1 / 23

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. Oxygen is present in two forms, O2 and O3

upton
Télécharger la présentation

E 4. Ozone depletion in stratosphere

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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 Oxygen is present in two forms, O2 and O3 protect life on the Earth’s surface from harmful ultraviolet (UV) radiation.

  3. Ozone • 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

  4. The bonds in oxygen and ozone are broken by UV of different wavelengths The bond in oxygen and ozone are both broken when they absorb UV radiation of sufficient energy. The double bond in O2 is stronger than the 1.5 bond in ozone and so is broken by radiation of shorter wavelengths.

  5. Worked Example The bond energy in ozone is 363 kJ mol-1. Calculate the wavelength of UV radiation needed to break the bond.

  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

  7. The natural formation of ozone The temperature of the atmosphere generally decreases with height But at 12km above the Earth’s surface, temperature starts to rise because the ultraviolet radiation is absorbed in a number of photo chemical reactions. This part of the atmosphere is called the stratosphere.

  8. Ozone: natural cycle (stratosphere) • formation of ozone: O2 + uv  O + O (λ = 242 nm) O2 + O O3 More energy needed to break double bond. • natural depletion of ozone O3 + O 2O2 O3 + uv  O2 + O (λ = 290 – 320 nm) Less energy needed to break ‘1.5 bond’ (delocalized pi bond). • rate of formation = equal to rate of depletion • = steady state • both types of reactions are slow

  9. Ozone: man-made depletion • nitrogen oxides: sources: combustion, airplanes, nitrogenous fertilizers

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

  11. Chlorofluorocarbons CFCs = chlorofluorocarbons end up in stratosphere as they are not broken down C-Cl bond is weakest; easily broken by UV: 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-suppressant • coolant in ACs and fridges • dry-cleaning agent

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

  14. Ozone: anthropogenic depletion

  15. Ozone depletion:alternatives to CFCs • hydrocarbons such as propane and 2- methyl propane as refrigerant coolants: no halogens • fluorocarbons: stronger C-F bonds don’t break • hydrochlorofluorocarbons: hydrogen makes it more stable; fewer halogen free radicals released • hydrofluorocarbons: stronger C-F bonds don’t break

  16. Ozone depletion: alternatives to CFCs • propane and 2- methyl propane as refrigerant coolants: greenhouse gases/flammable; highly hazardous • fluorocarbons: greenhouse gases but not flammable; non-hazardous • Hydrofluorocarbons: greenhouse gas, non-flammable, low toxicity • hydrochlorofluorocarbons: still some depletion as has Cl, and also greenhouse gases; hazardous

  17. 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

  18. Ozone: evidence of depletion

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

  20. Ozone depletion at the poles • Special mechanism causes esp high ozone depletion around poles • During winter, poles become very cold, with circular winds preventing warm air from entering • Forms ice in clouds, which act as heterogeneous catalysts for ozone depletion • HCl and ClNO3 combine to form Cl2, which then disassociates in summer to form Cl*

More Related