Ch3: Ozone Layer and UV

Stratospheric Ozone: Location, Function & Relevance

  • The ozone layer is located in the STRATOSPHERE, the second major layer of Earth’s atmosphere.
  • Acts as a “shield” that absorbs harmful ultraviolet (UV) radiation.
  • Stratospheric ozone (≈ 10–50 km altitude): beneficial; absorbs UV.
  • Tropospheric ozone (ground → ≈ 10 km): secondary air pollutant.

UV Radiation: Types, Energies & Atmospheric Absorption

  • UV radiation subdivides into three spectral regions (shorter λ\lambda ⇒ higher E):
    • UVA: 320–400 nm320\text{–}400\, \text{nm} (lowest energy; reaches surface almost unimpeded).
    • UVB: 280–320 nm280\text{–}320\, \text{nm} (moderate energy; partially absorbed by ozone; fraction reaches surface).
    • UVC: 200–280 nm200\text{–}280\, \text{nm} (highest energy; almost completely absorbed in stratosphere by O2\text{2} and O3\text{3}).
  • Relationship recalled: E∝1λE \propto \frac{1}{\lambda} (inverse proportionality between energy & wavelength).

The Chapman Cycle (Natural Ozone–Oxygen Photochemistry)

  • Four fundamental photochemical / collisional steps establish a dynamic equilibrium:
    1. Photodissociation of molecular oxygen - O2\text{2} + hν\nu (UVC) ⟶\longrightarrow 2O∙\text{O}^\bullet
    2. Ozone formation (three‐body collision) - O•\text{•} + O2\text{2} →M\xrightarrow{\text{M}} O3\text{3}
    3. Photodissociation of ozone (mainly by UVB) - O3\text{3} + hν\nu (UVB) ⟶\longrightarrow O2\text{2} + O•\text{•}
    4. O-atom + ozone recombination - O•\text{•} + O3\text{3} ⟶\longrightarrow 2O2\text{2}.
  • Net result: Continuous creation & destruction but steady-state concentration (“dynamic equilibrium”).

Why Only UVC Breaks O2\text{2}: Bond-Energy Calculation Example

  • Bond dissociation energy for O=O double bond: D0D_0 = 498 kJ mol−1498\,\text{kJ mol}^{-1}.
  • Calculation shows that the maximum wavelength photon with enough energy to break the O2\text{2} bond is approximately 240 nm240\,\text{nm}, which lies in the UVC band.

Terminology & Chemical Concepts

  • Free radical: atom or molecule with an unpaired electron (highly reactive). O•\text{•} generated in step 1.
  • Allotrope: different structural forms of the same element in the same physical state (e.g., O2\text{2} and O3\text{3}).

Human Impact: Ozone‐Hole Formation

  • Anthropogenic chemicals (CFCs, halons, etc.) introduce Cl & Br radicals that catalytically destroy O3\text{3} → ozone hole (observed 1980s–1990s, particularly over Antarctica).
  • Result: diminished column ozone, elevated UVB at surface, ecological & health consequences (