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E 4. Ozone depletion in stratosphere

E 4. Ozone depletion in stratosphere. Describe the formation and depletion of ozone in the stratosphere by natural processes. List the ozone-depleting pollutants and their sources. Discuss the alternatives to CFCs in terms of their properties. Ozone depletion. O 3 very pale bluish gas

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E 4. Ozone depletion in stratosphere

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  1. E 4. Ozone depletion in stratosphere Describe the formation and depletion of ozone in the stratosphere by natural processes. List the ozone-depleting pollutants and their sources. Discuss the alternatives to CFCs in terms of their properties.

  2. Ozone depletion • O3 • very pale bluish gas • very powerful oxidising agent • pungent smelling odor • absorbs UV light • detection: [O3] in a sample of air can be measured using UV spectroscopy; the more UV is absorbed the higher [O3] • in upper stratosphere; 15 to 45 km

  3. Ozone depletion Two functions • absorbs UV – 290 – 320 nm; UV • causes sunburn, skin cancer, eye cataracts (=clouding of the eye – can lead to blindness) • reduces plant growth as O3 destroys apparatus for photosynthesis • can cause genetic mutations • causes loss of plankton • Ozone production releases energy which produces an increase in temperature in stratosphere which gives it stability

  4. Ozone: natural cycle (stratosphere) • formation of ozone: O2 + uv  O + O (uv = 242 nm) O2 + O O3 • natural depletion of ozone O3 + O 2O2 O3 + uv  O2 + O (uv = 290 – 320 nm) • rate of formation = equal to rate of depletion • = steady state • both types of reactions are slow

  5. Antartica, autumn 2003 ozone hole = area having less than 220 Dobson units (if 100 DU of ozone were brought to the Earth's surface, it would form a layer 1 millimeter thick) Ozone: evidence for depletion

  6. Ozone: evidence of depletion

  7. Ozone: depletion http://www.epa.gov/ozone/science/hole/size.html

  8. Ozone: man-made depletion • nitrogen oxides: sources: combustion, airplanes, nitrogenous fertilisers • CFCs = chlorofluorocarbons • used in: refrigerators, air conditioners, blowing agents, solvents, dry cleaning agents • chemically stable, low toxicity, volatile, insulating, fire suppressive, low cost • end up in stratosphere as they are not broken down • Cl free radical produced by uv - photodissociation • Cl acts as catalyst in ozone depletion – catalytic depletion

  9. ChloroFluoroCarbons:useful compounds • chemically stable; long atmospheric life-time • low toxicity • low cost to manufacture • volatile liquids • good solvents • insulating • fire-oppressive

  10. Ozone: anthropogenic depletion

  11. Ozone depletion: equations • photodissociation: C- Cl is weakest bond CCl2F2  CClF2  + Cl • catalytic depletion: Cl + O3  ClO + O2   ClO + O Cl + O2

  12. Ozone depletion: equations • catalytic depletion: NO + O3  NO2 + O2   NO2 + O NO + O2 When added: O3 + O 2O2

  13. Ozone depletion:alternatives to CFCs • hydrocarbons such as propane and 2- methyl propane as refrigerant coolants: no halogens • fluorocarbons: stronger C-Hal bonds • hydrochlorofluorocarbons: hydrogen makes it more stable; fewer halogen free radicals released • hydrofluorocarbons: stronger C-F bond

  14. Ozone depletion: alternatives to CFCs • Alternatives have all useful properties of CFS’s but some issues: • propane and 2- methyl propane as refrigerant coolants: greenhouse gases/flammable • fluorocarbons: greenhouse gases but not flammable • hydrochlorofluorocarbons: still some depletion as has Cl, and also greenhouse gases

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