Acid Deposition (Cambridge (CIE) AS Environmental Management)

Acid Deposition

• Acid deposition is a significant environmental issue caused by human activities

• It occurs when sulfur dioxide and nitrogen oxides are released into the atmosphere primarily from the combustion of fossil fuels

• These gases can undergo chemical reactions and transform into secondary pollutants, resulting in both dry and wet acid deposition

  

Primary Pollutants From the Combustion of Fossil Fuels

• Fossil fuel combustion in power plants, industrial processes and motor vehicles releases sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), such as nitrogen monoxide (NO), into the atmosphere:

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

  • Sulfur dioxide is released when fossil fuels containing sulfur compounds are burned and contributes to the formation of acid rain directly

  • Oxides of nitrogen are produced through high-temperature combustion processes, such as those occurring in vehicle engines and industrial furnaces

Formation of Secondary Pollutants

• Sulfur dioxide and nitrogen monoxide gases can undergo chemical reactions with other compounds already present in the atmosphere, such as oxygen, water vapour and various pollutants:

  • These reactions lead to the formation of secondary pollutants, including sulfuric acid and nitric acid

Dry Deposition of Secondary Pollutants

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

  • Dry deposition occurs when these particles settle on surfaces, including vegetation, buildings and soil, causing damage over time

Wet Deposition of Secondary Pollutants

• Sulfuric acid and nitric acid can dissolve in rainwater or snowflakes, forming acidic precipitation:

  • Wet deposition occurs when this acidic precipitation falls to the ground (e.g. as snow, rain, hail or fog), affecting surface water and soils

• These processes of conversion from primary pollutants to secondary pollutants contribute to the phenomenon of acid deposition

• Understanding the causes of acid deposition is crucial in developing effective strategies to reduce emissions, mitigate its environmental impacts and promote sustainable practices

Impacts of Acid Deposition

• Acid deposition has significant environmental effects that can impact various ecosystems and regions

• The effects of acid deposition are diverse and can have direct and indirect consequences on aquatic organisms, forests and nutrient cycling

Direct Effects

• Acid deposition can react with metals and rocks (such as limestone) damaging buildings and statues:

  • Limestone and marble both contain calcium carbonate (CaCO3)

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

• Acidic deposition, particularly in bodies of water, directly affects aquatic organisms such as fish, amphibians and invertebrates:

  • Acidic deposition lowers the pH of the water, making it difficult for some species to survive and reproduce

• As well as acid deposition directly falling on leaves, causing leaf damage and eventually killing plants, acid particulates can also block stomata (plant pores), preventing gaseous exchange:

  • This can damage crops and reduce crop yields

• Coniferous forests, such as those composed of pine or spruce trees, are sensitive to acid deposition due to their shallow root systems and thin bark:

  • Acid rain also damages their foliage and inhibits nutrient absorption

Indirect Toxic Effects on Fish

• Acid deposition can increase the solubility of metals like aluminium in water bodies:

  • High levels of aluminium ions become toxic to fish, affecting their respiratory systems (e.g. their gills), impairing growth and even causing death

Indirect Nutrient Effects

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

  • Leaching of nutrients reduces their availability for plants, leading to nutrient deficiencies and impacting plant growth and overall ecosystem productivity

Geographic and Transboundary Impacts

• The impacts of acid deposition are not confined to the immediate vicinity of emission sources:

  • Areas downwind of major industrial regions are more likely to experience the adverse effects of acid deposition

  • It is also possible for the impacts of acid deposition to extend across national boundaries, affecting regions in different countries than the source of emissions

• These effects of acid deposition highlight the environmental risks associated with high levels of sulfur dioxide and nitrogen oxides emissions

• Efforts to reduce acid deposition involve emission control measures, such as the use of cleaner technologies and regulations on industrial and transportation emissions

• International cooperation and agreements are also necessary to address transboundary impacts