IB ESS SL8.3 Urban Air PollutionPaper 1 & 2~14 min read
Acid Deposition and Its Effects
This is the topic where pollution stops being a local problem. Sulfur dioxide leaves a chimney in one country, spends days in the atmosphere turning into acid, and falls on a forest or a lake hundreds of kilometres away in another. Nobody who caused it has to live with it.
📘 What you need to know
Normal rainwater has a pH between 5 and 5.5, so it is already slightly acidic.
Acid rain has a pH below 5 and is usually the result of human activity.
Nitrogen oxides (NOx) and sulfur dioxide (SO2) are the gases responsible.
They react with water and oxygen in the atmosphere to form nitric acid and sulfuric acid.
Wet deposition is acidic rain, snow or fog. Dry deposition is acidic dust and gases settling on surfaces.
Terrestrial impacts: leaching of calcium, magnesium and potassium, aluminium mobilisation, and direct foliage damage.
Freshwater impacts: solubilisation of aluminium, which is toxic to fish and invertebrates.
Buildings: calcium carbonate in marble and limestone is corroded. The Taj Mahal is the named example.
Health: nitrate and sulfate particles cause respiratory problems; acid rain does not harm people directly.
Management works at three levels: alter human activity, control the release, clean up and restore.
What counts as acid rain
Acid rain is rainfall with a pH lower than normal rainwater. Note what that means: normal rain is not neutral to begin with.
Regular rain has a pH between 5 and 5.5, so it is naturally slightly acidic.
Acid rain is more acidic still, with a pH below 5, and is frequently the result of human activity.
pH is a logarithmic scale, so rain at pH 4 is not “a bit” more acidic than rain at pH 5. It is ten times more acidic.
The chemistry
Nitrogen oxides and sulfur dioxide are the main gases responsible. Both react with water and oxygen in the atmosphere to form acids.
Formation of nitric acid
Nitrogen oxides are mainly produced from vehicle exhausts. Nitrogen monoxide reacts with oxygen to form nitrogen dioxide:
Step 1
2NO + O2 → 2NO2
The nitrogen dioxide then reacts with water and oxygen in the air to produce nitric acid:
Step 2
4NO2 + O2 + 2H2O → 4HNO3
Formation of sulfuric acid
Sulfur dioxide is produced by burning fossil fuels. It dissolves in rainwater to produce sulfurous acid:
Step 1
SO2 + H2O → H2SO3
That sulfurous acid is then oxidised by oxygen in the air to produce sulfuric acid:
Step 2
2H2SO3 + O2 → 2H2SO4
🧩
Two gases, two acids
NOx gives nitric acid. SO2 gives sulfuric acid. The first letter carries straight through, and both need water plus oxygen to get there.
Wet and dry deposition
Wet deposition is acidic precipitation falling to Earth as rain, snow or fog.
Sulfuric and nitric acid can also combine with ash and other particles in the air to form dry particles — acidic dust and gases.
Dry deposition happens when those particles settle on surfaces: vegetation, buildings, cars and soil.
Dry deposition lands close to the source; wet deposition travels. That is why acid rain became an international dispute rather than a local one.
Why this matters politically. Because the winds carry the gases, the country suffering the damage is often not the country that burned the fuel. A government has little incentive to pay for scrubbers when the benefit lands somewhere else, which is exactly why international agreements were needed to deal with acid rain.
Impacts on ecosystems
Terrestrial habitats
Leaching of nutrients. Acidic deposition speeds up the leaching of essential nutrients from soil, such as calcium, magnesium and potassium. That reduces their availability to plants, causing nutrient deficiencies, which reduces plant growth and overall ecosystem productivity.
Increased soil toxicity. Acid rain mobilises harmful metals such as aluminium, which damages plant roots and affects their ability to absorb water and nutrients.
Direct damage to foliage. This weakens trees, making them more vulnerable to disease and harsh weather.
Coniferous forests such as pine and spruce are especially sensitive, because of their shallow root systems and thin bark. Acid rain also damages their foliage and inhibits nutrient absorption.
Notice that acid rain attacks a tree from two directions at once — the leaves above and the roots below. That is why damage can be so severe: the tree loses its ability to photosynthesise and its ability to take up water and nutrients at the same time.
Freshwater habitats
Acid rain makes water bodies more acidic, through a process called solubilisation of aluminium: aluminium that is normally bound in the soil dissolves and enters nearby water bodies.
That aluminium is toxic to aquatic life. Fish gills become coated with it, which makes it harder for them to breathe.
Invertebrates with exoskeletons struggle too. They rely on calcium to build and maintain their hard outer shells, and increased acidity reduces the availability of calcium and other minerals, so they cannot properly develop or maintain their exoskeletons.
Impacts on buildings and infrastructure
Acid rain erodes marble, limestone, steel and paint used in buildings and monuments.
Marble and limestone both contain calcium carbonate (CaCO3), which reacts with sulfuric or nitric acid, so the stonework corrodes and weakens.
The Taj Mahal in India, built of marble, shows erosion and discolouration caused by acid rain.
Historical statues and structures in Rome and Greece have been affected in the same way.
Impacts on human health
Acid rain does not directly harm humans — you can stand in it safely. The harm comes from the particles.
Nitrate and sulfate particles associated with acid rain cause respiratory problems.
These are PM2.5 particles, small enough to enter the lungs, where they cause tissue damage and lung inflammation.
That raises the risk of conditions such as asthma and bronchitis.
Areas with heavy industrial activity, such as parts of China and Eastern Europe, face greater respiratory health risks as a result.
Managing acid deposition
There are three levels of pollution management, and they apply to acid deposition just as they do to any other pollutant.
The red boxes are the stages of the pollutant’s journey. Each management level intervenes at a different stage of it.
1. Altering human activity
Reducing the consumption of fossil fuels is the key strategy for minimising acid rain.
Encouraging renewable energy significantly reduces emissions of sulfur dioxide and nitrogen oxides.
International agreements and national governments promote sustainable practices, support clean technologies and lobby for emissions reductions.
2. Regulating and monitoring pollutant release
Government regulations and monitoring systems control and reduce the release of the pollutants that cause acid rain.
Coal-burning power plants and vehicles are the major sources of sulfur dioxide and nitrogen oxide emissions.
Scrubbers and catalytic converters remove these pollutants from emissions before they reach the atmosphere.
3. Clean-up and restoration
Spreading ground limestone or lime in acidified lakes and rivers neutralises the acidity and restores the water’s pH balance.
Damaged ecosystems can be restored through re-colonisation, such as planting acid-tolerant vegetation, which helps restore ecological balance.
Liming a lake treats the symptom. The acid keeps arriving, so the lime has to keep being added, and none of it helps the soil in the surrounding catchment. Point that out in an evaluation and you show you understand why level 1 sits at the top.
Worked examples
WE 1
Formation of acid rain
Explain how sulfur dioxide released from a power station leads to acid rain. (3 marks)
Step 1: the source
Sulfur dioxide is produced by burning fossil fuels, mainly coal and oil, in power stations.
Step 2: it dissolves
In the atmosphere it dissolves in rainwater to form sulfurous acid: SO2 + H2O gives H2SO3.
Step 3: it is oxidised
The sulfurous acid is then oxidised by oxygen in the air to produce sulfuric acid: 2H2SO3 + O2 gives 2H2SO4. This falls as rain with a pH below 5.
SO₂ to sulfurous acid to sulfuric acid, then down as raintwo steps, two equations — do not stop at sulfurous acid
WE 2
Effects on a lake
Explain how acid deposition can reduce fish populations in a freshwater lake. (4 marks)
Step 1: the water acidifies
Acid rain falling on the lake and running off the surrounding land lowers the pH of the water.
Step 2: aluminium is released
The acidity causes solubilisation of aluminium: aluminium normally bound in the soil dissolves and enters the lake.
Step 3: the direct effect on fish
Aluminium is toxic to aquatic life. It coats fish gills, which makes it harder for them to breathe.
Step 4: the effect on their food
Invertebrates with exoskeletons cannot get enough calcium in acidic water to build and maintain their shells, so the fish also lose part of their food supply.
Lower pH releases aluminium, which suffocates the fish and removes their preybringing in the invertebrates shows the ecosystem link and lifts the answer
WE 3
Evaluating management levels
Evaluate the three levels of pollution management as strategies for dealing with acid deposition. (4 marks)
Level 1: altering human activity
Reducing fossil fuel use and switching to renewables prevents SO2 and NOx being produced at all, so it is the most effective level. It is also the slowest and needs international agreement.
Level 2: controlling the release
Scrubbers and catalytic converters remove pollutants from emissions, which works where the activity cannot be stopped. It costs money to install and the activity continues.
Level 3: clean-up and restoration
Liming lakes and replanting acid-tolerant vegetation repairs damage, but the acid keeps arriving, so it must be repeated indefinitely.
Judgement
Level 1 is preferable because it removes the cause, but since no country can stop all emissions immediately, all three are used together.
Prevention beats treatment, but in practice all three run at oncejudge which is best and say why, then acknowledge why the others are still used
💡 Exam tips
Quote the pH figures: normal rain 5 to 5.5, acid rain below 5.
Learn both pairs of equations. They are short and they are worth easy marks.
Keep wet and dry deposition separate, and say what form each takes.
For ecosystem impacts, use the terms leaching, mobilisation and solubilisation of aluminium.
Name the example: the Taj Mahal, and mention calcium carbonate when discussing buildings.
Say acid rain does not harm people directly — the harm comes from nitrate and sulfate particles.
Use the three levels as a ready-made structure for any management question.
⚠ Common mistakes
Saying normal rain is pH 7. It is naturally acidic, between 5 and 5.5.
Treating pH as a linear scale. Each step is a factor of ten.
Saying acid rain burns people. It does not harm humans directly; the particles cause respiratory problems.
Forgetting the aluminium. Most fish deaths come from dissolved aluminium, not from the acidity alone.
Assuming the damage is local. Prevailing winds carry the gases across borders.
Calling liming a solution. It treats the symptom and has to be repeated.
Mixing up sulfurous and sulfuric acid. Sulfurous acid forms first, then is oxidised.
That completes Topic 8: Human Populations & Urban Systems. Read back across all ten notes and one argument runs through them: more people, concentrated into cities, drawing in resources and pushing out waste. Everything else — sprawl, smog, acid rain, planning — is a consequence of that one shift, and a chance to manage it better.
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