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

5 Ozone: evidence for depletion
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)

6 Ozone: evidence of depletion

7 Ozone: depletion

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: O 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

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