- Describe the vertical structure of the atmosphere and the physical properties of each layer.
- Explain the chemistry and physics of the greenhouse effect and name the greenhouse gases with their sources.
- Account for the catalytic destruction of ozone by CFCs and the formation of the Antarctic ozone hole.
- Relate atmospheric change to climate, sea level and living systems.
🌳 Concept tree
The Atmosphere
├── Layers (by temperature profile)
- │ ├── Troposphere 0–12 km · weather · Lapse rate 6.5 °C/km
- │ ├── Stratosphere 12–50 km · ozone layer · temperature rises
- │ ├── Mesosphere 50–85 km · coldest layer · meteors burn
- │ ├── Thermosphere 85–600 km · ionosphere · aurora · very hot, very thin
- │ └── Exosphere 600–10,000 km · merges with space
├── Composition
- │ ├── N₂ 78% · O₂ 21% · Ar 0.93% · CO₂ 0.04% + water vapour and dust
│ └── Variable gases and aerosols
├── The greenhouse effect
│ ├── Shortwave solar radiation passes in
│ ├── Earth re-emits longwave infrared
- │ └── GHGs absorb and re-emit → the surface stays warm
├── Ozone chemistry
| Comparison aspect | Main idea | Keywords |
|---|
| │ ├── Chapman cycle | - O₂ + hν → 2O·
- O· + O₂ → O₃
| |
| │ ├── CFC photolysis | CFC + hν → Cl· | |
| • | - │ └── Cl· + O₃ → ClO· + O₂
- ClO· + O → Cl· + O₂ (catalytic)
| catalytic |
└── Consequences
- ├── Sea-level rise, extreme weather, desertification
- └── UV-B increase skin cancer, cataracts, phytoplankton damage
| Comparison aspect | Main idea | Keywords |
|---|
| Lapse rate | - the rate at
- temperature falls with altitude in the troposphere
- about 6.5 °C per km
| temperaturefallsaltitudetroposphere |
| Troposphere | - the lowest layer
- 0–12 km
- containing about 75% of the mass of the atmosphere and practically all water vapour — the layer of weather
| lowestlayercontainingmass |
| Stratosphere | - 12–50 km
- temperature rises with height because ozone absorbs ultraviolet
- aircraft fly here for stable air
| temperaturerisesheightbecause |
| Ozone layer | - the region of maximum O₃ concentration
- roughly 15–35 km
- which absorbs most of the solar UV-B and UV-C
| regionmaximumconcentrationroughly |
| Ionosphere | the ionised part of the thermosphere that reflects radio waves and produces the aurora | ionisedpartthermospherereflects |
| Greenhouse gas (GHG) | - a gas that absorbs and re-emits long-wave infrared radiation and therefore warms the surface: water vapour
- CO₂
- CH₄
- N₂O
- O₃ and CFCs
| absorbsemitslongwave |
| Global warming potential (GWP) | - the heat trapped by a mass of a gas relative to CO₂ over 100 years — CH₄ ≈ 28
- N₂O ≈ 265
- some CFCs exceed 10
- 000
| heattrappedmassrelative |
| Dobson unit (DU) | - the unit of total ozone column
- 300 DU is a normal value and below 220 DU defines an ozone “hole”
| totalozonecolumnnormal |
| CFC (chlorofluorocarbon) | - a stable
- non-toxic
- volatile organic compound of carbon
- chlorine and fluorine
- once used as a refrigerant
- propellant and solvent
| stabletoxicvolatileorganic |
| Comparison aspect | Main idea | Keywords |
|---|
| Pressure decreases with altitude | - about 50% of the mass of the atmosphere lies below 5.5 km
- at 12 km the pressure is a quarter of sea-level value
| massatmosphereliesbelow |
| Wien’s law | λmax = 2.9×106/T(nm·K). The Sun (≈ 5800 K) peaks at 0.5 µm (visible); the Earth (≈ 288 K) peaks at 10 µm (infrared) — that difference is why the atmosphere is transparent to the incoming radiation and opaque to the outgoing one. | λmax5800peaksvisible |
| Enhanced greenhouse effect | - CO₂ has risen from about 280 ppm (pre-industrial) to more than 420 ppm today
- a rise of about 50%
- with a global mean temperature increase of roughly 1.1 °C
| risenindustrialtodayrise |
| Carbon budgets | 1 ppm of CO₂ ≈ 2.12 Gt of carbon in the atmosphere | atmosphere |
| Ozone stoichiometry | - one chlorine radical destroys up to 100
- 000 ozone molecules before it is removed from the stratosphere
| chlorineradicaldestroysmolecules |
| Comparison aspect | Main idea | Keywords |
|---|
| • | - Temperature
- not composition
- is what divides the atmosphere into layers
| Temperaturecompositionwhatdivides |
| Troposphere (0–12 km) | - temperature falls from about 15 °C to −56 °C
- convection
- clouds
- rain and all weather occur here
- combustion products and dust are trapped in it
| temperaturefallsconvectionclouds |
| Stratosphere (12–50 km) | - temperature rises from −56 °C to about −2 °C because ozone absorbs UV and releases heat
- the air is stable and dry
- so jet aircraft and balloons operate here
| temperaturerisesbecauseozone |
| Mesosphere (50–85 km) | - temperature falls again to about −90 °C — the coldest layer
- meteors burn up in it
| temperaturefallsagaincoldest |
| Thermosphere (85–600 km) | - temperature climbs above 1
- 000 °C because the few molecules absorb intense UV and X-rays
- the air is so thin that the heat would not warm an object. Contains the ionosphere
- the International Space Station orbits here
| temperatureclimbsabovebecause |
| Exosphere (600 km and above) | - particles escape into space
- merges with the magnetosphere
| particlesescapespacemerges |
Composition of dry air→N₂ 78.08%, O₂ 20.95%, Ar 0.93%, CO₂ 0.04%, with neon, helium, krypton, xenon and variable water vapour (0–4%) and dust that acts as nuclei for condensation.
- Short-wave solar radiation passes through the atmosphere with little absorption and heats the surface.
- The warmed surface emits long-wave infrared. Molecules that change dipole moment when they vibrate — CO₂, CH₄, H₂O, N₂O, O₃ — absorb that radiation and re-emit it in all directions, part of it back to the ground.
- N₂ and O₂, being symmetric diatomics, do not absorb infrared and are not greenhouse gases — a favourite exam distinction.
- The natural greenhouse effect keeps the Earth near 15 °C instead of −18 °C; the problem is the enhanced effect caused by extra gases.
- Sources of the main gases CO₂ from burning fossil fuels, cement manufacture and deforestation · CH₄ from rice paddies, ruminants, landfills and gas leaks · N₂O from fertilisers and biomass burning · CFCs from old refrigerants and solvents · tropospheric O₃ from the photochemical reaction of NOx with hydrocarbons in sunlight (photochemical smog).
- Feedbacks amplify the change melting ice lowers albedo, warmer air holds more water vapour, and thawing permafrost releases methane.
Chapman cycle — natural balance
O₂ + hν (UV-C) → 2 O·
O· + O₂ + M → O₃ + M
O₃ + hν (UV-B) → O₂ + O· · O₃ + O· → 2 O₂
- In the undisturbed stratosphere
- ozone is made and destroyed at the same rate.
| Comparison aspect | Main idea | Keywords |
|---|
| • | - CFCs are chemically inert in the troposphere
- so they drift unbroken up to the stratosphere — that very stability is the reason they reach the ozone layer
| CFCschemicallyinerttroposphere |
| There, strong UV splits them | CFCl₃ + hν → CFCl₂· + Cl· | CFCl₃CFCl₂ |
| Cl· + O₃ → ClO· + O₂ | - and ClO· + O· → Cl· + O₂: the chlorine atom is regenerated
- so it acts as a catalyst and one radical can remove up to 100
- 000 ozone molecules
| chlorineatomregeneratedacts |
| Net reaction | O₃ + O· → 2 O₂ — the ozone is gone while the chlorine is untouched | ozonegonewhilechlorine |
- In the polar winter the air is isolated and colder than −78 °C; polar stratospheric clouds form on whose surfaces inactive chlorine reservoirs (HCl, ClONO₂) are converted into photochemically active Cl₂.
- When sunlight returns in September, that chlorine is released all at once and the ozone column collapses — a seasonal hole, deepest in September–October.
- The Montreal Protocol (1987) and its amendments phased out CFCs and halons; the ozone layer is now recovering, with a full expected recovery near the middle of this century — the clearest example of successful international environmental action.
| Comparison aspect | Main idea | Keywords |
|---|
| Higher UV-B reaching the surface | - skin cancer and cataracts in humans
- suppressed immunity
- reduced photosynthesis and phytoplankton production — the base of the marine food web and a major carbon sink
| skincancercataractshumans |
| Warming | - sea-level rise from thermal expansion and from melting of the Greenland and Antarctic ice sheets
- coastal erosion and salt-water intrusion into the Nile Delta aquifer
| levelrisethermalexpansion |
| Weather extremes | - heat waves
- flash floods
- longer droughts and shifts in rainfall belts that reduce crop yield
| heatwavesflashfloods |
| Acid rain | SO₂ and NOx oxidise and dissolve to H₂SO₄ and HNO₃, which lower the pH of lakes, leach aluminium from soil and corrode limestone buildings and monuments. | oxidisedissolveH₂SO₄HNO₃ |
| Photochemical smog | - NO₂ plus hydrocarbons under sunlight give ozone and PAN
- which irritate the eyes and lungs and damage vegetation in cities
| plushydrocarbonsundersunlight |
| Desertification and loss of biodiversity | - species that cannot move or adapt disappear
- the more diverse an ecosystem
- the more resistant it is
| speciescannotmoveadapt |
- The temperature at sea level is 21 °C. Find it at 3 km on average lapse rate. — 21 − 3 × 6.5 = 1.5 °C.
- Using Wien’s law, find the wavelength of maximum emission of the Earth at 288 K, and state the region. — λ = 2.9×106/288 ≈ 10.1 µm, infrared.
- A town’s CO₂ emission is 50 Mt of CO₂ a year. How many ppm does that add globally per year? — 50 Mt CO₂ = 13.6 Mt C; 1 ppm ≈ 2.12 Gt C ⟹ ≈ 0.0064 ppm — which shows why national totals matter only when summed globally.
- Explain why a CFC is stable at ground level but reactive in the stratosphere. — Because the high-energy UV-C that breaks the C–Cl bond exists only above the ozone layer, where it is not already filtered out.
- State two gases responsible for the greenhouse effect and one that is not, with the reason. — CO₂ and CH₄ are; N₂ is not, because its electron distribution is symmetrical and its vibration creates no changing dipole, so it cannot absorb infrared.
| Comparison aspect | Main idea | Keywords |
|---|
| Temperature does not fall with height everywhere. | - It falls in the troposphere and mesosphere
- rises in the stratosphere and thermosphere
| fallstropospheremesosphererises |
| The ozone layer is in the stratosphere | - not the troposphere
- tropospheric ozone is a pollutant
- even though the same molecule
| tropospheretroposphericpollutanteven |
| Chlorine is a catalyst | do not write that it is “used up” — state that it is regenerated | writeusedstateregenerated |
| CO₂ is not the main greenhouse gas by total effect | - water vapour contributes most of the natural effect
- CO₂ is the important one because human activity changes its amount
| watervapourcontributesmost |
| O₂ and N₂ are not greenhouse gases | do not list them | listthem |
| The ozone hole is not the cause of global warming | they share CFCs as one common factor but are two different mechanisms — questions love to mix them | shareCFCscommonfactor |
| 220 DU is the hole threshold | not 300 DU | |
