Earth's Atmosphere
5. Acid Rain
Learning outcomes
- I can explain how acid rain forms.
- I can identify the pollutants responsible for acid rain.
- I can describe the effects of acid rain on ecosystems and infrastructure.
- I can explain how air pollution contributes to acid deposition.
- I can evaluate methods used to reduce acid rain.
Introduction
Rainwater is naturally slightly acidic because it absorbs a small amount of carbon dioxide from the atmosphere, forming a weak solution of carbonic acid. Normally, rain has a pH of about 5.6, which does not significantly harm the environment.
However, when pollutants such as sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) are released into the atmosphere, they react with water vapour and oxygen to form much stronger acids. These acids fall back to Earth as acid rain or other forms of acid deposition, damaging forests, lakes, buildings, and wildlife.
What Is Acid Rain?
Acid rain is precipitation that has a lower-than-normal pH because it contains sulfuric acid and nitric acid formed from air pollutants.
Acid rain may fall as:
- rain
- snow
- sleet
- hail
- fog
Not all acid deposition is wet. Some acidic particles settle directly onto surfaces without precipitation.
Definition:
Acid rain is precipitation made unusually acidic by sulfuric and nitric acids formed from atmospheric pollutants.
Normal Rain vs Acid Rain
| Type of Rain. | Typical pH |
|---|---|
| Pure water | 7.0 |
| Normal rain | About 5.6 |
| Acid rain | Below 5.6 |
The lower the pH, the more acidic the rain.
How Does Acid Rain Form?
The formation of acid rain begins with the release of pollutants into the atmosphere.
Step 1 – Pollutants Are Released
The main pollutants are:
- sulfur dioxide (SO₂)
- nitrogen oxides (NOₓ)
These gases are produced mainly by:
- coal-fired power stations
- factories
- vehicle engines
- industrial processes
Volcanoes can also release sulfur dioxide naturally.
Step 2 – Chemical Reactions in the Atmosphere
The pollutants react with:
- oxygen
- water vapour
These reactions produce acids such as:
- sulfuric acid (H₂SO₄)
- nitric acid (HNO₃)
Step 3 – Acid Deposition
The acids return to Earth's surface as:
- rain
- snow
- fog
- dry acidic particles
Together, these are known as acid deposition.
Pollutants Responsible for Acid Rain
Sulfur Dioxide (SO₂)
Major sources:
- burning coal
- oil-fired power stations
- metal smelting
- volcanic eruptions
Sulfur dioxide reacts in the atmosphere to form sulfuric acid.
Nitrogen Oxides (NOₓ)
Major sources:
- vehicle exhaust
- aircraft
- power stations
- industrial combustion
Nitrogen oxides react with oxygen and water to produce nitric acid.
Summary Table
| Pollutant | Main Human Sources | Acid Produced |
|---|---|---|
| Sulfur dioxide (SO₂) | Coal, industry | Sulfuric acid (H₂SO₄) |
| Nitrogen oxides (NOₓ). | Vehicles, power stations. | Nitric acid (HNO₃) |
Effects on Ecosystems
Acid rain can damage both aquatic and terrestrial ecosystems.
Lakes and Rivers
Acid rain lowers the pH of water bodies.
As lakes become more acidic:
- fish eggs fail to hatch
- aquatic plants decline
- insects disappear
- biodiversity decreases
Some lakes have become too acidic to support fish populations.
Forests
Acid rain damages forests by:
- harming leaves and needles
- reducing photosynthesis
- weakening trees
- making plants more vulnerable to disease and pests
It also removes important nutrients such as:
- calcium
- magnesium
- potassium
from the soil.
Effects on Buildings and Infrastructure
Acid rain also affects human-made structures.
It reacts with materials such as:
- limestone
- marble
- concrete
- certain metals
This can cause:
- erosion of stone buildings
- corrosion of bridges
- damage to statues and monuments
- increased maintenance costs
Historic buildings are particularly vulnerable because many are constructed from limestone or marble.
Real-World Example
Famous landmarks, including parts of the Parthenon in Greece and buildings in many European cities, have suffered damage from decades of acid rain and air pollution.
Air Pollution and Acid Deposition
Air pollutants released in one country can travel hundreds or even thousands of kilometres before falling as acid rain.
This means that:
- pollution often crosses national borders,
- countries may experience acid rain even if they produce relatively little pollution themselves,
- international cooperation is often needed to solve the problem.
Prevailing winds play a major role in transporting pollutants across regions.
Reducing Acid Rain
Many countries have successfully reduced acid rain by lowering emissions of sulfur dioxide and nitrogen oxides.
Cleaner Energy
Using renewable energy sources such as:
- solar
- wind
- hydroelectric
reduces the burning of fossil fuels.
Flue Gas Desulfurization
Many power stations install scrubbers that remove sulfur dioxide from exhaust gases before they enter the atmosphere.
Catalytic Converters
Most modern vehicles use catalytic converters that reduce nitrogen oxide emissions.
Low-Sulfur Fuels
Using fuels with lower sulfur content greatly reduces sulfur dioxide emissions.
Environmental Regulations
Governments have introduced:
- emission standards
- clean-air laws
- international agreements
- pollution monitoring
These policies have significantly reduced acid rain in many developed countries.
Worked Example
A coal-fired power station installs equipment that removes most of its sulfur dioxide emissions.
Question
How is this likely to affect acid rain?
Solution
Sulfur dioxide is one of the main pollutants responsible for acid rain.
Removing sulfur dioxide means:
- less sulfuric acid forms in the atmosphere,
- less acid rain is produced,
- ecosystems and buildings experience less damage.
Therefore, acid rain is expected to decrease.
Real-World Connection
During the 1970s and 1980s, acid rain caused widespread damage across Europe and North America. Since then, stricter air-quality regulations, cleaner fuels, catalytic converters, and improved industrial technologies have dramatically reduced sulfur dioxide emissions in many countries. This has allowed many lakes, forests, and ecosystems to begin recovering, demonstrating that environmental policies can successfully address large-scale pollution problems.
Did You Know?
One of the world's most famous examples of acid rain damage is the Taj Mahal in India. Air pollution from nearby industries and vehicles has contributed to the gradual discolouration and deterioration of its white marble exterior. Conservation efforts now include stricter pollution controls around the monument to help preserve this UNESCO World Heritage Site.
Key Terms
- Acid rain — precipitation made unusually acidic by sulfuric and nitric acids.
- Acid deposition — the transfer of acidic substances to Earth's surface through wet or dry processes.
- Sulfur dioxide (SO₂) — a pollutant that forms sulfuric acid in the atmosphere.
- Nitrogen oxides (NOₓ) — pollutants that form nitric acid in the atmosphere.
- Sulfuric acid (H₂SO₄) — a strong acid formed from sulfur dioxide.
- Nitric acid (HNO₃) — a strong acid formed from nitrogen oxides.
- Scrubber — equipment used to remove sulfur dioxide from industrial emissions.
- Catalytic converter — a device that reduces harmful vehicle exhaust emissions.
Key Takeaways
- Acid rain forms when sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) react with water vapour and oxygen in the atmosphere.
- The resulting sulfuric acid and nitric acid return to Earth through wet and dry deposition.
- Acid rain damages lakes, forests, soils, buildings, monuments, and infrastructure.
- Air pollution can travel long distances, making acid rain an international environmental issue.
- Technologies such as scrubbers, catalytic converters, cleaner fuels, renewable energy, and environmental regulations have significantly reduced acid rain in many parts of the world.
- Acid rain is an example of how controlling air pollution can protect both ecosystems and human-built environments.