Sulfur Dioxide and Acid Rain

Sources of Sulfur Dioxide

Sulfur dioxide (SO₂) is a colourless gas with a sharp, choking smell. It is a major atmospheric pollutant and mainly comes from:

• Burning fossil fuels: Coal and oil often contain sulfur compounds. When these fuels are burned in power stations, factories, or vehicles, sulfur reacts with oxygen to form sulfur dioxide.
• Volcanic activity: Volcanoes naturally release sulfur dioxide during eruptions, contributing to atmospheric SO₂.
• Industrial processes: Certain industries, such as metal smelting and refining, release sulfur dioxide as a by-product.

The burning of fossil fuels is the largest human-made source of sulfur dioxide in the UK and worldwide.

Formation of Acid Rain

Sulfur dioxide released into the atmosphere can dissolve in water droplets in clouds. This leads to the formation of acid rain through the following process:

• SO₂ dissolves in water (rain, fog, or dew) forming sulfurous acid, which is weakly acidic.
• Further oxidation in the atmosphere converts sulfurous acid into sulfuric acid, a much stronger acid.
• When these acidic droplets fall as precipitation, they are called acid rain.

Acid rain has a pH less than 5.6 (normal rain is slightly acidic with pH around 5.6 due to dissolved CO₂).

For instance, when sulfur dioxide dissolves in water:

$\mathrm{SO_2 + H_2O \rightarrow H_2SO_3}$ (sulfurous acid)

Then, sulfurous acid can be oxidised to sulfuric acid:

$2\mathrm{H_2SO_3 + O_2 \rightarrow 2H_2SO_4}$

Effects of Acid Rain

Acid rain causes several environmental and structural problems:

• Damage to plants and trees: Acid rain leaches important nutrients like calcium and magnesium from the soil, making it harder for plants to absorb them. It also damages leaves, reducing photosynthesis.
• Acidification of lakes and rivers: Acid rain lowers the pH of freshwater, making it too acidic for many aquatic animals and plants to survive. This can lead to a loss of biodiversity.
• Corrosion of buildings and metals: Acid rain reacts with materials like limestone, marble, and metals, causing them to weaken and decay. This damages historic buildings and infrastructure.

Reducing Sulfur Dioxide Emissions

To reduce the harmful effects of sulfur dioxide and acid rain, several methods are used:

• Use of low sulfur fuels: Switching to fuels with less sulfur content, such as natural gas instead of coal, reduces SO₂ emissions.
• Flue gas desulfurization (FGD): This is a technology used in power stations to remove sulfur dioxide from exhaust gases before they are released into the atmosphere. It often involves spraying a limestone slurry that reacts with SO₂ to form harmless calcium sulfate.
• Legislation and regulations: Governments have introduced laws to limit sulfur dioxide emissions from factories, power stations, and vehicles. For example, the UK 27s Clean Air Acts and EU directives have helped reduce SO₂ levels significantly.

For example, in flue gas desulfurization:

$\mathrm{SO_2 + CaCO_3 + \frac{1}{2}O_2 + 2H_2O \rightarrow CaSO_4 \cdot 2H_2O + CO_2}$

Here, calcium carbonate (limestone) reacts with sulfur dioxide and oxygen to produce calcium sulfate (gypsum), which is a solid and can be removed.

For example, a simple inline calculation: If normal rain has a pH of 5.6 and acid rain has a pH of 4.0, the acid rain is about $10^{(5.6-4.0)} = 10^{1.6} \approx 40$ times more acidic.