The Atmosphere: Layers, the Greenhouse Effect and Ozone Depletion

• Lapse rate: the rate at which temperature falls with altitude in the troposphere, about 6.5 °C per km.
• 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.
• Stratosphere: 12–50 km, in which temperature rises with height because ozone absorbs ultraviolet; aircraft fly here for stable air.
• Ozone layer: the region of maximum O₃ concentration, roughly 15–35 km, which absorbs most of the solar UV-B and UV-C.
• Ionosphere: the ionised part of the thermosphere that reflects radio waves and produces the aurora.
• 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.
• 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.
• Dobson unit (DU): the unit of total ozone column; 300 DU is a normal value and below 220 DU defines an ozone "hole".
• CFC (chlorofluorocarbon): a stable, non-toxic, volatile organic compound of carbon, chlorine and fluorine, once used as a refrigerant, propellant and solvent.

• 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.
• Wien's law: λ_max = 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.
• 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.
• Carbon budgets: 1 ppm of CO₂ ≈ 2.12 Gt of carbon in the atmosphere.
• Ozone stoichiometry: one chlorine radical destroys up to 100,000 ozone molecules before it is removed from the stratosphere.

Temperature, not composition, is what divides the atmosphere into layers.

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 NO_x 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.

How CFCs destroy it

• 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.
• There, strong UV splits them: CFCl₃ + hν → CFCl₂· + Cl·
• 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.
• Net reaction: O₃ + O· → 2 O₂ — the ozone is gone while the chlorine is untouched.

Why the hole is Antarctic

• 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.

1. The temperature at sea level is 21 °C. Find it at 3 km on average lapse rate. — 21 − 3 × 6.5 = 1.5 °C.
2. 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.
3. 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.
4. 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.
5. 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.