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Describe acid deposition and its effects on the environment.

Causes of Acid Deposition

Before we discuss how acid deposition forms, we need to review how acids and bases work. Click through the presentation below to refresh your understanding of acids and bases:

Now that we understand acids and bases a little more, we can talk more about acid deposition and acid precipitation (rain).

A visual representation of the formation and effects of acid rain, highlighting the chemical reactions involved and their detrimental impact on trees. — **Acid rain forms when SO₂ and NO_x mix with water and sunlight in the atmosphere.**

Acid precipitation is any precipitation (rain, fog, mist, or snow) that is more acidic than usual. Acid deposition is a combination of precipitation and dry particle fallout. Acid rain usually has a pH of 5.6 or below. In most of the eastern United States, rain now has a pH below 4.6. Acid rain can be reliably found in the United States, Europe, Russia, China, and Japan.

Acid rain is usually composed of two acids: sulfuric and nitric acids. These acids are formed when SO₂ and NO_x are released into the atmosphere. and can have natural or anthropogenic sources, but of course, the anthropogenic sources are increasing.

Natural sources of SO₂ are volcanoes, sea spray, and microbial processes. Natural sources of NO_x are lightning, burning of biomass (natural burning, such as forest fires), and microbial processes. These natural sources are spread throughout the globe and are not concentrated in particular areas.

Anthropogenic sources of SO₂ and NO_x are burning fossil fuels (stationary and vehicular) and biomass (for fuel or slash and burn agriculture). Anthropogenic sources of SO₂ and NO_x are concentrated in industrialized regions. For example, when sulfur dioxide and nitrogen oxides are released into the atmosphere from burning coal, they can combine with moisture to form sulfuric acid and nitric acid. These acids can then fall to the ground as acid rain, snow, or fog, impacting the environment and communities below.

Essential Knowledge

Acid rain and deposition are due to nitrogen oxides and sulfur oxides from anthropogenic and natural sources in the atmosphere.

Nitric oxides that cause acid deposition come from motor vehicles and coal-burning power plants. Sulfur dioxides that cause acid deposition come from coal-burning power plants.

Impacts of Acid Deposition

Acid deposition has a variety of environmental effects. Explore the tabs to learn how acid rain can affect each part of the biosphere.

Land
Aquatic
Atmospheric
Health

Impacts on Land

A forest of dead trees — **Acid rain can cause trees to lose their leaves and needles.**

Acid rain alters soil pH, leading to soil acidification. Acidic soils affect nutrient availability, hinder plant growth, and harm soil-dwelling organisms.

Forests that have been exposed to acid deposition are more susceptible to freezing in the winter and it causes pine trees to lose their needles. Acid rain weakens tree bark, making trees more susceptible to environmental stressors like drought, extreme temperatures, and pests. Sugar maples, especially, are extremely susceptible to acid deposition, with losses ranging from 20-80%.

In human areas, acid deposition can cause paints and fabrics to deteriorate more quickly. It can also cause rubber products to crack and metal to corrode. Stonework exposed to acid deposition can weather and deteriorate faster. These effects are particularly noticeable because acid deposition tends to occur near urban areas, which are where humans are as well.

Aquatic Impacts

Metals leaching into water, changing the color of the water — **Acid rain can cause heavy metals to leach into waterways.**

Healthy aquatic systems generally have a pH between 6-8 and the organisms in these systems are adapted to this. Exposure to a pH that is consistently low, overwhelms the regulatory mechanisms in most organisms.

Acid rain acidifies lakes, rivers, and streams. This harms aquatic life, including fish, insects, and amphibians.

Acid deposition also exacerbates leaching of other toxic elements (for example, aluminum), leading to changes in metabolic activity in exposed organisms. Acidification disrupts nutrient cycling, food webs, and overall ecosystem health.

The regional differences in soils and bedrock play a crucial role in determining the impact of acid deposition on a particular area. For example, regions with limestone bedrock have a natural advantage as limestone has the ability to neutralize the effects of acid rain on lakes and ponds. The alkaline properties of limestone can help balance the acidity of the water bodies, preventing harmful consequences on aquatic life.

Atmospheric Impacts

Acid rain contributes to atmospheric mercury deposition, affecting forests and ecosystems.

Health Impacts

Acid deposition also corrodes the living tissues of all organisms. Inhalation of acid rain particles can exacerbate respiratory conditions, especially for vulnerable populations.

Direct contact with acid rain can cause skin irritation.

Essential Knowledge

Acid deposition mainly affects communities that are downwind from coal-burning power plants.

Acid rain and deposition can lead to the acidification of soils and bodies of water and corrosion of human-made structures.

Regional differences in soils and bedrock affect the impact that acid deposition has on the region - such as limestone bedrock's ability to neutralize the effect of acid rain on lakes and ponds.

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