Atmosphere and Climate

• Structure of the atmosphere — four layers and their key characteristics
• Composition of the troposphere and how greenhouse gases trap heat
• Global warming potential of CO₂, CH₄, N₂O, and CFCs
• Ozone layer formation and destruction by CFC-derived chlorine radicals
• Formation of acid rain and the chemical reactions involved

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Structure of the Atmosphere

Troposphere composition (dry air):

• N₂: 78%, O₂: 21%, Ar: 0.93%, CO₂: ~0.04%, trace gases

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Greenhouse Effect and Global Warming

Natural greenhouse effect (beneficial): Atmosphere absorbs outgoing infrared (IR) radiation → keeps Earth warm (~15°C average). Without it, Earth would be −18°C.

Enhanced greenhouse effect (harmful): Increased concentrations of greenhouse gases → more IR trapped → global warming.

Greenhouse Gases

GWP measures how much heat a gas traps relative to CO₂ over 100 years. CFCs are the most potent greenhouse gases per molecule.

Consequences of global warming:

• Rise in sea level (thermal expansion + glacier melting)
• Increase in frequency of extreme weather events
• Shift in agricultural zones; disruption of monsoons
• Acidification of oceans (CO₂ + H₂O → H₂CO₃)
• Bleaching of coral reefs

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Ozone Layer — Formation and Function

Located at 23–25 km in the stratosphere. Absorbs harmful UV-B (280–315 nm) and UV-C (100–280 nm) radiation.

Ozone formation (Chapman mechanism):

Step 1: O₂ + UV (λ < 240 nm) → 2O•        (photodissociation)
Step 2: O• + O₂ + M → O₃ + M             (M = third body, takes away energy)

Net: 3O₂ + UV → 2O₃

Natural equilibrium (without pollutants):
O₃ + UV → O₂ + O•
O• + O₃ → 2O₂                           (ozone destroyed and re-formed continuously)

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Ozone Depletion — CFC Chain Reaction

CFCs (Chlorofluorocarbons): e.g., CCl₂F₂ (Freon-12), CCl₃F (Freon-11). Inert at lower atmosphere but UV breaks them in stratosphere.

CFC depletion mechanism:

Step 1: CCl₂F₂ + UV → •CClF₂ + Cl•        (Cl radical released)
Step 2: Cl• + O₃ → ClO• + O₂             (ozone destroyed)
Step 3: ClO• + O• → Cl• + O₂             (Cl• regenerated — CHAIN REACTION)

NET: O₃ + O• → 2O₂                       (Cl• acts as catalyst — NOT consumed)

Each Cl atom can destroy ~100,000 ozone molecules before being permanently deactivated.

Ozone hole over Antarctica:

• Southern polar vortex traps cold air in winter
• Polar stratospheric clouds (PSCs) provide surfaces for heterogeneous CFC reactions
• Spring sunlight provides UV → rapid ozone depletion → ozone "hole" (thinning, not complete absence)
• First observed 1985; largest in late September/October

Montreal Protocol (1987): International agreement to phase out CFCs. Replaced by HCFCs, then HFCs (which have lower/zero ODP but are greenhouse gases), then HFOs.

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Acid Rain

Normal rain pH ≈ 5.6 (slightly acidic due to dissolved CO₂ forming H₂CO₃).
Acid rain pH < 5.6 (typically 4.2–4.4 in polluted areas).

Causes:

Sulphur dioxide pathway (coal burning, smelting):

S + O₂ → SO₂                             (combustion of sulphur in coal)
2SO₂ + O₂ → 2SO₃                         (catalysed by particulates/ozone)
SO₂ + H₂O → H₂SO₃                        (sulphurous acid — weak)
SO₃ + H₂O → H₂SO₄                        (sulphuric acid — strong; main contributor)

Nitrogen oxides pathway (vehicles, lightning):

N₂ + O₂ → 2NO                            (high temperature combustion)
2NO + O₂ → 2NO₂
3NO₂ + H₂O → 2HNO₃ + NO                  (nitric acid formation)

Effects of acid rain:

• Damages limestone/marble buildings: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂
• Acidifies lakes → kills aquatic life (fish die when pH < 4.5)
• Leaches nutrients (Ca²⁺, Mg²⁺) from soil → forest decline
• Corrodes metal structures
• "Black crust" on buildings (CaSO₄ deposit)

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Why CO₂ is a Greenhouse Gas — Molecular Explanation

Greenhouse gases absorb IR radiation because they have asymmetric stretching or bending vibration modes that create a changing dipole moment.

• N₂ and O₂: symmetric, homonuclear diatomic → NO dipole change → NOT greenhouse gases
• CO₂: although linear and symmetric, the O=C=O asymmetric stretch and bending modes → changing dipole → absorbs IR
• H₂O, CH₄, N₂O: polar/asymmetric molecules → multiple IR-active modes

CFC Alternatives — ODP vs GWP Trade-off

ODP (Ozone Depletion Potential): relative ability to destroy stratospheric ozone vs CFC-11 = 1.

Photochemical Smog vs London Smog

PAN = Peroxyacetyl Nitrate = CH₃CO–O–O–NO₂ (eye irritant, damages plants)

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Example 1: Ozone depletion chain reaction

Problem: Show how a single CFC molecule (CCl₂F₂) can destroy many ozone
molecules. Write the stepwise mechanism.

Step 1: CFC photolysis in stratosphere (λ < 240 nm UV)
  CCl₂F₂ + hν → •CClF₂ + Cl•
  
  The C–Cl bond is weaker than C–F bond → preferentially broken by UV

Step 2: Cl radical attacks ozone
  Cl• + O₃ → ClO• + O₂     ... (i)
  
  Ozone is destroyed; ClO• (chlorine monoxide radical) formed

Step 3: ClO• reacts with atomic oxygen
  ClO• + O• → Cl• + O₂     ... (ii)
  
  Cl• is REGENERATED (not consumed) → acts as catalyst

Step 4: Net reaction (add i + ii)
  O₃ + O• → 2O₂
  
  Cl• is a catalyst → each Cl• can repeat steps (i) and (ii)
  approximately 100,000 times before being permanently removed

Step 5: Chain termination
  Cl• + CH₄ → HCl + CH₃•   (HCl = reservoir species, temporarily deactivates Cl•)
  HCl + OH• → Cl• + H₂O    (Cl• reactivated — cycle continues)

Conclusion: Chain reaction mechanism; CFCs are long-lived in stratosphere
(inert at lower atmosphere), so small amounts cause massive ozone destruction.

Example 2: Acid rain chemistry calculation

Problem: Write ALL chemical reactions involved when SO₂ from coal combustion
eventually forms acid rain. Which acid causes more damage and why?

Step 1: SO₂ generation
  C (coal contains S as impurity) + O₂ → CO₂
  S + O₂ → SO₂        (sulphur combustion)

Step 2: SO₂ → H₂SO₃ (in presence of rainwater)
  SO₂(g) + H₂O(l) → H₂SO₃(aq)
  Sulphurous acid: weak acid, Ka₁ = 1.5 × 10⁻²

Step 3: SO₂ → SO₃ (catalytic oxidation by O₂ in presence of particulates/O₃)
  2SO₂ + O₂ → 2SO₃    (slower but significant in polluted atmosphere)

Step 4: SO₃ → H₂SO₄
  SO₃ + H₂O → H₂SO₄
  Sulphuric acid: strong acid, fully dissociates → lower pH → more damage

Step 5: Compare H₂SO₃ vs H₂SO₄
  H₂SO₄ (sulphuric acid): strong acid (pKa₁ ≈ −3), fully dissociates
  H₂SO₃ (sulphurous acid): weak acid (pKa₁ ≈ 1.8), partially dissociates
  
  At same concentration, H₂SO₄ gives ~10⁵ times more H⁺ than H₂SO₃
  → H₂SO₄ is primary cause of acidic damage

  H₂SO₄ also reacts with limestone buildings:
  CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂↑

Answer: SO₂ forms both H₂SO₃ (direct dissolution) and H₂SO₄
(via SO₃). H₂SO₄ causes more damage as it is a strong acid, fully
dissociating to provide much higher [H⁺] in rainwater.

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Q1. The ozone layer is located in which atmospheric layer?

A) Troposphere
B) Mesosphere
C) Thermosphere
D) Stratosphere

Q2. Which greenhouse gas has the highest Global Warming Potential (GWP) per molecule?

A) CO₂
B) CH₄
C) CFCs
D) N₂O

Q3. In CFC-mediated ozone depletion, the chlorine radical (Cl•) acts as:

A) A reactant that is consumed
B) A product of the final reaction
C) A catalyst (regenerated after each cycle)
D) A stabilising agent for ozone

Q4. The main acid responsible for damage to marble/limestone monuments by acid rain is:

A) H₂SO₃
B) H₂SO₄
C) HNO₂
D) H₂CO₃

Q5. Normal rain has a pH of approximately 5.6 because:

A) Dissolved NO₂ from the atmosphere forms nitric acid
B) Dissolved CO₂ forms a weak carbonic acid
C) Dissolved SO₂ from industrial sources forms sulphurous acid
D) It contains trace amounts of HCl from volcanic emissions

Answer Key: 1-D | 2-C | 3-C | 4-B | 5-B

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• NCERT Class 12 Chemistry — Chapter 14: Environmental Chemistry
• Pages 408–420 (atmosphere and climate)
• Key sections: 14.2 Atmospheric pollution (14.2.1 Tropospheric pollution, 14.2.2 Stratospheric pollution)
• Figure 14.1: Regions of atmosphere
• Table 14.1: Global warming potential of greenhouse gases
• Cross-reference: Class 11 Chapter 5 (States of Matter) for gas laws context

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🎯 Exam Tips

• CFC chain reaction mechanism: Cl• + O₃ → ClO• + O₂; ClO• + O• → Cl• + O₂ — write both steps, know Cl• is catalyst
• GWP order: CFCs >> N₂O > CH₄ > CO₂ — frequently tested in MCQs
• Acid rain equations: remember SO₃ + H₂O → H₂SO₄ (not direct SO₂ + H₂O); distinguish both pathways
• Ozone layer altitude (23–25 km, stratosphere) is tested as a simple recall question

📊 Weightage

• Environmental Chemistry: 1–2 questions per JEE Main paper (~4–8 marks)
• Atmosphere/Climate topic is the higher-weightage sub-topic within this chapter
• Ozone depletion and greenhouse effect together account for majority of chapter questions

🔗 Related Topics

• Redox Reactions (oxidation states of S, N in atmospheric reactions)
• Chemical Kinetics (chain reaction mechanism — Cl• catalyst concept)
• Surface Chemistry (PSC surfaces for heterogeneous CFC reactions)
• Environmental Pollution (secondary pollutants from atmospheric chemistry)

📝 Revision Checklist

• Name the four atmospheric layers with altitude ranges
• List 4 greenhouse gases with sources and relative GWP
• Write ozone formation reactions (Chapman mechanism)
• Write CFC photolysis and chain reaction (3 steps + net reaction)
• Explain why ozone hole forms over Antarctica specifically
• Write acid rain formation reactions for both SO₂ and NOₓ pathways
• State why N₂ and O₂ are NOT greenhouse gases (molecular symmetry)