The definition and nature of transboundary pollution (TBP)

Transboundary pollution (TBP) refers to pollution that impacts a wide area or crosses the borders of more than one country.

Characteristics of transboundary pollution

• Wide-reaching impact - Affects large regions, often spanning multiple countries, due to the movement of pollutants through air or water.
• Cross-border challenges - Creates issues for neighbouring countries that may not be the source of the pollution but still suffer its consequences.
• Examples of TBP - Includes phenomena like acid rain, air pollution from industrial emissions, and river contamination that flows across borders.

The formation and types of acid deposition

Acid deposition is a significant form of transboundary pollution, occurring when pollutants are released into the atmosphere and later deposited on the Earth's surface. It can take two primary forms, depending on the mechanism of deposition, and often travels long distances from its source.

Processes involved in acid deposition formation

• Emission of pollutants - Factories, power stations, and vehicles release sulphur dioxide (SO₂) and nitrogen oxides (NOx) into the atmosphere.
• Transport by wind - These pollutants are carried by winds over distances ranging from tens to thousands of kilometres, often crossing international borders.
• Mixing and transformation - Pollutants mix with atmospheric moisture, undergoing oxidation and dissolution in clouds, forming weak sulphuric and nitric acids.
• Deposition to the surface - The acids are deposited back to the ground either as dry particles or within precipitation.

Types of acid deposition

• Dry deposition - Occurs close to the pollution source, typically within 12-32 km, where pollutants settle as dry particles or gases directly onto surfaces like soil or vegetation.
• Wet deposition - Happens when pollutants dissolve in precipitation, such as rain or snow, and fall to the ground, often far from the source, over hundreds or even thousands of kilometres. This is a key form of transboundary pollution.

The causes and effects of acid rain

Acid rain is a type of wet deposition where rainfall becomes more acidic than normal due to human activities. It is defined as having a pH below 5.5, compared to natural rainfall which has a pH of 5 to 6 due to the absorption of carbon dioxide forming weak carbonic acid.

Causes of acid rain

• Primary pollutants - The main contributors are sulphur dioxide (SO₂) and nitrogen oxides (NOx), released from burning fossil fuels like coal, oil, and gas.
• Major sources - Coal-fired power stations are the largest producers of SO₂, while vehicles, particularly cars, contribute most of the NOx emissions.
• Relative impact - Sulphur oxides account for approximately 65% of acid rain formation, though global SO₂ emissions are decreasing, while NOx emissions are rising due to increasing vehicle numbers.

Effects of acid rain

• Damage to infrastructure - Accelerates the weathering and corrosion of buildings and monuments, especially those made of limestone or other sensitive materials.
• Impact on water bodies - Acidic water mobilises metals like iron and aluminium, carrying them into rivers and lakes, where aluminium can harm fish by damaging their gills.
• Harm to vegetation - Reduces tree growth by increasing soil acidity, making it harder for plants to absorb nutrients.
• Soil degradation - Increases soil acidity, disrupting ecosystems and affecting agricultural productivity.
• Aquatic life decline - Acidification of lakes and rivers harms aquatic organisms, reducing biodiversity.
• Potential health links - Though not fully proven, there are suggested connections between acid deposition and increased cases of senile dementia due to metal mobilisation.
• Regional variations - Effects are more severe in areas with high rainfall, where more acidity is deposited, and in regions with acidic rocks that cannot neutralise the deposited acids.

Strategies for reducing the impacts of acid deposition

Addressing acid deposition involves both short-term mitigation and long-term prevention strategies. While immediate actions can help reduce damage, sustainable solutions focus on cutting down the emission of harmful pollutants.

Short-term mitigation measures

Adding powdered limestone to affected lakes and rivers raises their pH and reduces acidity, protecting aquatic ecosystems temporarily.

Long-term prevention strategies

• Reducing fossil fuel use - Decreasing reliance on coal, oil, and gas by improving energy efficiency and promoting conservation practices.
• Switching to cleaner fuels - Using fossil fuels with lower sulphur content to minimise SO₂ emissions during combustion.
• Adopting alternative energy sources - Investing in renewable energy options like solar, wind, and hydropower, which do not produce nitrate or sulphate gases.
• Emission control technologies - Installing systems to remove pollutants from industrial exhausts before they are released into the atmosphere, such as scrubbers in power stations.