Acid Rain and Global Dimming

This lesson covers two important environmental consequences of atmospheric pollution: acid rain and global dimming, as required by AQA GCSE Chemistry specification (5.9.3). You need to understand how these phenomena are caused, what their effects are, and how they can be reduced. Both topics are frequently examined and require you to link chemical knowledge to real-world environmental issues.

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What Is Acid Rain?

Acid rain is rain (or any form of precipitation) that is more acidic than normal. Normal rain is slightly acidic (pH approximately 5.6) because carbon dioxide in the atmosphere dissolves in rainwater to form carbonic acid:

CO₂ + H₂O → H₂CO₃

Acid rain has a pH significantly lower than 5.6, typically between pH 3 and pH 5, due to the presence of stronger acids formed from atmospheric pollutants.

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Causes of Acid Rain

Acid rain is caused primarily by two pollutant gases dissolving in rainwater:

1. Sulfur Dioxide (SO₂)

Sulfur dioxide is produced when fossil fuels containing sulfur impurities are burned, especially coal. In the atmosphere, SO₂ reacts with water and oxygen to form sulfuric acid (H₂SO₄):

SO₂ + H₂O → H₂SO₃ (sulfurous acid)

Further oxidation produces the stronger sulfuric acid:

2SO₂ + O₂ + 2H₂O → 2H₂SO₄

2. Nitrogen Oxides (NOₓ)

Nitrogen oxides are produced in vehicle engines and power stations at high temperatures. In the atmosphere, nitrogen dioxide (NO₂) reacts with water and oxygen to form nitric acid (HNO₃):

4NO₂ + O₂ + 2H₂O → 4HNO₃

These acids dissolve in cloud droplets and fall to the ground as acid rain, sleet, snow, or fog. Acid rain can also be deposited in dry form — as acidic gases and particles that settle on surfaces.

flowchart TD
    A["Burning Fossil Fuels"] --> B["SO₂ released<br/>(from sulfur impurities)"]
    A --> C["NOₓ released<br/>(from N₂ + O₂ at high temp)"]
    B --> D["SO₂ dissolves in<br/>rainwater"]
    C --> E["NO₂ dissolves in<br/>rainwater"]
    D --> F["Sulfuric acid<br/>H₂SO₄"]
    E --> G["Nitric acid<br/>HNO₃"]
    F --> H["Acid Rain<br/>pH 3–5"]
    G --> H

    style A fill:#e74c3c,color:#fff
    style H fill:#8e44ad,color:#fff

Exam Tip: Acid rain is caused by sulfur dioxide AND nitrogen oxides — you must mention both. A common mistake is to only mention one. Remember: SO₂ forms sulfuric acid, and NOₓ forms nitric acid. Learn these two acid names and link them to the correct pollutant.

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Effects of Acid Rain

Acid rain causes damage to the natural environment, buildings, and human health.

Environmental Effects

Effect	Explanation
Damage to trees and forests	Acid rain damages leaves, weakens trees by leaching nutrients from the soil, and releases toxic metals (e.g. aluminium) from the soil that harm roots.
Acidification of lakes and rivers	Acid rain lowers the pH of water bodies. Many aquatic organisms (fish, invertebrates, amphibians) cannot survive in acidic conditions. Below pH 4.5, most fish species die.
Leaching of soil nutrients	Acids dissolve essential minerals (calcium, magnesium, potassium) from the soil, making it less fertile.
Release of toxic metals	Acid rain dissolves aluminium and other toxic metals from the soil, which then wash into waterways and harm aquatic life.

Effects on Buildings and Materials

Material	Effect of Acid Rain
Limestone and marble	These are forms of calcium carbonate (CaCO₃). Acid rain reacts with them: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂. This causes erosion and dissolving of stone, damaging buildings, statues, and monuments.
Iron and steel	Acid rain accelerates corrosion (rusting) of iron and steel structures.
Paint and coatings	Acid rain can damage paint on buildings and vehicles.

Health Effects

• Acid rain itself does not directly harm humans at the pH levels typically observed.
• However, the pollutant gases (SO₂ and NOₓ) that cause acid rain can irritate the respiratory system, worsen asthma, and cause bronchitis.
• Contaminated water supplies (with dissolved toxic metals) can also pose health risks.

Exam Tip: If asked for the equation showing acid rain attacking limestone, use: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂. This is a neutralisation reaction — the acid reacts with the carbonate to produce a salt, water, and carbon dioxide.

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Reducing Acid Rain

Several strategies can reduce acid rain:

Strategy	How It Works
Flue gas desulfurisation (FGD)	Removes SO₂ from power station exhaust gases using limestone or lime. Reduces SO₂ emissions by up to 95%.
Catalytic converters	Convert NOₓ to harmless nitrogen gas (N₂) in vehicle exhaust systems.
Using low-sulfur fuels	Natural gas contains very little sulfur compared to coal. Switching from coal to gas reduces SO₂ emissions.
Desulfurisation of fuels	Removing sulfur from crude oil and coal before burning them.
Renewable energy	Using wind, solar, nuclear, or hydroelectric power instead of fossil fuels eliminates SO₂ and NOₓ emissions entirely.
Liming	Adding calcium carbonate (limestone) to acidified lakes and soils to neutralise the acid and raise the pH. This treats the symptom, not the cause.

Success Stories

International action to reduce acid rain has been successful in many countries. For example:

• The UK has reduced SO₂ emissions by over 95% since the 1970s, mainly through the switch from coal to gas for electricity generation and the use of FGD.
• The US Clean Air Act (1990) introduced emissions trading for SO₂, leading to significant reductions.
• Acid rain damage to forests and lakes in Scandinavia and eastern Canada has decreased as a result of reduced emissions in Europe and North America.

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What Is Global Dimming?

Global dimming is the gradual reduction in the amount of sunlight reaching the Earth's surface. It is caused by particulates (soot and other solid particles) and aerosols in the atmosphere, which reflect and scatter incoming sunlight back into space.

How Global Dimming Works

1. Particulates from incomplete combustion (especially from diesel engines, coal-fired power stations, and biomass burning) are released into the atmosphere.
2. These particles reflect some sunlight directly back into space.
3. Particulates also act as cloud condensation nuclei — water droplets form around them, creating clouds with more but smaller droplets. These clouds are more reflective (higher albedo) than normal clouds.
4. The result is that less solar radiation reaches the Earth's surface.

Evidence for Global Dimming

Scientists have measured a decrease in solar radiation reaching the surface in many parts of the world since the 1950s. Some studies suggest a reduction of 4–5% per decade between the 1960s and 1990s. However, since the 1990s, global dimming has reversed in many regions due to cleaner air (reduced particulate emissions).

Exam Tip: Global dimming is caused by particulates (soot), NOT by greenhouse gases. This is a common source of confusion. Particulates block sunlight (causing cooling), while greenhouse gases trap heat (causing warming). These two effects can partially cancel each other out.

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Global Dimming vs Climate Change

Global dimming and climate change (global warming) have opposing effects on temperature: