhaust to pass though it. This converter looks like a dense honeycomb, but it is coated with either platimun, palladium, or rhodium.
This converts nitrogen oxides, carbon dioxides and unburned hydrocarbons into a cleaner state. To reduce sulfur dioxide emission, utility plants are required to do several steps by the Clean Air Act Amendment. Before combustion, these utilities plants have to go through a process call coal cleaning. Another way to reduce sulfur dioxide before combustion is by burning coal with low sulfur content. Low sulfur content coals are called subituminous coal. This process in reducing sulfur dioxide is very expensive due to the high demand of subituminous coal. During combustion, a process called Fluidized Bed Combustion, is used to reduce sulfur dioxide emissions into the atmosphere. This process contains limestone or a sandstone bed that are crushed and diluted into the fuel.
Flue gas is the mixture of gases resulting from combustion and other reactions in a chamber. This enables the limestones to react with sulfur dioxide and reduce emission by 90 percent. After combustion, a process known as wet flue gas desulfurization is taken into action. This process requires a web scrubber at the downward end of the boiler. This process is very similar to Fluidized Bed Combustion. This scrubber can be made of either limestone or sodium hydroxide. Limestone is more commonly used. As sulfur dioxide enters this area it reacts with the limestone. After being scrubbed, which is the term used for the phase after coal has past the wet scrubber, the flue gas is re-emmited and the waste solids are disposed. Acid rain is an issue that can not be over looked. This phenomenon destroys anything it touches or interacts with it. When acid rain damages the forest or the environment it affects humans in the long run.
Once forests are totally destroyed and lakes are totally polluted animal populations begin to decrease because of lack of food and shelter. If all the animals, which are our food source, die out, humans too would die out. Acid rain can also destroy our homes and monuments that we hold dearly. What humans can do, as citizens, to reduce sulfur and nitrogen dioxide emission is to reduce the use of fossil fuels. Car pools, public transportation, or walking can reduce tons of nitrogen oxide emissions. Using less energy benefits the environment because the energy used comes from fossil fuels which can lead to acid rain. For example, turning off lights not being used, and reduce air conditioning and heat usage. Replacing old appliances and electronics with newer energy efficient products is also an excellent idea.
An alternative power source can also be used in power plants to reduce emissions. These alternatives are: geothermal energy, solar power energy, wind energy, and water energy. In conclusion, the two primary sources of acid rain is sulfur dioxide and nitrogen oxide. Automobiles are the main source of nitrogen oxide emissions, and utility factories are the main source for sulfur dioxide emissions. These gases evaporate into the atmosphere and then oxidized in clouds to form nitric or nitrous acid and sulfuric acid. When these acids fall back to the earth they do not cause damage to just the environment but also to human health. Acid rain kills plant life and destroys life in lakes and ponds.
The pollutants in acid rain causes problem in human respiratory systems. The pollutants attack humans indirectly through the foods they consumed. They effected human health directly when humans inhale the pollutants. Governments have passed laws to reduce emissions of sulfur dioxide and nitrogen oxide, but it is no use unless people start to work together in stopping the release of these pollutants. If the acid rain destroys our environment, eventually it will destroy us as well.