Acid rain is precipitation with a pH of around 5.2 or lower, mainly caused by sulfur dioxide and nitrogen oxides released from burning fossil fuels. Power plants, vehicles, and industrial facilities emit these pollutants, which then mix with water and other chemicals in the atmosphere to form sulfuric and nitric acids. Acid rain harms natural and human-made environments. It can lower the pH of surface waters, reduce biodiversity, weaken trees, and corrode buildings and monuments. It also depletes soil of plant nutrients and can release toxic aluminum. Although human activities are the primary source, volcanic eruptions and lightning can also contribute to acid rain.

What causes acid rain to form?

Acid rain, more scientifically known as acid deposition, happens when acidity moves from the atmosphere to the Earth's surface in various forms, including rain, snow, fog, and dry acidic particles. The primary culprits are sulfur dioxide ((SO_2)) and nitrogen oxides ((NO_x)), released mainly from burning fossil fuels in power plants, vehicles, and industrial operations. These gases react with water, oxygen, and other chemicals in the atmosphere to form sulfuric acid ((H_2SO_4)) and nitric acid ((HNO_3)). These acids mix with cloud moisture, reducing the pH of precipitation and resulting in acid rain. Although natural sources such as volcanic eruptions and lightning strikes can also contribute, human activities are the dominant cause.

How does air pollution relate to acid rain?

Air pollution is a major contributor to acid rain . The combustion of fossil fuels releases sulfur dioxide ((SO_2)) and nitrogen oxides ((NO_x)) into the atmosphere, which are the main causes of acid rain. These gases react with water, oxygen, and other chemicals to form sulfuric acid ((H_2SO_4)) and nitric acid ((HNO_3)). Acid rain and deposition can occur as rain, snow, sleet, or dry deposition. These acids mix with cloud moisture and fall to the ground as acid rain, which can damage the environment.

What are some effects of acid rain on the environment?

Acid rain, caused primarily by sulfur dioxide and nitrogen oxides from burning fossil fuels, has several harmful environmental effects. It can lower the pH of surface waters, reducing biodiversity and harming aquatic life. Acid rain also weakens trees, making them more vulnerable to stressors like drought and pests, and it depletes soil nutrients such as calcium and magnesium while releasing toxic aluminum. Furthermore, acid deposition contributes to the corrosion of buildings and monuments, especially those made of limestone and marble. Though regulations have reduced acid rain in some regions, it remains an issue in areas with weaker emission controls.

How does acid rain affect buildings and human-made structures?

Acid rain, resulting from sulfur dioxide and nitrogen oxides emissions, significantly affects human-made structures. The most pronounced effects are observed on limestone and marble , common materials in historic buildings, monuments, and gravestones. Sulfur dioxide, a precursor to acid rain, interacts with limestone in the presence of water, leading to the formation of gypsum, which can flake off or dissolve. Acid rain can also directly dissolve limestone and marble upon contact, causing deterioration.

What steps can people and governments take to reduce acid rain?

Acid rain, caused primarily by sulfur dioxide and nitrogen oxides from burning fossil fuels, harms ecosystems and infrastructure. To reduce acid rain, governments and individuals can: * Enact and enforce stricter air pollution regulations to lower sulfur dioxide ($SO_2$) and nitrogen oxides ($NO_x$) emissions. * Use flue gas treatment technologies, such as scrubbers and electrostatic precipitators, to remove pollutants from industrial emissions. * Explore alternative energy sources, such as wind, solar, and hydropower, to reduce reliance on fossil fuels. * Implement emissions trading programs, setting caps on pollution and allowing companies to trade emission allowances. * Support international agreements for coordinated efforts to reduce emissions on a global scale.

Acid rain

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Learn about the causes and effects of acid rain

Diagram illustrating the nitrogen cycle, showing how nitrogen moves between the atmosphere, biosphere, and soil through natural processes like nitrogen fixation, assimilation, ammonification, nitrification, and denitrification, as well as through human activities such as fertilizer use and industrial emissions. Blue arrows indicate natural activities, and red arrows indicate human activities.

Diagram illustrating the sulfur cycle, showing how sulfur moves through the biosphere and atmosphere. Arrows trace sulfur emissions from fossil fuel burning and decomposition, deposition from the air, plant uptake, runoff into water, and sediment formation.

Two pie charts compare sources of sulfur dioxide and nitrogen oxides emissions contributing to acid rain. The first chart shows electric utilities produce 66% of SO2, other sources 29%, and vehicles 5%. The second chart shows vehicles produce 58% of NOx, other sources 24%, and electric utilities 18%.

Severely defoliated spruce trees stand amid snow in Karkonosze National Park, Poland.

World map highlighting regions with high acid sensitivity in orange and areas of acid deposition with purple hatching. Notable high sensitivity regions appear in eastern North America, northern and central Europe, parts of South America, Africa, and Asia, with some overlap between acid sensitivity and deposition zones. Continents and major oceans are labeled for reference.

Weathered statue with a damaged face and surface, showing significant erosion from acid rain and pollution.

Learn how pollution from industrialization in Europe damaged the limestone of Cologne Cathedral

Line graph showing U.S. emissions of ammonia (NH3), sulfur dioxide (SO2), and nitrogen oxides (NOx) from 1970 to 2008. SO2 and NOx emissions decline steadily, while NH3 remains nearly constant. The graph relates to acid rain.

At a Glance

acid rain summary

Acid rain

pollution

Also known as: acid deposition, acid precipitation

Written by

Gene E. LikensAll

Fact-checked by

Britannica Editors

Last updated

Nov. 10, 2026 •History

Top Questions

What is acid rain?
Acid rain is precipitation with a pH of around 5.2 or lower, mainly caused by sulfur dioxide and nitrogen oxides released from burning fossil fuels. Power plants, vehicles, and industrial facilities emit these pollutants, which then mix with water and other chemicals in the atmosphere to form sulfuric and nitric acids.

Acid rain harms natural and human-made environments. It can lower the pH of surface waters, reduce biodiversity, weaken trees, and corrode buildings and monuments. It also depletes soil of plant nutrients and can release toxic aluminum. Although human activities are the primary source, volcanic eruptions and lightning can also contribute to acid rain.
2 Britannica Sources
1. What Causes Acid Rain?
2. acid rain
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What causes acid rain to form?
Acid rain, more scientifically known as acid deposition, happens when acidity moves from the atmosphere to the Earth's surface in various forms, including rain, snow, fog, and dry acidic particles. The primary culprits are sulfur dioxide ((SO_2)) and nitrogen oxides ((NO_x)), released mainly from burning fossil fuels in power plants, vehicles, and industrial operations.

These gases react with water, oxygen, and other chemicals in the atmosphere to form sulfuric acid ((H_2SO_4)) and nitric acid ((HNO_3)). These acids mix with cloud moisture, reducing the pH of precipitation and resulting in acid rain. Although natural sources such as volcanic eruptions and lightning strikes can also contribute, human activities are the dominant cause.
2 Britannica Sources
1. What Causes Acid Rain?
2. acid rain: Chemistry of acid deposition
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How does air pollution relate to acid rain?
Air pollution is a major contributor to acid rain. The combustion of fossil fuels releases sulfur dioxide ((SO_2)) and nitrogen oxides ((NO_x)) into the atmosphere, which are the main causes of acid rain. These gases react with water, oxygen, and other chemicals to form sulfuric acid ((H_2SO_4)) and nitric acid ((HNO_3)). Acid rain and deposition can occur as rain, snow, sleet, or dry deposition. These acids mix with cloud moisture and fall to the ground as acid rain, which can damage the environment.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are some effects of acid rain on the environment?
Acid rain, caused primarily by sulfur dioxide and nitrogen oxides from burning fossil fuels, has several harmful environmental effects. It can lower the pH of surface waters, reducing biodiversity and harming aquatic life. Acid rain also weakens trees, making them more vulnerable to stressors like drought and pests, and it depletes soil nutrients such as calcium and magnesium while releasing toxic aluminum. Furthermore, acid deposition contributes to the corrosion of buildings and monuments, especially those made of limestone and marble. Though regulations have reduced acid rain in some regions, it remains an issue in areas with weaker emission controls.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How does acid rain affect buildings and human-made structures?
Acid rain, resulting from sulfur dioxide and nitrogen oxides emissions, significantly affects human-made structures.

The most pronounced effects are observed on limestone and marble, common materials in historic buildings, monuments, and gravestones. Sulfur dioxide, a precursor to acid rain, interacts with limestone in the presence of water, leading to the formation of gypsum, which can flake off or dissolve. Acid rain can also directly dissolve limestone and marble upon contact, causing deterioration.
2 Britannica Sources
1. acid rain: Effects on human-made structures
2. What Happened to Acid Rain?
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What steps can people and governments take to reduce acid rain?
Acid rain, caused primarily by sulfur dioxide and nitrogen oxides from burning fossil fuels, harms ecosystems and infrastructure.

To reduce acid rain, governments and individuals can:
* Enact and enforce stricter air pollution regulations to lower sulfur dioxide ($SO_2$) and nitrogen oxides ($NO_x$) emissions.
* Use flue gas treatment technologies, such as scrubbers and electrostatic precipitators, to remove pollutants from industrial emissions.
* Explore alternative energy sources, such as wind, solar, and hydropower, to reduce reliance on fossil fuels.
* Implement emissions trading programs, setting caps on pollution and allowing companies to trade emission allowances.
* Support international agreements for coordinated efforts to reduce emissions on a global scale.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Acid rain, precipitation possessing a pH of about 5.2 or below primarily produced from the emission of sulfur dioxide (SO₂) and nitrogen oxides (NO_x; the combination of NO and NO₂) from human activities, mostly the combustion of fossil fuels. In acid-sensitive landscapes, acid deposition can reduce the pH of surface waters and lower biodiversity. It weakens trees and increases their susceptibility to damage from other stressors, such as drought, extreme cold, and pests. In acid-sensitive areas, acid rain also depletes soil of important plant nutrients and buffers, such as calcium and magnesium, and can release aluminum, bound to soil particles and rock, in its toxic dissolved form. Acid rain contributes to the corrosion of surfaces exposed to air pollution and is responsible for the deterioration of limestone and marble buildings and monuments.

The phrase acid rain was first used in 1852 by Scottish chemist Robert Angus Smith during his investigation of rainwater chemistry near industrial cities in England and Scotland. The phenomenon became an important part of his book Air and Rain: The Beginnings of a Chemical Climatology (1872). It was not until the late 1960s and early 1970s, however, that acid rain was recognized as a regional environmental issue affecting large areas of western Europe and eastern North America. Acid rain also occurs in Asia and parts of Africa, South America, and Australia. As a global environmental issue, it is frequently overshadowed by climate change. Although the problem of acid rain has been significantly reduced in some areas, it remains an important environmental issue within and downwind from major industrial and industrial agricultural regions worldwide.