E 4. Ozone depletion in stratosphere

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. CFCs alternatives must have similar positive properties but without the production of Cl radicals or any other radical than can cause it. Advantages of these alternatives: reduced production of Cl free radical. These alternatives should be/have: low boiling points, non-toxic, non-reactive, non-flammable and not acts as greenhouse gases.

14. Ozone depletion:alternatives to CFCs • Hydrocarbons such as propane and 2- methyl propane as refrigerant coolants: no halogens; more flammable + also greenhouse effect. • Fluorocarbons: stronger C- F bond; greenhouse effect. • Hydrochlorofluorocarbons: H makes it more stable but still has C- Cl bonds; fewer halogen free radicals released – also greenhouse gas. • Hydrofluorocarbons: stronger C-F bond.

15. 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