Causes & Impacts of Acid Rain (Cambridge (CIE) IGCSE Environmental Management): Revision Note

How does acid rain form?

• Acid rain is precipitation or atmospheric deposits with a pH value below 5

• It occurs when sulphur dioxide and nitrogen oxides are released into the atmosphere through human activities such as car emissions and burning coal

• It can also come from natural sources, such as volcanic eruptions

• The equation for this process is: 

sulphur dioxide + water + oxygen → sulphuric acid

• Most acid rain is caused by fossil fuel combustion in factories and power stations, and motor vehicles, releasing sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) into the atmosphere

  • Power stations that burn coal, oil, or natural gas are major contributors to SO₂ emissions, while NOₓ emissions mostly come from vehicle exhausts

  • Sulphur dioxide is released when fossil fuels containing sulphur compounds are burned

  • Oxides of nitrogen are produced when nitrogen from the atmosphere reacts with oxygen at the high temperatures of vehicle engines and industrial furnaces

  • These acidic gases settle directly onto surfaces (buildings, vegetation, soil) without rain

  • This is known as dry deposition

• When NOₓ and SO₂ react with water vapour and oxygen in the atmosphere, they form weak solutions of nitric acid (HNO₃) and sulphuric acid (H₂SO₄)

• Sulphuric acid and nitric acid can dissolve in rainwater or snowflakes, forming acid rain

• Acid rain can be moved by winds for hundreds of kilometres before it falls to the ground, so it can occur at some distance from the source; this is known as wet deposition

Impacts of acid rain

• Acid rain is not restricted to the immediate area of emission

• Acid rain is more likely to affect locations downwind of big industrial regions because winds can carry particles for long distances, causing transboundary pollution to occur in other countries

• Acid rain affects both the living and the non-living

Water

• Acidic rain directly affects bodies of water, aquatic food webs, and aquatic organisms such as fish, amphibians, and invertebrates

• When acid rain falls on bodies of water, pH levels fall and cannot support life, causing water acidification

• Food web destruction starts at the lowest level of the food chain, where tiny microorganisms that are food die

• As food sources dwindle, more and larger fish die

• This cascades through food webs, which further reduces water quality

• Acid rain can leach copper, aluminium, and other heavy metals out of the soil and into runoff and drinking water

Soil and plants

• Acid rain decreases the pH of the soil, causing its acidity to increase, which decreases the level of important nutrients found in the soil

  • This makes it harder for plants to absorb nutrients and grow

• Acid rain negatively impacts forests and trees as sulphur dioxide interferes with the process of photosynthesis

  • When acid rain directly falls on leaves, it damages the protective waxy coating of leaves, interrupting water and gas exchange, which eventually kills the plant

• Acid rain also damages foliage, causing defoliation and reduces nutrient absorption

  • Acid rain is especially harmful to coniferous forests such as pine and spruce due to their shallow root systems and thin bark

• Removing a whole tree can be equivalent to the effect of 60 years of acid rain

Buildings

• Acid rain can react with metals and stone (such as limestone), damaging buildings and statues

• Limestone and marble both contain calcium carbonate (CaCO₃)

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