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13.7: Acid Rain: Air Pollution Water Pollution

  • Page ID
    177628
  • Learning Objective

    • Describe acid rain and its effects.

    Acid rain, or acid deposition, is a broad term that includes any form of precipitation with acidic components, such as sulfuric or nitric acid that fall to the ground from the atmosphere in wet or dry forms. This can include rain, snow, fog, hail or even dust that is acidic. Normal rain has a pH of about 5.6; it is slightly acidic because carbon dioxide (CO2) dissolves into it forming weak carbonic acid. Acid rain usually has a pH between 4.2 and 4.4.

    What Causes Acid Rain?

    Acid rain (Figure \(\PageIndex{1}\) results when sulfur dioxide (SO2) and nitrogen oxides (NOX) are emitted into the atmosphere and transported by wind and air currents. The SO2 and NOX react with clipboard_ecf490907dd8a610192a196b7682c12c9.pngwater, oxygen and other chemicals to form sulfuric and nitric acids. These then mix with water and other materials before falling to the ground.

    While a small portion of the SO2 and NOX that cause acid rain is from natural sources such as volcanoes, most of it comes from the burning of fossil fuels. The major sources of SO2 and NOX in the atmosphere are:

    · Burning of fossil fuels to generate electricity. Two thirds of SO2 and one fourth of NOX in the atmosphere come from electric power generators.

    · Vehicles and heavy equipment.

    · Manufacturing, oil refineries and other industries.

    Winds can blow SO2 and NOX over long distances and across borders making acid rain a problem for everyone and not just those who live close to these sources.

    Figure \(\PageIndex{1}\): The acid rain pathway. Image from EPA

    Forms of Acid Deposition

    Wet Deposition

    Wet deposition is what we most commonly think of as acid rain. The sulfuric and nitric acids formed in the atmosphere fall to the ground mixed with rain, snow, fog, or hail.

    Dry Deposition

    Acidic particles and gases can also deposit from the atmosphere in the absence of moisture as dry deposition. The acidic particles and gases may deposit to surfaces (water bodies, vegetation, buildings) quickly or may react during atmospheric transport to form larger particles that can be harmful to human health. When the accumulated acids are washed off a surface by the next rain, this acidic water flows over and through the ground, and can harm plants and wildlife, such as insects and fish.

    The amount of acidity in the atmosphere that deposits to earth through dry deposition depends on the amount of rainfall an area receives. For example, in desert areas the ratio of dry to wet deposition is higher than an area that receives several inches of rain each year.

    The Effects of Acid Rain on Ecosystems

    An ecosystem is a community of plants, animals and other organisms along with their environment including the air, water and soil. Everything in an ecosystem is connected. If something harms one part of an ecosystem – one species of plant or animal, the soil or the water – it can have an impact on everything else.

    Effects of Acid Rain on Fish and Wildlife

    The ecological effects of acid rain are most clearly seen in aquatic environments, such as streams, lakes, and marshes where it can be harmful to fish and other wildlife. As it flows through the soil, acidic rain water can leach aluminum from soil clay particles and then flow into streams and lakes. The more acid that is introduced to the ecosystem, the more aluminum is released.

    Some types of plants and animals are able to tolerate acidic waters and moderate amounts of aluminum. Others, however, are acid-sensitive and will be lost as the pH declines. clipboard_ee61df3698b102479768a178e5b637b4c.pngGenerally, the young of most species are more sensitive to environmental conditions than adults. Figure \(\PageIndex{2}\) illustrates the pH level at which key organisms may be lost as their environment becomes more acidic. Not all fish, shellfish, or the insects that they eat can tolerate the same amount of acid.At pH 5, most fish eggs cannot hatch. At lower pH levels, some adult fish die. Some acidic lakes have no fish. Even if a species of fish or animal can tolerate moderately acidic water, the animals or plants it eats might not. For example, frogs have a critical pH around 4, but the mayflies they eat are more sensitive and may not survive pH below 5.5.

    Figure \(\PageIndex{2}\)

    Level of acidity that is tolerable to various species of most aquatic life.

    Image from EPA

    Effects of Acid Rain on Plants and Trees

    Dead or dying trees are a common sight in areas effected by acid rain (Figure \(\PageIndex{2}\)) . Acid rain leaches aluminum from the soil. That aluminum may be harmful to plants as well as animals. Acid rain also removes minerals and nutrients from the soil that trees need to grow.

    At high elevations, acidic fog and clouds might strip nutrients from trees’ foliage, leaving them with brown or dead leaves and needles. The trees are then less able to absorb sunlight, which makes them weak and less able to withstand freezing temperatures.

    Buffering Capacity

    Many forests, streams, and lakes that experience acid rain don’t suffer effects because the soil in those areas can buffer the acid rain by neutralizing the acidity in the rainwater flowing through it. This capacity depends on the thickness and composition of the soil and the type of bedrock underneath it. In areas such as mountainous parts of the Northeast United States, the soil is thin and lacks the ability to adequately neutralize the acid in the rain water. As a result, these areas are particularly vulnerable and the acid and aluminum can accumulate in the soil, streams, or lakes.

    Episodic Acidification

    Melting snow and heavy rain downpours can result in what is known as episodic acidification. Lakes that do not normally have a high level of acidity may temporarily experience effects of acid rain when the melting snow or downpour brings greater amounts of acidic deposition and the soil can’t buffer it. This short duration of higher acidity (i.e., lower pH) can result in a short-term stress on the ecosystem where a variety of organisms or species may be injured or killed.

    Nitrogen Pollution

    It’s not just the acidity of acid rain that can cause problems. Acid rain also contains nitrogen, and this can have an impact on some ecosystems. For example, nitrogen pollution in our coastal waters is partially responsible for declining fish and shellfish populations in some areas. In addition to agriculture and wastewater, much of the nitrogen produced by human activity that reaches coastal waters comes from the atmosphere.

    Figure \(\PageIndex{3}\): Effects of acid rain on trees, Jizera Mountains, Czech Republic. Image from Wikimedia commons, Public Domain. https://commons.wikimedia.org/wiki/C...:Acid_rain#/media/File:Acid_rain_woods1.JPG (left) and on a monument in Greenwood cemetery, Brooklyn New York. Photo by James P. Fischer. https://commons.wikimedia.org/wiki/C...rain#/media/File:Charlotte_Canda_Memorial_(Angel).jpg(right)

    Summary

    • Acid rain results when sulfur dioxide (SO2) and nitrogen oxides (NOX) are emitted into the atmosphere and react with water, oxygen and other chemicals to form sulfuric and nitric acids.
    • Acidic rain water in the soil, streams, lakes, and marshes (and other bodies of water) can be harmful to trees, plants, animals, especially aquatic plants and animals.

    Source

    US Environmental Protection Agency