Acid Rain: How is Acid Rain Formed? Effects, Causes, Definition, Facts

Definition of acid rain

Air pollution produced when acid chemicals are incorporated into rain, snow, fog or mist. The "acid" in acid rain comes from sulfur oxides and nitrogen oxides, products of burning coal and other fuels and from certain industrial processes. The sulfur oxides and nitrogen oxides are related to two strong acids: sulfuric acid and nitric acid. When sulfur dioxide and nitrogen oxides are released from power plants and other sources, winds blow them far from their source. If the acid chemicals in the air are blown into areas where the weather is wet, the acids can fall to Earth in the rain, snow, fog or mist. In areas where the weather is dry, the acid chemicals may become incorporated into dusts or smokes. Acid rain can damage the environment, human health and property.

Effects of Acid Rain - Surface Waters and Aquatic Animals

The ecological effects of acid rain are most clearly seen in the aquatic, or water, environments, such as streams, lakes, and marshes. Acid rain flows into streams, lakes, and marshes after falling on forests, fields, buildings, and roads. Acid rain also falls directly on aquatic habitats. Most lakes and streams have a pH between 6 and 8, although some lakes are naturally acidic even without the effects of acid rain. Acid rain primarily affects sensitive bodies of water, which are located in watersheds whose soils have a limited ability to neutralize acidic compounds (called “buffering capacity”). Lakes and streams become acidic (i.e., the pH value goes down) when the water itself and its surrounding soil cannot buffer the acid rain enough to neutralize it. In areas where buffering capacity is low, acid rain releases aluminum from soils into lakes and streams; aluminum is highly toxic to many species of aquatic organisms.

Where Does Acid Rain Affect Lakes and Streams?

Many lakes and streams examined in a National Surface Water Survey (NSWS) suffer from chronic acidity, a condition in which water has a constant low pH level. The survey investigated the effects of acidic deposition in over 1,000 lakes larger than 10 acres and in thousands of miles of streams believed to be sensitive to acidification. Of the lakes and streams surveyed, acid rain caused acidity in 75 percent of the acidic lakes and about 50 percent of the acidic streams. Several regions in the U.S. were identified as containing many of the surface waters sensitive to acidification. They include the Adirondacks and Catskill Mountains in New York state, the mid-Appalachian highlands along the east coast, the upper Midwest, and mountainous areas of the Western United States. In areas like the Northeastern United States, where soil-buffering capacity is poor, some lakes now have a pH value of less than 5. One of the most acidic lakes reported is Little Echo Pond in Franklin, New York. Little Echo Pond has a pH of 4.2.

Acidification is also a problem in lakes that were not surveyed in federal research projects. For example, although lakes smaller than 10 acres were not included in the NSWS, there are from one to four times as many of these small lakes as there are larger lakes. In the Adirondacks, the percentage of acidic lakes is significantly higher when it includes smaller lakes.

Streams flowing over soil with low buffering capacity are as susceptible to damage from acid rain as lakes. Approximately 580 of the streams in the Mid-Atlantic Coastal Plain are acidic primarily due to acidic deposition. In the New Jersey Pine Barrens, for example, over 90 percent of the streams are acidic, which is the highest rate of acidic streams in the nation. Over 1,350 of the streams in the Mid-Atlantic Highlands (mid-Appalachia) are acidic, primarily due to acidic deposition.

The acidification problem in both the U.S. and Canada grows in magnitude if “episodic acidification” is taken into account. Episodic acidification refers to brief periods during which pH levels decrease due to runoff from melting snow or heavy downpours. Lakes and streams in many areas throughout the U.S. are sensitive to episodic acidification. In the Mid-Appalachians, the Mid-Atlantic Coastal Plain, and the Adirondack Mountains, many additional lakes and streams become temporarily acidic during storms and spring snowmelt. For example, approximately 70 percent of sensitive lakes in the Adirondacks are at risk of episodic acidification. This amount is over three times the amount of chronically acidic lakes. In the mid-Appalachians, approximately 30 percent of sensitive streams are likely to become acidic during an episode. This level is seven times the number of chronically acidic streams in that area. Episodic acidification can cause “fish kills.”

Emissions from U.S. sources also contribute to acidic deposition in eastern Canada, where the soil is very similar to the soil of the Adirondack Mountains, and the lakes are consequently extremely vulnerable to chronic acidification problems. The Canadian government has estimated that 14,000 lakes in eastern Canada are acidic.

How Does Acid Rain Affect Fish and Other Aquatic Organisms?

Acid rain causes a cascade of effects that harm or kill individual fish, reduce fish population numbers, completely eliminate fish species from a waterbody, and decrease biodiversity. As acid rain flows through soils in a watershed, aluminum is released from soils into the lakes and streams located in that watershed. So, as pH in a lake or stream decreases, aluminum levels increase. Both low pH and increased aluminum levels are directly toxic to fish. In addition, low pH and increased aluminum levels cause chronic stress that may not kill individual fish, but leads to lower body weight and smaller size and makes fish less able to compete for food and habitat.

Some types of plants and animals are able to tolerate acidic waters. Others, however, are acid-sensitive and will be lost as the pH declines. Generally, the young of most species are more sensitive to environmental conditions than adults. At pH 5, most fish eggs cannot hatch. At lower pH levels, some adult fish die. Some acid lakes have no fish. The chart below shows that not all fish, shellfish, or the insects that they eat can tolerate the same amount of acid; for example, frogs can tolerate water that is more acidic (i.e., has a lower pH) than trout.

How Does Acid Rain Affect Ecosystems?

Together, biological organisms and the environment in which they live are called an ecosystem. The plants and animals living within an ecosystem are highly interdependent. For example, frogs may tolerate relatively high levels of acidity, but if they eat insects like the mayfly, they may be affected because part of their food supply may disappear. Because of the connections between the many fish, plants, and other organisms living in an aquatic ecosystem, changes in pH or aluminum levels affect biodiversity as well. Thus, as lakes and streams become more acidic, the numbers and types of fish and other aquatic plants and animals that live in these waters decrease.

How Is Acid Rain Formed?

Acid rain is caused by the release of the gases SO2 (sulphur dioxide) and NOX (nitrous oxides). The main sources of NOX emissions are vehicles and fuel combustion.

Sulphur dioxide reacts with water vapour and sunlight to form sulphuric acid. Likewise NOX form nitric acid in the air. These reactions takes hours, or even days, during which polluted air may move hundreds of kilometers. Thus acid rain can fall far from the source of pollution.

When mist or fog droplets condense they will remove pollutants from the air and can become more strongly acid than acid rain. Even snow can be acid. Gases and particles, not dissolved in water, with a low pH can also be deposited directly onto soil, grass and leaves. It is possible that even more acidity is deposited in this way than by rain. Not much is known about this process, and it is particularly difficult to study.

Effects Felt Through the Food Chain

Acid rain describes any form of precipitation with high levels of nitric and sulfuric acids. It can also occur in the form of snow, fog, and tiny bits of dry material that settle to Earth.

Rotting vegetation and erupting volcanoes release some chemicals that can cause acid rain, but most acid rain falls because of human activities. The biggest culprit is the burning of fossil fuels by coal-burning power plants, factories, and automobiles.

When humans burn fossil fuels, sulfur dioxide (SO2) and nitrogen oxides (NOx) are released into the atmosphere. These chemical gases react with water, oxygen, and other substances to form mild solutions of sulfuric and nitric acid. Winds may spread these acidic solutions across the atmosphere and over hundreds of miles. When acid rain reaches Earth, it flows across the surface in runoff water, enters water systems, and sinks into the soil.

Acid rain has many ecological effects, but none is greater than its impact on lakes, streams, wetlands, and other aquatic environments. Acid rain makes waters acidic and causes them to absorb the aluminum that makes its way from soil into lakes and streams. This combination makes waters toxic to crayfish, clams, fish, and other aquatic animals.

Some species can tolerate acidic waters better than others. However, in an interconnected ecosystem, what impacts some species eventually impacts many more throughout the food chain—including non-aquatic species such as birds.

Acid rain also damages forests, especially those at higher elevations. It robs the soil of essential nutrients and releases aluminum in the soil, which makes it hard for trees to take up water. Trees' leaves and needles are also harmed by acids.

The effects of acid rain, combined with other environmental stressors, leave trees and plants less able to withstand cold temperatures, insects, and disease. The pollutants may also inhibit trees' ability to reproduce. Some soils are better able to neutralize acids than others. In areas where the soil's "buffering capacity" is low, the harmful effects of acid rain are much greater.

The only way to fight acid rain is by curbing the release of the pollutants that cause it. This means burning fewer fossil fuels. Many governments have tried to curb emissions by cleaning up industry smokestacks and promoting alternative fuel sources. These efforts have met with mixed results. But even if acid rain could be stopped today, it would still take many years for its harmful effects to disappear.

Individuals can also help prevent acid rain by conserving energy. The less electricity people use in their homes, the fewer chemicals power plants will emit. Vehicles are also major fossil fuel users, so drivers can reduce emissions by using public transportation, carpooling, biking, or simply walking wherever possible.

Causes of Acid Rain

The main cause of acid rain is air pollution. There are several man-made as well as natural causes which lead to air pollution. Some of the man-made causes I am sure you must be knowing; burning things, exhaust from cars, smoking, burning of fuel etc. The natural causes are usually the chemicals released from a volcano. The volcano releases sulfur dioxide which causes acid rain. Smoke released from factories or cars, contains some invisible gases which are harmful to the environment. Some of the main gases are nitrogen oxide and sulfur dioxide and when they mix with the water droplets that are present in the atmosphere, they form nitric acid and sulfuric acid, and when they fall in the form of rain, they are called acid rain. Now that we know what causes acid rain, let us now learn about its various forms.

Forms of Acid Rain

Some people think that acid rain is actually 'rain'. But I am sorry to say it's not. Depending upon the temperature of a particular location, the 'rain' might also be 'snow'. Wet regions have acid rain, while very cold areas have acid snow or acid fog. Dry regions generally get acid gas or maybe acid dust. These are the various forms of acid rain.

Impact on Forestation

Acid rain has a major impact on forestation as well as on crops. Trees are greatly harmed by it. Some experts have pointed out that in several parts of the world, the trees in the forests are dying away due to it. Leaves of the plants have a waxy layered protection on them. The acid rain damages this layer and causes harm to the trees. In the process of photosynthesis, water is turned into gas, but when acid rain starts destroying the plant, gas is seeping into the leaves, thus taking the place of the water. What happens next is that the plant cannot take in carbon dioxide and thus, it starts dying. Now you know about the acid rain effects on plants.


The acid rain deposits certain harmful or toxic metals in the forest and it also has a major impact on the soil. The rain consists of some toxic metals such as zinc, lead, chromium, copper etc. When there is acid rain, these metals are released and they stunt the growth of the trees which include the moss, the algae, and even the nitrogen-fixing bacteria. These are the ones that help the forest to grow. The soil contains important nutrients that are required for the plant to grow. Acid rain releases aluminum in the soil which destroys these nutrients and thus destroying the plants.

Impact on Water

Acid rain shows a pH balance below the 5.6 level. When the pH balance goes below 7, it means that the substance is more acidic. When acid rain mixes with water, it is harmful for the sea animals. It kills them and when they die, they decompose and remain at the bottom of the sea. This causes further harm to the water and also to its inhabitants.

Impact on Humans

Another acid rain effect is on human health. It causes respiratory problems in humans. Whatever we eat or drink or even breathe, might have come in contact with the acid and when we take this in, it causes harm to our health. Records show that each year, in the United States and Canada, nearly 550 premature deaths take place due to acid rain.

Facts About Acid Rain

  • Acid rain can corrode buildings, statues, monuments and stone. It also corrodes marbles. Hence, the Taj Mahal in India is at a great risk.

  • Some areas can become very foggy mainly due to acid rain.

  • No matter how healthy a person is acid rain can cause health problems to anyone.

  • Most acid rain problems occurs mainly because of the coal burner plants in the Western countries.

  • Acid rain carries the chemicals sulphur dioxide and nitrogen oxide.

  • Acid "rain" isn't just rain; acid snow, acid fog or mist, acid gas, and acid dust all have the same effect as acid rain.

  • Lakes and streams are normally acidic, but acid rain can make them so acidic that it damages animal and plant life.

  • Acid rain can cause a mass killing to trees.

  • Area's can become very hazy and foggy due to acid rain.

  • Acid rain (which is linked with acid air pollution) has been linked to breathing and lung problems in children, and people who have asthma.

  • Even people in exceptional health could have their lungs damaged by this acid air.

What You Can Do To Prevent Acid Rains

Government agencies and scientists are not the only ones that can take action to stop acid rain. You can become part of the solution, too!

Understand the Problem

The first step you can take is to understand the problem and its solutions.

Conserve Energy

Since energy production creates large amounts of the pollutants that cause acid rain, one important step you can take is to conserve energy. You can do this in a number of ways:

  • Turn off lights, computers, televisions, and other electrical equipment when you're not using them.

  • Buy equipment that uses less electricity, including lights, air conditioners, heaters, refrigerators, and washing machines. Such equipment might have the Energy Star label.

  • Try to limit the use of air conditioning.

  • Adjust the thermostat (the device used to control the temperature in your home) when you go on vacation.

Minimize the Miles

Driving cars and trucks also produces large amounts of nitrogen oxides, which cause acid rain. To help cut down on air pollution from cars, you can carpool or take public transportation, such as buses and trains. You can also walk or bike to a nearby store or friend’s house instead of driving.

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