Earth's Chemical Shield: Formation of Ozone, Dobson Units, UV Classification, and Ozone Pollutants
The ozone layer is Earth's biological sunscreen, safeguarding all terrestrial life from lethal cosmic and solar ultraviolet (UV) radiation. An ozone molecule (O3) is formed in the stratosphere through a two-step photochemical reaction: first, molecular oxygen (O2) absorbs high-energy UV rays, severing its double covalent bond into two free oxygen atoms (O + O); then, each free atom combines with an intact oxygen molecule to synthesize ozone (O + O2 -> O3). English scientist G. M. B. Dobson deduced that under Standard Temperature and Pressure (STP), the entire stratospheric ozone layer would compress into a thin sheet only 3 mm thick, defining this normal baseline as 300 Dobson Units (DU). We study the three categories of UV radiation and investigate the four major chemical pollutants responsible for the 'Ozone Hole' above the South Pole.
1. Ozone Formation & UV Spectrum
Synthesis equations and radiation penetration:
- Chemical Synthesis:
• Step 1: O&sub2; →UV O + O (breaking double covalent bond).
• Step 2: O + O&sub2; → O&sub3; (ozone molecule formation). - Thickness at STP: 3 mm, standardized at 300 Dobson Units (DU).
- UV Radiation Penetration Levels:
• Far UV (100 - 280 nm): 100% absorbed (0% penetrates; highly lethal).
• Medium UV (280 - 315 nm): 95% absorbed (only 5% penetrates; harmful).
• Near UV (315 - 400 nm): 100% penetrates (beneficial; stimulates vitamin D synthesis in bones).
2. The Ozone Hole & The 4 Pollutants
Chemical threats damaging the stratospheric shield:
- The Ozone Hole: Thinning of the ozone layer over the South Pole, expanding markedly every September due to polar vortex wind currents carrying black pollutant clouds.
- Four Major Ozone Pollutants:
• Chlorofluorocarbons (CFCs / Freons): Cooling agents in refrigerators and air conditioners, aerosol propellants, and foam blowing agents.
• Halons: Chemical fire extinguishers used for oil and electrical fires.
• Methyl Bromide: Insecticide gas used to preserve grain crops in storage silos.
• Nitrogen Oxides: High-altitude exhaust emissions from supersonic Concorde airplanes.
Ozone Depletion Calculation Clinic:
Solution:
1. Normal ozone level = 300 DU.
2. Degree of erosion = 300 - 120 = 180 DU.
3. Depletion percentage = (180 / 300) × 100% = 60%.
Answer: Nitrogen Oxides (NOx).
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