Acid Rain (HL IB Environmental Systems & Societies (ESS))

Acid Rain Formation

• Acid rain refers rainfall that has a pH lower than normal rainwater

  • Regular rain has a pH between 5 and 5.5, meaning it is naturally slightly acidic

  • Acid rain is more acidic, has a pH lower than 5, and is frequently the result of human activity

Chemical reactions leading to acid rain

• Nitrogen oxides (NOx) and sulphur dioxide (SO₂) are the main gases responsible for acid rain

  • These gases react with water and oxygen in the atmosphere to form nitric acid and sulfuric acid

Formation of nitric acid

• Nitrogen oxides are mainly produced from vehicle exhausts

• The reactions are as follows:

  • Nitrogen monoxide (NO) reacts with oxygen (O₂) to form nitrogen dioxide (NO₂)

2NO + O₂ → 2NO₂

• The nitrogen dioxide then reacts with water (H₂O) and oxygen in the air to produce nitric acid (HNO₃)

4NO₂ + O₂ + 2H₂O → 4HNO₃

Formation of sulphuric acid

• Sulphur dioxide is produced by burning fossil fuels and reacts with water in the atmosphere

• The reactions are as follows:

  • Sulphur dioxide (SO2) dissolves in rainwater, producing sulphurous acid (H₂SO₃)

SO₂ ​+ H₂​O → H₂​SO₃​

• The sulphurous acid is then oxidised by oxygen in the air to produce sulfuric acid (H₂SO₄)

2H₂SO₃ + O₂ → 2H₂SO₄

Types of deposition

• Wet deposition refers to acidic precipitation falling to Earth in the form of rain, snow, or fog

• Sulphuric acid and nitric acid can also combine with ash and other particles present in the air, forming dry particles (i.e. acidic dust and gases)

  • Dry deposition occurs when these particles settle on surfaces, including vegetation, buildings, cars and soil

Acid Rain Impacts

Impacts on ecology

Impacts on terrestrial habitats

• Acidic deposition from acid rain accelerates the leaching of essential nutrients from soil, such as calcium, magnesium and potassium

  • Leaching of these nutrients reduces their availability for plants

  • This leads to nutrient deficiencies

  • This reduces plant growth and overall ecosystem productivity

• Acidic rain can increase soil toxicity

  • This can occur by mobilising harmful metals like aluminium

  • This damages plant roots and affects their ability to absorb water and nutrients

• Acid rain causes direct damage to foliage

  • This weakens trees, making them more vulnerable to disease and harsh weather

• Coniferous forests, e.g. forests of pine or spruce trees, are sensitive to acid rain

  • This is due to their shallow root systems and thin bark

  • Acid rain also damages their foliage and inhibits nutrient absorption

Impacts on freshwater habitats

• Acid rain can make water bodies more acidic

• This is due to a process referred to as solubilisation of aluminium

  • Acid rain causes aluminium, which is normally bound in the soil, to dissolve

  • This allows the aluminium to enter nearby water bodies

• This aluminium is toxic to aquatic life, such as fish and freshwater invertebrates

  • Fish gills can become coated with aluminium

    • This makes it harder for them to breathe

  • Some invertebrates with exoskeletons may have difficulty maintaining their protective shells

    • They rely on calcium to build and maintain their hard outer shells

    • When acid rain increases the acidity of water, it reduces the availability of calcium and other minerals that these organisms need

    • This makes it harder for them to properly develop or maintain their exoskeletons

Impacts on buildings and infrastructure

Corrosion of construction materials

• Acid rain erodes materials like marble, limestone, steel, and paint used in buildings and monuments

• Marble and limestone both contain calcium carbonate (CaCO₃)

• The calcium carbonate reacts with sulphuric acid or nitric acid, causing stonework to corrode and weaken

  • For example, the Taj Mahal in India, made of marble, has shown signs of erosion and discolouration due to acid rain

  • Acid rain has also had an impact on historical statues and structures, such as those in Rome and Greece

Impacts on human health

Respiratory issues

• Acid rain does not directly harm humans

• However, nitrate and sulphate particles from acid rain can cause respiratory problems

  • PM2.5 particles (tiny air pollutants) from acid rain can enter the lungs

  • This leads to:

    • Tissue damage

    • Lung inflammation

    • An increased risk of conditions such as asthma and bronchitis

  • As a result, areas with heavy industrial activity, such as parts of China and Eastern Europe, experience greater respiratory health risks

Acid Rain Management Strategies

• There are three main levels of pollution management strategies:

  1. Changing human activity

  2. Regulating and reducing quantities of pollutants released at the point of emission

  3. Cleaning up the pollutants and restoring the ecosystem after pollution has occurred

• These levels can also be applied to acid rain management strategies

  • Acid rain requires effective pollution management strategies to mitigate its harmful effects on the environment and human health

1. Altering human activity

• Reducing the consumption of fossil fuels is a key strategy to minimise acid rain

  • Encourage the use of alternative energy sources, such as renewable energy, can significantly reduce emissions of sulphur dioxide and nitrogen oxides

• International agreements and national governments play a vital role in:

  • Promoting sustainable practices

  • Supporting the development of clean technologies

  • Lobbying for emissions reductions

2. Regulating and monitoring pollutant release

• Government regulations and monitoring systems are essential to control and reduce the release of pollutants that contribute to acid rain

  • Coal-burning power plants and vehicles are major sources of sulphur dioxide and nitrogen oxide emissions

  • Installing pollution control devices such as scrubbers and catalytic converters can effectively remove these pollutants from emissions

3. Clean-up and restoration measures

• In areas heavily affected by acid rain, certain strategies may be used to mitigate the damage caused

  • For example, spreading ground limestone or lime in acidified lakes and rivers can neutralise acidity and restore the water's pH balance

• Restoring damaged ecosystems can also be achieved through re-colonisation efforts, such as planting acid-tolerant vegetation

  • This can help restore ecological balance to these damaged ecosystems