Study Notes on Photochemical Smog

Introduction to Photochemical Smog

  • Focus on formation of nitrogen oxides (NO, NO₂) and ozone as key pollutants in the troposphere.
  • Understanding role of nitrogen oxides in ozone formation.
  • Discuss harmful effects of nitrogen oxides and ozone.
  • Catalytic converters reduce nitrogen oxides in vehicles.

Formation of Nitrogen Oxides

  • Nitrogen (
    ) and oxygen (
    ) form nitrogen oxides when their bonds break.
  • Natural occurrences: lightning, volcanic activity, bushfires.
  • Anthropogenic sources: internal combustion engines, jet engines, industrial kilns.

Troposphere Overview

  • Closest atmospheric layer to Earth, heavily impacted by human activities.
  • Ozone in lower troposphere acts as a pollutant.

Photochemical Smog

  • Complex mixture of chemicals: includes primary (directly from source) and secondary pollutants (formed from reactions).
  • Example of primary pollutant: nitrogen dioxide (from hydrocarbons).
  • Example of secondary pollutant: ozone (formed from nitrogen oxides and oxygen).
  • Requires: high pollutant concentration, sunlight (UV radiation), still air mass, temperature inversion.

Formation of Pollutants

  • Primary pollutant formation: N₂ + O₂ → 2NO (nitric oxide).
  • NO₂ formation: 2NO + O₂ → 2NO₂.
  • Secondary pollutant (ozone) formation:
    • NO₂ photodissociates under UV: NO₂ → NO + O.
    • Reactive oxygen radical (O) + O₂ → O₃.

Effects of Photochemical Smog

Human Impact

  • Eye irritation due to nitrogen dioxide.
  • Ozone causes respiratory distress, damaging lung tissue, affecting oxygen diffusion.

Effects on Plants

  • Ozone disrupts stomata, hindering gas exchange and photosynthesis, resulting in growth limitation and increased disease susceptibility.

Effects on Polymers

  • Ozone damages elastomers, compromising structural integrity (oxidation of C=C to C-C bonds).

Catalytic Converters

  • Reduce pollutants in exhaust emissions from cars.
  • Convert nitrogen oxides into less harmful substances.

Three-Way Catalytic Converter

  • Converts carbon monoxide (CO), nitrogen oxides, unburnt hydrocarbons into less harmful substances.
  • Reactions:
    1. CO + 1/2 O₂ → CO₂.
    2. CₓHₓ + O₂ → H₂O + CO₂ (hydrocarbon balancing varies).
    3. 2NO + 2CO → N₂ + 2CO₂ (removes nitrogen oxide).

Conclusion

  • Summary of key pollutants and reduction methods.
  • Encourage review of material for understanding.