Acid Rain Causes and Effects
During the early 1980s and 90s, dramatic visions of acid precipitation corroding buildings and killing trees captured the minds of the public. Since then, the threat of “acid rain” has lost some of its celebrity appeal, due in part to government policies aimed to mitigate the problem and environmental issues, such as global warming, that have taken centerstage. But acid deposition continues to affect ecological systems and is likely contributing to forest degradation, fish kills, and tainted water quality. But what exactly is acid deposition and where does it come from? What are its short-term and long-term ecological effects?
What is Acid Rain?
Acid deposition, or acid rain as it is commonly known, occurs when emissions of sulfur dioxide (SO2) and oxides of nitrogen (NOx) react in the atmosphere with water, oxygen, and oxidants to form various acidic compounds. These compounds then fall to the earth in either dry form (such as gas and particles) or wet form (such as rain, snow, and fog).
Two Forms of Acid Deposition
Wet deposition is any form of precipitation that removes acids from the atmosphere and deposits them on the Earth’s surface. Dry deposition polluting particles and gases stick to the ground via dust and smoke in the absence of precipitation. This form of deposition is dangerous however because precipitation can eventually wash acid pollutants into streams, lakes, and rivers.
Acid Base Balance
Acidity itself is determined based on the pH level of the water droplets. PH is the scale measuring the amount of acid in the water and liquid. The pH scale ranges from 0 to 14 with lower pH being more acidic while a high pH is alkaline; seven is neutral. Normal rain water is slightly acidic and has a pH range of 5.3-6.0. Acid deposition is anything below that scale. It is also important to note that the pH scale is logarithmic and each whole number on the scale represents a 10-fold change.
Today, acid deposition is present in the northeastern United States, southeastern Canada, and much of Europe including portions of Sweden, Norway, and Germany. In addition, parts of North East Asia, South Africa, Sri Lanka, and Southern India are all in danger of being impacted by acid deposition in the future.
Acid Rain Causes and History
Acid Deposition can be caused both naturally and through human activity
Acid deposition can occur via natural sources like volcanoes but it is mainly caused by the release of sulfur dioxide and nitrogen oxide during fossil fuel combustion. When these gases are discharged into the atmosphere they react with the water, oxygen, and other gases already present there to form sulfuric acid, ammonium nitrate, and nitric acid. These acids then disperse over large areas because of wind patterns and fall back to the ground as acid rain or other forms of precipitation.
Acid Deposition ocurrs through power generation
The gases responsible for acid deposition are normally a byproduct of electric power generation and the burning of coal. As such, it began entering the atmosphere in large amounts during the Industrial Revolution and was first discovered by a Scottish chemist, Robert Angus Smith, in 1852. In that year, he discovered the relationship between acid rain and atmospheric pollution in Manchester, England.
Although it was discovered in the 1800s, acid deposition did not gain significant public attention until the 1960s and the term acid rain was coined in 1972. Public attention further increased in the 1970s when the New York Times published reports about problems occurring in the Hubbard Brook Experimental Forest in New Hampshire.
In the United States today, roughly 2/3 of all (SO2) and 1/4 of all (NOx) come from electric power generation that relies on burning fossil fuels, like coal.
Acid Rain Effects
Acid Rain Effect of Aquatic Ecosystem
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After studying the Hubbard Brook Forest and other areas today, there are several important impacts of acid deposition on both natural and man-made environments. Aquatic settings are the most clearly impacted by acid deposition though because acidic precipitation falls directly into them. Both dry and wet deposition also runs off of forests, fields, and roads and flows into lakes, rivers, and streams.
As this acidic liquid flows into larger bodies of water, it is diluted but over time, acids can accrue and lower the overall pH of the body. Acidic water leaches aluminum and other toxic metals from rocks, soil and decaying plants (see Adirondack Council Report p. 15), where it otherwise would have remained bound to other elements in a harmless state. If the pH of a lake drops below 4.8, fish are unable to absorb oxygen from the water, slowly suffocating. It is estimated that around 50,000 lakes in the United States and Canada have a pH below normal (about 5.3 for water). Several hundred of these have a pH too low to support any aquatic life.
Acid Rain Effect on Trees and Plants
Aside from aquatic bodies, acid deposition can significantly impact forests. As acid rain falls on trees, it can make them lose their leaves, damage their bark, and stunt their growth. By damaging these parts of the tree, it makes them vulnerable to disease, extreme weather, and insects. Acid falling on a forest’s soil is also harmful because it disrupts soil nutrients, kills microorganisms in the soil, and can sometimes cause a calcium deficiency. Trees at high altitudes are also susceptible to problems induced by acidic cloud cover as the moisture in the clouds blankets them.
Damage to forests by acid rain is seen all over the world, but the most advanced cases are in Eastern Europe. It’s estimated that in Germany and Poland, half of the forests are damaged, while 30% in Switzerland have been affected.
Acid Rain Effect on Buildings
Finally, acid deposition also has an impact on architecture and art because of its ability to corrode certain materials. As acid lands on buildings (especially those constructed with limestone) it reacts with minerals in the stones sometimes causing it to disintegrate and wash away. Acid deposition can also corrode modern buildings, cars, railroad tracks, airplanes, steel bridges, and pipes above and below ground.
Acid rain is rain consisting of water droplets that are unusually acidic because of atmospheric pollution – most notably the excessive amounts of sulfur and nitrogen released by cars and industrial processes. Acid rain is also called acid deposition because this term includes other forms of acidic precipitation such as snow.
Where is Acid Deposition a Problem?
The U.S. Geological Survey (USGS) is the lead federal agency for the monitoring of wet atmospheric deposition in the United States from 250 active sites. In the U.S., the ecological effects of acid deposition vary from region to region.
Acid Rain Effects in Eastern North America
In the eastern U.S., ecological damage from acidification (nitric and sulfuric acid deposition) is widely apparent in the Adirondack and Catskill Mountains of New York. Many lakes and streams in these areas are no longer able to maintain fish populations, and water quality in these areas has been highly degraded. Similar situations are also reported in the Smoky Mountains, Shenandoah National Park, and in several Midwestern states. Eastern Canadian ecosystems have also been similarly hard hit by acid deposition.
The Further West Acid Rain Affects the Lower Elevations
In the western U.S., nitrogen leaching in high elevation ecosystems has just recently been detected in the Colorado Rockies Front Range. Further West, the rate of nitrogen leaching into streams surrounding chaparral forests of the Los Angeles Basin, CA, is one of the highest recorded in the country. Nitric acid deposition is of greater concern in the West than sulfuric acid. In some coastal ecosystems deposition of nitric acid is a key contributor to poor water quality conditions and massive fish kills.
Outside the U.S., scientists predict that future NOx and SO2 emissions will substantially increase in the developing world, especially in Asia, making acidification a greater ecological problem.
Acid Rain Effects on Human Health
Acid rain looks, feels, and tastes just like clean rain. The harm to people from acid rain is not direct. Walking in acid rain, or even swimming in an acid lake, is no more dangerous than walking or swimming in clean water.
However, the pollutants that cause acid rain-sulfur dioxide (SO2) and nitrogen oxides (NOx)-do damage human health. These gases interact in the atmosphere to form fine sulfate and nitrate particles that can be transported long distances by winds and inhaled deep into people’s lungs. Fine particles can also penetrate indoors.
A large number of epidemiological studies over the past 10 to 20 years show an association between ambient fine particle concentrations (e.g. sulfates and nitrates formed in the atmosphere from SO2 and NOx emissions) and health effects, such as increased numbers of hospital admissions and emergency room visits for heart and lung disease, increased incidences of respiratory disease and symptoms (such as asthma), decreased lung function, and even premature death. Children, the elderly, and individuals with existing cardiovascular or lung conditions, such as asthma, are especially vulnerable to the effects of particles, according to the National Acid Assessment Report to congress 2005 (p. 62).
Laws & Treaties
Clean Air Act (CAA)
The Environmental Protection Agency provides the CAA as amended in 1990. Also included is a history of the Clean Air Act and a description that is easy to understand.
For The Classroom
Acid Rain Research
This Access Excellence inquiry lab developed by teacher Connie Jones, allows students to collect samples of rainfall, cloud condensation, and soil-filtered water so that they may test for pH, conductivity, and the presence of sulfates, nitrates, calcium, and magnesium. [Grades 9-12]
Acid Rain and Plant Growth
The University of Georgia’s College of Agricultural and Environmental Sciences developed a lab exercise on the effects of acid rain. Instructions, sample questions, and goals are provided for different grade levels.