How acid rain forms at Higher Level: The same statement as SL, with different real cases: India's coal power stations, Robert Angus Smith in Manchester, Kilauea's vog and the smelters of Norilsk. At HL, link acid rain across borders to who should pay for the damage, and to the international law that followed.
Practise this as you read
- Write the equations for sulfuric and nitric acid.
- Explain the pattern of damage on a map.
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Two gases from burning: Acid rain starts with two gases: sulfur dioxide and nitrogen oxides.
Where the gases come from
- Sulfur dioxide (SO₂) comes from burning coal and oil, which contain sulfur, and from smelting metal ores.
- Nitrogen oxides (NOₓ) form in the heat of vehicle engines and power stations, from the nitrogen and oxygen in air.
- The main sources: coal-burning industry, electricity generation and transport.
- Farming adds nitrogen gases too: making and using fertilisers, and livestock.
- Natural sources: volcanoes (SO₂), lightning and soil microbes (NOₓ).
- SO₂ and NOₓ are primary pollutants: they come straight out of the source.
Remember it as: Sulfur from the fuel, nitrogen from the air.
Real example: India's electricity still comes mostly from coal. Many of its coal power stations had no equipment to clean their smoke, and by 2019 India had become the world's largest emitter of sulfur dioxide from human activities.
Sulfur v nitrogen: SO₂ depends on the fuel: low-sulfur fuel makes little. NOₓ depends on the heat: any very hot burning makes it, even of clean fuel.
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In the air the two gases take in more oxygen and dissolve in water droplets. That turns them into strong acids.
The chemistry of acid rain
- SO₂ and NOₓ react with water and oxygen in the air.
- Sulfur dioxide becomes sulfuric acid (H₂SO₄); nitrogen oxides become nitric acid (HNO₃).
- These acids are secondary pollutants: they form in the air, not at the chimney.
- Unpolluted rain is already slightly acidic, about pH 5.6, because CO₂ dissolves in it.
- Acid rain has a pH below 5.6; polluted rain often falls to about pH 4.
- Each step down the pH scale is 10 times more acidic.
In words: sulfur dioxide + oxygen + water → sulfuric acid; nitrogen dioxide + oxygen + water → nitric acid.
Real example: in 1852 the chemist Robert Angus Smith found that rain in Manchester, a city of coal-burning mills, contained sulfuric acid. He was the first to call it 'acid rain'.
Wet or dry, it still lands: Acid deposition is the full name. Acid rain is only one part of it.
Wet and dry deposition
- Acid deposition is every way the acids reach the ground.
- Wet deposition: acids dissolved in rain, snow, fog and mist.
- Dry deposition: SO₂, NOₓ and acid particles settle out of the air, mostly near the source.
- Rain washes SO₂ out of the air: it dissolves in the drops, so SO₂ is lower on wet days.
- Dry deposition left on leaves and soil turns to acid when it next gets wet.
Real example: the Kilauea volcano in Hawaii releases sulfur dioxide that turns into a haze of tiny acid droplets, called vog. Downwind it falls as acid rain.
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Acid deposition rarely stays where it was made. A map of damage around a source shows where the wind took it.
Carried by the wind
- Prevailing winds carry SO₂ and NOₓ downwind, often hundreds of kilometres.
- So acid deposition crosses borders: one country's smoke falls on another.
- Deposition and damage are greatest near the source and fall with distance.
- Further away the gases are diluted, so there is less damage.
- Valleys channel the wind, so pollution travels along them; rivers and lakes spread it.
- Tall chimneys reduce local pollution but send the gases further.
Real example: around the nickel smelters of Norilsk, dead and damaged forest stretches far downwind. In the 1970s and 1980s, most of the acid falling on southern Norway and Sweden came from the UK and central Europe.
Explain, do not describe: When a map shows damage, describing the zones scores nothing. Explain them: the wind direction, the distance from the source, the dilution, the valleys.
How this comes up: Paper 1 gives a map of damage around a source and asks you to explain its pattern; Paper 2 asks short parts on a graph of SO₂.
A map shows vegetation damage around a copper smelter. The damage is total next to the smelter, and the severe and moderate zones stretch 60 km to the east, along a river valley. The prevailing wind blows from the west.
Explain the distribution of vegetation damage shown on the map.
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A Section A part asks you to read a graph of SO₂ in one line.
Monitors in a port city show that sulfur dioxide is lower in the rainy season than in the dry season, although the factories burn the same fuel all year.
Outline why sulfur dioxide levels are lower in the rainy season.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.