Lecture Notes: Stratospheric Chemistry and the Ozone Layer

Introduction

• Global Environmental Problem: Stratospheric ozone depletion.
• Local/Regional Problem: Ground-level air pollution.
• Precautionary Principle: Manufacturers must ensure new products/processes cause no harm.
• Historical Context: Replacement of sulfur dioxide with CFCs in refrigerators; later discovery of CFCs' role in ozone depletion.

Regions of the Atmosphere

• Troposphere: Ground level to ~15 km; contains 85% of atmosphere's mass.
• Stratosphere: ~15-50 km; contains the ozone layer.
• Ozone Layer: Bottom half of the stratosphere; filters harmful UV rays.

Environmental Concentration Units for Atmospheric Gases

• Absolute Concentrations: Molecules per cubic centimeter, partial pressure.
• Relative Concentrations: Mole fraction, parts per million (ppm), parts per billion (ppb), parts per trillion (ppt).

The Physics, Chemistry, and Biology of UV

• UV Light: High-energy, short-wavelength light (50-400 nm).
• UV Regions: UV-C (200-280 nm), UV-B (280-320 nm), UV-A (320-400 nm).
• Health Effects: UV-B causes skin cancer, cataracts, immune system suppression; UV-A less harmful but penetrates deeper.

Absorption of Light by Molecules

• Electromagnetic Spectrum: X-rays, UV, visible, infrared.
• Absorption Spectrum: Shows relative fraction of light absorbed by a molecule as a function of wavelength.
• Photochemical Reactions: Reactions initiated by light energy.

Filtering of Sunlight's UV Component by Atmospheric O2 and O3

• O2: Filters UV light from 120-220 nm.
• O3: Filters UV light from 220-320 nm; ineffective in UV-A region.

Deleterious Effects of UV Light on Human Skin

• Skin Cancer: Most skin cancers due to UV-B overexposure.
• Melanoma: Linked to short periods of high UV exposure; higher incidence in fair-skinned individuals.
• Protection: Sunscreens, clothing, avoiding peak sunlight hours.

Sunscreens

• Types: Reflect/scatter sunlight (zinc oxide, titanium dioxide) or absorb UV component (complex organic compounds).
• SPF: Sun Protection Factor; measures effectiveness against UV-B.
• UV-A Protection: Star system (4 stars = highest protection).

Other Environmental Effects of UV Light

• Eye Damage: Cataracts, macular degeneration.
• Immune System: Suppression, increased infectious diseases.
• Plants and Animals: Interference with photosynthesis, damage to aquatic life.

Variation in Light's Energy with Wavelength

• Photon Energy: E = hν or E = hc/λ.
• Energy Relationships: UV-C > UV-B > UV-A > visible > infrared.
• Photochemical Dissociation: Molecules absorb photons and dissociate if energy is sufficient.

Creation of Ozone in the Stratosphere

• Photochemical Decomposition: O2 + UV-C → 2O.
• Ozone Formation: O + O2 + M → O3 + M + heat.
• Temperature Inversion: Stratosphere warmer than air below or above due to ozone formation.

Destruction of Stratospheric Ozone

• Photochemical Decomposition: O3 + UV photon (λ < 320 nm) → O2* + O*.
• Catalytic Destruction: O3 + O → 2O2.
• Chapman Mechanism: Series of reactions for ozone production and destruction.

Catalytic Processes of Ozone Destruction

• Mechanism I: X + O3 → XO + O2; XO + O → X + O2.
• Mechanism II: X + O3 → XO + O2; X' + O3 → X'O + O2; XO + X'O → [XOOX'] → X + X' + O2.
• Catalysts: NO, OH, Cl, Br.

Atomic Chlorine and Bromine as X Catalysts

• Sources: Natural (CH3Cl, CH3Br) and synthetic (CFCs).
• Catalytic Efficiency: Bromine more efficient than chlorine.
• Inactive Forms: HCl, ClONO2, HBr, BrONO2.

Steady-State Analysis of Atmospheric Reactions

• Steady-State Approximation: Rate of formation = rate of destruction.
• Chapman Mechanism Analysis: Derivation of steady-state concentrations of O and O3.

Review Questions and Problems

• Factuals: Identify key gases, altitudes, wavelengths, and reactions.
• Conceptuals: Explain terms, mechanisms, and environmental impacts.
• Problems: Calculate rates, concentrations, and photon energies.

Teaching Tips

• Visual Aids: Use diagrams (e.g., Figure 1-1, 1-2, 1-3) to illustrate atmospheric regions, UV spectrum, and absorption spectra.
• Interactive Activities: Engage students with activities like the WHO database analysis on melanoma mortality rates.
• Real-World Connections: Discuss current events related to ozone depletion, CFC replacements, and environmental policy.
• Problem-Solving: Assign problems that require students to apply concepts to new scenarios, fostering critical thinking.
• Discussion: Encourage discussion on the precautionary principle, the role of international agreements, and the future of environmental chemistry.