Acid Rain: Definition Causes, and Effects | Acid Rain

What is acid rain?

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.  

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 in an area that receives several inches of rain each year.

Acid Rain

What Causes Acid Rain?

Acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. These substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain.

Acid rain 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 water, 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 rains a problem for everyone and not just those who live close to these sources.

Effects of acid rain

1. The Effects of Acid Rain on Ecosystems

This figure 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.

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.

2. Effects of Acid Rain on Fish and Wildlife

The ecological effects of acid rains 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 rainwater 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. 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 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.

3. Effects of Acid Rain on Plants and Trees

Dead or dying trees are a common sight in areas affected by acid rain. It leaches aluminum from the soil.  That aluminum may be harmful to plants as well as animals. It 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 doesn’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 rainwater. 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.

4. Effects of Acid Rain on Materials

Not all acidic deposition is wet. Sometimes dust particles can become acidic as well, and this is called dry deposition. When acid rain and dry acidic particles fall to earth, the nitric and sulfuric acid that make the particles acidic can land on statues, buildings, and other manmade structures, and damage their surfaces.

The acidic particles corrode metal and cause paint and stone to deteriorate more quickly. They also dirty the surfaces of buildings and other structures such as monuments.

The consequences of this damage can be costly:

  • damaged materials that need to be repaired or replaced,
  • increased maintenance costs, and
  • loss of detail on stone and metal statues, monuments and tombstones.

5. Other Effects of SO2 and NOX

Visibility:

In the atmosphere, SO2 and NOX gases can be transformed into sulfate and nitrate particles, while some NOX can also react with other pollutants to form ozone. These particles and ozone make the air hazy and difficult to see through. This affects our enjoyment of national parks that we visit for the scenic view such as Shenandoah and the Great Smoky Mountains.

Human Health:

Walking in acid rain, or even swimming in a lake affected by acid rain, is no more dangerous to humans than walking in normal rain or swimming in non-acidic lakes. However, when the pollutants that cause acid rain SO2 and NOX, as well as sulfate and nitrate particles are in the air, they can be harmful to humans.

SO2 and NOX react in the atmosphere to form fine sulfate and nitrate particles that people can inhale into their lungs.  Many scientific studies have shown a relationship between these particles and effects on heart function, such as heart attacks resulting in death for people with increased heart disease risk, and effects on lung function, such as breathing difficulties for people with asthma.

Measuring Acid Rain

A diagram showing where various substances fall on the pH scale.Acidity and alkalinity are measured using a pH scale for which 7.0 is neutral. The lower a substance’s pH (less than 7), the more acidic it is; the higher a substance’s pH (greater than 7), the more alkaline it is.

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.

Policymakers, research scientists, ecologists, and modelers rely on the National Atmospheric Deposition Program’s (NADP) National Trends Network (NTN) for measurements of wet deposition. The NADP/NTN collects acid rain at more than 250 monitoring sites throughout the US, Canada, Alaska, Hawaii and the US Virgin Islands.

Measuring Acid Rain

Unlike wet deposition, dry deposition is difficult and expensive to measure. Dry deposition estimates for nitrogen and sulfur pollutants are provided by the Clean Air Status and Trends Network (CASTNET). Air concentrations are measured by CASTNET at more than 90 locations.

When an acid deposition is washed into lakes and streams, it can cause some to turn acidic. The Long-Term Monitoring (LTM) Network measures and monitors surface water chemistry at over 280 sites to provide valuable information on aquatic ecosystem health and how water bodies respond to changes in acid-causing emissions and acid deposition.

How to prevent acid rain?

A great way to reduce acid rain is to produce energy without using fossil fuels. Instead, people can use renewable energy sources, such as solar and wind power. Renewable energy sources help reduce acid rain because they produce much less pollution. These energy sources can be used to power machinery and produce electricity.

Simple and small steps can be adopted to reduce and prevent acid rains:

  • Use alternative modes of transport or public transport, use the carpooling system more and of course be more walking and healthier.
  • Reduce the use of air conditioners, use lights and fans only when needed, and reduce the use of home appliances to conserve energy.
  • Reduce the use of non-renewable sources of energy and use more renewable sources such as wind energy, water energy, and solar energy.
  • Use of coal that has low sulfur content.

FAQ

What is Acid Rain?

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.  

What Causes Acid Rain?

Acid rain is caused by a chemical reaction that begins when compounds like sulfur dioxide and nitrogen oxides are released into the air. These substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain.

what are the effect of Acid rain?

When acid rain and dry acidic particles fall to earth, the nitric and sulfuric acid that makes the particles acidic can land on statues, buildings, and other manmade structures, and damage their surfaces. The acidic particles corrode metal and cause paint and stone to deteriorate more quickly.

How to prevent acid rain?

A great way to reduce acid rain is to produce energy without using fossil fuels. Instead, people can use renewable energy sources, such as solar and wind power. Renewable energy sources help reduce acid rain because they produce much less pollution. These energy sources can be used to power machinery and produce electricity.

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