IB ESS HLTopic 8 — Urban Air PollutionPaper 1 & 2Core skill~12 min read
Acid Deposition and Its Effects
Sulphur dioxide and nitrogen oxides do not stay where they are released. They travel with the wind, react with water and oxygen, and come back down as acid — often hundreds of kilometres away, in a country that burned none of the fuel. That is what makes acid deposition a political problem as well as a chemical one.
📚 What you need to know
Normal rain has a pH of about 5 to 5.6, already slightly acidic because carbon dioxide dissolves in it.
Acid rain has a pH below 5 and is largely caused by human activity.
Nitrogen oxides (mainly from vehicles) form nitric acid; sulphur dioxide (mainly from burning coal and oil) forms sulphuric acid.
Wet deposition is acid falling as rain, snow or fog. Dry deposition is acidic particles and gases settling directly onto surfaces.
Effects: leaching of soil nutrients, release of toxic aluminium, damage to foliage, acidified lakes, corroded stonework and metal.
Humans are affected indirectly, through the fine nitrate and sulphate particles that damage the lungs.
Acid deposition is transboundary: the pollution crosses national borders.
What counts as acid rain
Rain is naturally acidic. Carbon dioxide in the air dissolves into raindrops to make weak carbonic acid, giving unpolluted rain a pH of roughly 5 to 5.6. Acid rain means rainfall with a pH below 5, which requires something stronger than carbon dioxide — and in practice that means nitric and sulphuric acid from human emissions.
Because the scale is logarithmic, a drop from pH 5.6 to pH 4.6 is a much bigger change than it looks on the bar.
The chemistry
Nitric acid
Nitrogen oxides come mainly from vehicle exhausts. Nitrogen monoxide is oxidised in the air to nitrogen dioxide, which then reacts with water and oxygen to give nitric acid.
If you can only remember one thing about the chemistry, remember which pollutant comes from where: nitrogen oxides from traffic, sulphur dioxide from coal. That single fact lets you predict which acid dominates in a given country, and which control measure would help most.
Wet and dry deposition
Wet deposition — the acids dissolve in cloud droplets and fall as acidic rain, snow or fog. This carries pollution furthest, because clouds travel.
Dry deposition — the acids combine with ash and other particles to form acidic dust and gases, which settle directly onto vegetation, soil, buildings and cars. This tends to happen closer to the source.
Dry deposition lands near the source; wet deposition travels with the cloud. The same chimney can damage a nearby car park and a forest in another country.
Effects on ecosystems
Soils and forests
Nutrient leaching. Acid washes calcium, magnesium and potassium out of the soil faster than they are replaced, so plants become nutrient deficient and growth slows.
Aluminium mobilised. Aluminium is normally locked into soil minerals. Acid dissolves it, and dissolved aluminium damages fine root hairs so trees cannot take up water and nutrients properly.
Direct foliage damage. Acid strips the waxy cuticle from leaves and needles, so trees lose water and are more vulnerable to frost, drought and disease.
Conifers are especially sensitive, because they have shallow roots, thin bark and needles that stay on the tree for years, accumulating damage.
Lakes and rivers
Water bodies become more acidic, particularly where the surrounding rock is granite and cannot neutralise the acid.
The dissolved aluminium washed from the soil is toxic to aquatic life. It irritates fish gills, which respond by producing mucus, and the coated gills can no longer absorb oxygen efficiently, so the fish effectively suffocate.
Invertebrates with exoskeletons such as snails and crayfish struggle, because acidic water reduces the availability of the calcium they need to build and maintain their shells.
Fish eggs and young fish are affected at higher pH values than adults, so a lake can lose its next generation long before the adults die.
Two mechanisms, not one. Acid harms life directly by lowering pH, and indirectly by dissolving aluminium out of the soil. The indirect route often does more damage, and mentioning both is what separates a full answer from a half one.
Buildings and infrastructure
Marble and limestone are made of calcium carbonate, which reacts with sulphuric acid. The stone surface dissolves, blisters and crumbles, and detail on carvings is lost permanently.
Metal structures corrode and rust faster, and paint degrades. The Taj Mahal in India, built of marble, has shown erosion and discolouration linked to acid deposition, and historic statues and buildings in Rome and Greece have suffered the same way.
Human health
Acid rain does not burn people. The harm is indirect: the same emissions produce fine nitrate and sulphate particles, which are PM2.5. These reach deep into the lungs, causing tissue damage and inflammation and raising the risk of asthma and bronchitis. Regions with heavy industry, including parts of China and Eastern Europe, carry higher respiratory risks as a result.
WORKED EXAMPLE
Rain over an industrial region is measured at pH 3.6. Unpolluted rain is about pH 5.6. Calculate how many times more acidic the polluted rain is, and explain why this matters for a lake.
Step 1: find the difference in pH units5.6 − 3.6 = 2 pH unitsStep 2: apply the logarithmic scale
Each unit is a ten-fold change in hydrogen ion concentration.
102 = 100100 times more acidicStep 3: why it matters
Most freshwater fish struggle below about pH 5, and at pH 4 or lower few species survive.
Dissolved aluminium also rises sharply as pH falls, so the water becomes toxic as well as acidic.
Lakes on granite have little natural buffering, so they acidify quickly; lakes on limestone resist for much longer.
WORKED EXAMPLE
Explain how emissions from a coal-fired power station in one country can kill fish in a lake in another country.
Step 1: emission
Burning coal releases sulphur dioxide from sulphur impurities in the fuel.
Step 2: transport
Tall chimneys push the gas high into the atmosphere, where prevailing winds carry it hundreds of kilometres across borders.
Step 3: reactionSO2 + H2O gives H2SO3, which is oxidised to H2SO4 in cloud droplets.
Step 4: deposition and effect
The acid falls as wet deposition, lowering lake pH and dissolving aluminium from surrounding soils. The aluminium irritates fish gills, which produce mucus and can no longer absorb oxygen.
The fish suffocate, in a country that burned none of the coalThe phrase to use is transboundary pollution. It is exactly what the question is testing.
Managing acid deposition
Use the same three levels from the previous page.
Level
Strategy
Comment
1. Alter human activity
Switch to renewables and nuclear; reduce fossil fuel demand; international agreements to cut emissions
The only permanent fix, but slow and politically difficult
2. Control the release
Scrubbers on power station chimneys; catalytic converters on vehicles; low-sulphur fuels; emission limits
Effective and fast; sulphur emissions in Europe and North America fell sharply this way
3. Clean up and restore
Spreading ground limestone or lime on acidified lakes and soils; replanting acid-tolerant vegetation
Treats the symptom; must be repeated, and does not stop the acid arriving
Acid deposition is one of the genuine environmental success stories. Sulphur dioxide emissions in Europe and North America fell dramatically from the 1980s onwards through scrubbers, low-sulphur fuel and international agreements, and many damaged lakes have recovered. It is worth saying so — it shows the management levels actually work when they are applied.
💡 Exam tip
Quote the pH values: normal rain about 5 to 5.6, acid rain below 5.
Learn which gas comes from which source: NOx from traffic, SO2 from coal.
Distinguish wet from dry deposition explicitly if the question mentions deposition.
Give both the direct acidity effect and the indirect aluminium effect for lakes and soils.
Use the word transboundary, and say it is why international agreements are needed.
Structure impacts as soils, freshwater, buildings, health. It prevents you from writing the same point twice.
⚠ Common mix-ups
Saying normal rain is pH 7. It is naturally acidic at about 5.6 because of dissolved carbon dioxide.
Treating pH as a linear scale. A drop of two units is a hundred-fold increase in acidity.
Claiming acid rain burns people or fish directly. The health effect comes from fine particles; fish deaths are largely caused by aluminium.
Mixing up sulphurous and sulphuric acid. Sulphurous acid, H2SO3, forms first and is then oxidised.
Forgetting dry deposition. Acid also arrives as particles and gases with no rain at all.
Saying liming solves the problem. It neutralises existing acidity and must be repeated; the emissions continue.
Up next: How Photochemical Smog Forms — the HL material on what sunlight does to traffic fumes, and why some cities get trapped under their own pollution.
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