Syllabus Edition
First teaching 2024
First exams 2026
Acid Rain (DP IB Environmental Systems & Societies (ESS))
Revision Note
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
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 + H2O → H2SO3
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:
Changing human activity
Regulating and reducing quantities of pollutants released at the point of emission
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
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