IB Diploma Environmental Systems & Societies HL · Atmospheric Systems and Societies
Mini-Lesson
Atmospheric Systems
This mini-lesson covers the thin blanket of gas that makes Earth habitable: the composition and structure of the atmosphere and its circulation, the double life of ozone (a life-saving shield high up, a pollutant near the ground), and the air pollution we create — primary and secondary pollutants, photochemical smog, acid deposition, and how these are managed.
Systems lens: the atmosphere is a shared global resource. Pollution released in one country routinely crosses borders, making air quality a transboundary — and political — problem.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Atmosphere · what it is
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.
Calculate
Your turn — the rest of the air
1Dry air is about 78% nitrogen and 21% oxygen by volume. Calculate the percentage made up by all the remaining gases (argon, carbon dioxide and others).
%
Hint: remaining % = 100 − 78 − 21.
Atmosphere · circulation
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.
Atmosphere · the ozone shield
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".
Quick check
Good ozone, bad ozone
?Ozone is described as both essential and a pollutant. Where is ozone beneficial to life?
Quick check
How CFCs do damage
?Why does a single chlorine atom from a CFC destroy so many ozone molecules?
Calculate
Your turn — phasing out ODS
2Global use of ozone-depleting substances fell from 1.5 million tonnes to 0.15 million tonnes after the Montreal Protocol. Calculate the percentage reduction.
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.
Quick check
Primary or secondary?
?Sunlight makes NOₓ and VOCs from traffic react to form ground-level ozone. What type of pollutant is this ozone?
Sort it
Sort the pollutants
Tap a substance, then tap the category it belongs to.
🏭 Primary pollutant
🌫️ Secondary pollutant
🧴 Ozone-depleting
Atmosphere · smog
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.
Atmosphere · acid deposition
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.
Quick check
What causes acid rain?
?Which pair of primary pollutants is chiefly responsible for acid deposition?
Calculate
Your turn — cleaning the air
3After fitting scrubbers to power stations, a country cut its SO₂ emissions from 25 million tonnes to 6 million tonnes. Calculate the percentage reduction in SO₂ emissions.
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.
Quick check
Best level to act
?Fitting catalytic converters to cars to reduce NOₓ leaving the exhaust is an example of pollution management at which level?
Match it
Match the atmosphere terms
Tap a statement on the left, then its matching answer on the right.