Acid Rain

Michael A. Hartmann

Acid Rain


Acid Rain


Acid rain is a form of air pollution that is mainly produced by human activity.  Damage from acid rain has been widespread in Eastern North America, throughout Europe, Japan, China and Southeast Asia.  These damages have affected the world’s soil, plants, animals, bodies of water and human made structures.  Many countries have identified acid rain as a real threat to the environment and are attempting to restrain pollutants to reduce the production of acid rain.

Fundamentals of Acid Rain

Acid rain is the catch all term that is generally used to describe ways that acids descend out of the atmosphere.  Environmental scientists use the expression “acid deposition” to more precisely describe these events.  Acid deposition is divided into two parts, wet deposition and dry deposition (U.S. Environmental Protection Agency, 2002).

Wet deposition is used to illustrate acidic precipitation in the form of rain, fog, dew or snow.  Wet deposition represents about half of the acidity deposited from the atmosphere.  While dry deposition may not be as frequently addressed in the media, they can introduce from 20% to 60% the remaining half of the acidity represented from the atmosphere.  Dry deposition is acidic gases and particles that can travel with the wind over hundreds of miles.  Rain or snow can combine with dry deposition to render the precipitation increasingly acidic (U.S. Environmental Protection Agency, 2002).

In order to understand the fundamentals of acid rain, it is necessary to understand what an acid is and how it is measured.  An acid was first defined by Svante Arrhenius in 1884 as a material that can release a proton or hydrogen ion.  Bases, or alkali, which are the opposite of acids are defined as a material that can donate a hydroxide ion.  In 1923, G.N. Lewis further broadened the definitions of acids as being electron pair acceptors and bases as electron pair donors (Chemtutor, 2003).

Acidity is calculated using a pH scale system with units ranging from 0 to 14.  Substances that possess the lower the pH number are increasingly acidic.  A substance with a pH number from 0 to 6 is considered to be an acid.  A substance such as lemon juice which has a pH number of 2 would be more acidic than tomato juice with a pH number of 4.  Substances with a pH number ranging from 8 to 14 are the opposite of being acidic, and are called bases, or alkalis.  Bleaches have a pH number of about 13 which is more basic and than baking soda with a pH number of 9.  A substance with a pH of 7 is neither an acid nor a base and is considered balanced or neutral.  Mixing an alkali with an acid will cause the reaction and the mixture to become less acidic (Encarta, 2003).  The reaction also creates a combination of salt and water (Chemtutor, 2003).

Acids have the characteristics that they release a hydrogen ion into a water solution.  Acids are corrosive to metals when they are introduced.  Acids also taste sour as is found in lemon juice and vinegar.  In contrast, bases release a hydroxide ion into a water solution.  Bases will denature, or destroy protein on contact.  Bases also have the distinction of tasting bitter and can be found in Milk of Magnesia (Chemtutor, 2003).

Litmus is the oldest known pH indicator.  Litmus is one of a number of organic compounds that can be used to indicate between an acidic substance and an alkaline.  Acids will turn blue litmus to red while bases will turn red litmus to blue.  The hydrogen-electrode method can also be used to identify the pH number of a substance when further information is required (Chemtutor, 2003).

Distilled or pure water has a pH neutral number of 7.  Rain that is unpolluted has a slightly acidic pH of 5.6.  Rain water is somewhat acidic because carbon dioxide (CO2) in the atmosphere reacts with water to form a weak acid called carbonic acid (H2CO3) (U.S. Geological Survey, 1997).

Rain is considered to be dubbed acid rain when precipitation is highly acidic, having a pH number lower than 5.6.  Rain becomes more acidic when mixed with air pollutants stemming from fossil fuels such as coal, natural gas, oil and from a selection of forms of manufacturing (Encarta, 2003).  The burning of these fossil fuels emits gaseous sulfur dioxide (SO2) and nitrogen oxides (NO, NO2) into the atmosphere.  Acid deposition 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 acidic solutions fall to the earth as acid rain (U.S. Environmental Protection Agency, 2002).

The most common source of sulfur dioxide comes from electric power plants.  Power plants contribute to 70 percent of sulfur dioxide in the United States.  Nitrogen oxides are introduced to the atmosphere from a combination of sources.  In the United States, automobiles account for 43 percent of the nitrogen oxides injected into the atmosphere.  These contaminants are able to travel hundreds of miles with the wind to contaminate regions that may have other wise been almost acid free (Encarta, 2003).  The Northeastern region of the United States is especially affected by acid rain due to population, industry and prevailing wind directions (U.S. Geological Survey, 1997).

Uncontaminated rain with a pH number of 5.6 or higher is not considered a threat to the environment.  This is because nature neutralizes normal rain by the alkaline chemicals in the environment.  These chemicals are found in rocks, soils, lakes and streams (Encarta, 2003).  These acid-buffering materials react with the rain to bring pH balance to the environment (Environment Canada, 2003).

Acid rain, containing higher acidity levels, upsets the balance between the alkaline chemicals on the earth’s surface.  The higher acidity makes it difficult for nature to neutralize and balance the acidity.  These acid buffers become depleted over time.  As the acidity builds up in an area, damage occurs in the form of soil and plant degradation.  Lakes, rivers and streams are also affected as well as the wildlife in the waters.  Building erosion has its taken toll on the nation’s most sacred buildings (U.S. Environmental Protection Agency, 2002).

Effects of Acid Rain


Acid rain dissolves and washes away nutrients that are present in soil that is needed by foliage.  Acid rain can also dissolve toxic substances in soil.  These naturally present substances such as aluminum and mercury are freed by the acids to progress on to pollute water or poison plants.  Some soils are relatively alkaline and can neutralize acid deposition for an indefinite period.   However, thin soils found in mountainous regions may only be able to buffer acids temporarily (Encarta, 2003).

In the Northeast of the United States, acid rain has leached calcium, magnesium, and aluminum substances from the soil. Calcium and magnesium are essential nutrients for foliage.  The absence of these nutrients also attributes to the ability of the soil to counter acid rain (Ember, 2001).

Trees and Plants

When nutrients are removed from the soil, acid rain slows down the growth of trees and plants.  Acid rain also directly affects trees by eating holes in the waxy coating of the leaves needles resulting in dead brown spots.  This causes trees to lose some of its ability to produce food through photosynthesis.  These impaired leaves are susceptible to disease from organisms.  Additionally, weakened trees and plants are more likely to be vulnerable to insect infestations, drought and cold temperatures (Encarta, 2003).  These conditions are blamed for a depleting forest ecosystem in the Northeast (Ember, 2001).


The vast majority of farm crops are less affected by acid rains than are forests.  The deep soils of many farm localities, such as those in the Midwestern United States, can soak up and deactivate hefty amounts of acid.  Mountain farms are more at risk because the slim soil in these higher elevations can not soak up as much acid.  Farmers can thwart acid rain damage by monitoring the condition of the soil and adding crushed limestone to the soil when needed.  Farmers can furthermore add nutrient-rich fertilizer if any nutrients have been exhausted (Encarta, 2003).

Surface Waters

Acid rain falls into and drains into streams, lakes, rivers and marshes.  Most natural waters are near pH 7, or neutral.  Acid rain has caused some waters in the Northeastern United States to contain a pH value of 5 or less.  This reveals that these bodies of water are ten times more acidic than they should be.  Many lakes with this level of acidity have lost some or all of certain species of fish (Encarta, 2003).

As acidity increases, progressively more species of plants and animals decline or disappear.  As the water draws near to the level of pH 6, crustaceans, insects, and some plankton species begin to fade away.  As pH 5 approaches, major changes in the composition of the plankton community transpire.  This produces a less desirable species of mosses and plankton that invade the body of water.  Fish populations may begin to deteriorate as the less acid tolerant fishes disappear first.  Water that plunges bellow pH 5 will be principally devoid of fish.  The bottom will be covered with undecayed matter and the territories nearby the shore may be subjugated by mosses (Environment Canada, 2003).


The consequences of acid rain on animals can be extensive.  If a population of one plant or animal is adversely affected by acid rain, the animals that feed on that organism may also suffer.  The ecosystem itself may become endangered.  In the Netherlands, songbirds are finding fewer snails to eat because snails are declining due to acid rain.  The eggs that are produced by these birds have been found to have weakened shells because the birds are receiving less calcium that they would ingest from these snails (Encarta, 2003).
Humans drink water and breathe air that has come into contact with acid deposition.  Acid deposition can increase the levels of toxic metals such as aluminum, copper, and mercury in untreated drinking water supplies.  Canadian and United States studies indicate that there is a connection between this pollution and respiration troubles in children and asthmatics (Environment Canada, 2003).


Many buildings and monuments are made of stone.  Granite is now the most widely used stones for buildings and bridges.  Limestone is the second most used building stone.  Sandstone was widely used in the Northeastern United States prior to the 20th century.  Marble and limestone are commonly used in the nation to build monuments.  Granite and some varieties are resistant to acid rain, however some sandstone dissolves easily when they come into contact with even weak acids.  Limestone and marble are also as vulnerable to acids.  Since monuments are composed of limestone and marble they are more likely to be damaged by acid precipitation (U.S. Geological Survey, 1997).

Since acid rain is higher in the Northeast where Washington D.C. is located, many of the nation’s monuments have suffered damage from acid rain.  The Capital building shows an example of building dissolution and blackened alteration.  Additional national treasures such as the Grant Memorial and the Jefferson Memorial are suffering the same fate (U.S. Geological Survey, 1997).

Controlling Acid Rain

The National Acid Precipitation Assessment Program (NAPAP), a federally sponsored program, has commissioned studies on how acid rain forms, and its effects.  Scientists are researching effective control technologies to limit emissions from power plants and automobiles.  The government has adapted changes in regulations to attempt to reduce air pollution that causes acid deposition (U.S. Geological Survey, 1997).

The 1990 Clean Air Act Amendments commenced a remarkable reduction in emissions of sulfur dioxide and nitrogen oxides by electric power plants.  Power plants are allotted an allowance of SO2 emissions each year.  These allowances can be traded by companies to each other like commodities. The Environmental Protection Agency oversees that companies adhere to these regulations (Burtraw, 1998).


Wet and dry deposition is an environmental concern and is a more proven scientific fact than other concerns such as ozone depletion or global warming which are highly debated.  This issue should continue to be a focus to limit or eliminate pollutants that cause acid rain.  In order to provide a balanced ecosystem for plants and animals alike to survive in the future, the world’s nations and its industries must strive to provide a healthy environment in which we all share.  Future generations must be insured that the current generations provide a balanced ecosystem to maintain a high quality environment in which to exist. However, acid deposition is only one of the misfortunes of developing into the modern world.  Nonetheless it is a good starting point to protect the future of all generations to come.


Burtraw, D. (1998). Costs and benefits of reducing air pollutants related to acid rain Contemporary Economic Policy, 16.  Retrieved October 12, 2003, from Proquest database
Chemtutor. (2003). Acids and bases.  Retrieved October 11, 2003, from
Ember L. (2001). Acid rain still a menace.  Chemical & Engineering News, 14.  Retrieved October 11, 2003, from Proquest database
Encarta. (2003). Acid rain.  Retrieved October 10, 2003, from
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U.S. Environmental Protection Agency. (2002, October).  Acid rain.  Retrieved October 12, 2003, from
U.S. Geological Survey. (1997, July).  What is acid rain?  Retrieved October 12, 2003, from


©2018 Michael A. Hartmann

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