Presentation on theme: "E 4. Ozone depletion in stratosphere"— Presentation transcript:
1E 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.
2Ozone depletion O3 very pale bluish gas very powerful oxidising agent pungent smelling odorabsorbs UV lightdetection: [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
3Ozone 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 photosynthesiscan cause genetic mutationscauses loss of planktonOzone production releases energy which produces an increase in temperature in stratosphere which gives it stability
4Ozone: natural cycle (stratosphere) formation of ozone:O uv O O (uv = 242 nm)O O O3natural depletion of ozoneO O 2O2O uv O O (uv = 290 – 320 nm)rate of formation = equal to rate of depletion= steady stateboth types of reactions are slow
5Ozone: evidence for depletion Antartica, autumn 2003ozone 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)
8Ozone: man-made depletion nitrogen oxides: sources: combustion, airplanes, nitrogenous fertilisersCFCs = chlorofluorocarbonsused in: refrigerators, air conditioners, blowing agents, solvents, dry cleaning agentschemically stable, low toxicity, volatile, insulating, fire suppressive, low costend up in stratosphere as they are not broken downCl free radical produced by uv - photodissociationCl acts as catalyst in ozone depletion – catalytic depletion
9ChloroFluoroCarbons: useful compounds chemically stable; long atmospheric life-timelow toxicitylow cost to manufacturevolatile liquidsgood solventsinsulatingfire-oppressive
13CFCs alternativesmust 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.
14Ozone 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.
15Ozone 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/flammablefluorocarbons: greenhouse gases but not flammablehydrochlorofluorocarbons: still some depletion as has Cl, and also greenhouse gases