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 protect life on the Earth’s surface from harmful ultraviolet (UV) radiation.

Ozone O 3 very pale bluish gas very powerful oxidising agent pungent smelling odor absorbs UV light detection: [O 3 ] in a sample of air can be measured using UV spectroscopy; the more UV is absorbed the higher [O 3 ] in upper stratosphere; 15 to 45 km

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.

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

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 O 3 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

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.

Ozone: Ozone: natural cycle (stratosphere) formation of ozone: O 2 + uv  O  + O  (λ = 242 nm) O 2 + O   O 3 More energy needed to break double bond. natural depletion of ozone O 3 + O   2O 2 O 3 + uv  O 2 + 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

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

Ozone depletion Ozone depletion: equations catalytic depletion: NO  + O 3  NO 2  + O 2 NO 2  + O   NO  + O 2 When added: O 3 + O   2O 2

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

ChloroFluoroCarbons: 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

Ozone depletion Ozone depletion: equations photodissociation: C- Cl is weakest bond CCl 2 F 2  CClF 2  + Cl  catalytic depletion: Cl  + O 3  ClO  + O 2 ClO  + O   Cl  + O 2

Ozone Ozone: anthropogenic depletion

Ozone depletion: 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

Ozone depletion: 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

Ozone: 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)

Ozone Ozone: evidence of depletion

Ozone Ozone: depletion

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 ClNO 3 combine to form Cl 2, which then disassociates in summer to form Cl*