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. The Structures of different forms of oxygen

4. UV Wavelengths Bonds in Oxygen and ozone are broken when they absorb UV radiation (only if sufficient energy) O2 is stronger than ozone so is broken by UV of shorter wavelengths

5. UV Wavelengths The energy Ephoton of a photon of light is related to its frequency f:

6. 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 • Ozone molecules in the Earths atmosphere absorbs UV light

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

8. 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

9. Ozone: evidence of depletion

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

11. Ozone: man-made depletion • nitrogen oxides: sources: combustion, airplanes, nitrogenous fertilisers NO(g) + O3(g) NO2(g) + O2(g) NO2(g) + O (g) NO(g) + O2(g) • 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

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

13. Ozone: anthropogenic depletion

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

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

16. 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

17. 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

18. Environmental Impact of Ozone Depletion • Ozone protects the Earths surface from dangerous high-energy UV radiation • Induces skin cancer and eye problems • Damages aquatic life