Composition and structure

Dry air is about 78% nitrogen, 21% oxygen and roughly 1% other gases (argon, carbon dioxide, and variable water vapour). Though tiny in amount, greenhouse gases like CO₂ have an outsized effect.

• Troposphere — the lowest layer (0–~12 km); where weather and most life occur.
• Stratosphere — above it (~12–50 km); contains the protective ozone layer.

Your turn — the rest of the air

Atmospheric circulation

Uneven heating drives global air movement. Intense sunlight at the equator warms air, which rises, cools and releases heavy rain (tropical rainforests), then sinks near 30° latitude, creating the world’s great deserts. These looping Hadley cells, plus the Earth’s rotation, set the pattern of winds, rainfall and climate zones.

Circulation also spreads pollutants: emissions can be carried thousands of kilometres, so no country breathes in isolation.

Stratospheric ozone and its depletion

In the stratosphere, ozone (O₃) absorbs harmful ultraviolet (UV) radiation, shielding life from skin cancer, cataracts and damage to ecosystems and crops.

Ozone-depleting substances (ODS) — chiefly CFCs (once used in aerosols, fridges and foams) — drift up to the stratosphere, where UV frees chlorine atoms. Each chlorine atom acts as a catalyst, destroying many thousands of ozone molecules before it is removed, thinning the layer and opening the Antarctic "ozone hole".

Good ozone, bad ozone

How CFCs do damage

Your turn — phasing out ODS

Primary and secondary pollutants

• Primary pollutants — emitted directly, mainly from combustion: carbon monoxide (CO), oxides of nitrogen (NOₓ), sulfur dioxide (SO₂), particulate matter and volatile organic compounds (VOCs).
• Secondary pollutants — formed in the atmosphere when primary pollutants react, often in sunlight: ground-level ozone, peroxyacetyl nitrates (PANs) and acids.

Tip: if it comes straight out of an exhaust or chimney it is primary; if it is made later by reactions in the air, it is secondary.

Primary or secondary?

Sort the pollutants

Tap a substance, then tap the category it belongs to.

Photochemical smog and thermal inversions

Photochemical smog forms in warm, sunny cities when NOₓ + VOCs + sunlight generate ground-level ozone and PANs. It irritates eyes and lungs, worsens asthma and damages crops.

A thermal (temperature) inversion — a layer of warm air trapping cooler air below — stops pollutants dispersing, so smog builds up (as in Los Angeles or Delhi). Topography, such as a surrounding basin of hills, makes it worse.

Acid deposition

Burning fossil fuels releases SO₂ and NOₓ, which react with water and oxygen to form sulfuric and nitric acids. These fall as acid deposition (acid rain, snow or dry particles), often far downwind of the source.

• Effects: acidifies lakes (killing fish), leaches nutrients and mobilises toxic aluminium in soils, damages tree foliage, and corrodes limestone buildings.
• Transboundary: UK emissions have acidified Scandinavian lakes — a classic international dispute.

What causes acid rain?

Your turn — cleaning the air

Managing air pollution

Pollution management works at three levels, and it is generally cheaper and more effective to act earlier:

• 1. Alter the human activity — burn less fossil fuel: public transport, renewables, energy efficiency.
• 2. Regulate and reduce at the point of release — catalytic converters, flue-gas scrubbers, low-sulfur fuels, emission standards.
• 3. Clean up and restore afterwards — e.g. adding lime to acidified lakes.

Success story: the Montreal Protocol (1987) phased out CFCs worldwide and the ozone layer is now slowly recovering — proof that global cooperation can work.

Best level to act

Match the atmosphere terms

Tap a statement on the left, then its matching answer on the right.

The big ideas to know

You can now explain both the ozone shield and the pollution we pump into the air. Press Finish to see your score.