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 depletionOzone 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

The Structures of different forms of oxygen

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

UV Wavelengths

The energy Ephoton of a photon of light is related to its frequency f:

Ozone depletionOzone depletionTwo 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

Ozone: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

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)

OzoneOzone: evidence of depletion

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

                                                                           

        

                                                                           

        

                                                                           

        

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

fertilisersNO(g) + O3(g) NO2(g) + O2(g)

NO2(g) + O (g) NO(g) + O2(g)

CFCs = chlorofluorocarbonsused in: refrigerators, air conditioners, blowing agents, solvents, dry cleaning agentschemically 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

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

OzoneOzone: anthropogenic depletion

Ozone depletionOzone depletion: equations photodissociation: C- Cl is weakest bond

CCl2F2 CClF2 + Cl catalytic depletion:   

Cl + O3 ClO + O2

  ClO + O Cl + O2

Ozone depletionOzone depletion: equations catalytic depletion:   

NO + O3 NO2 + O2

  NO2 + O NO + O2

When added:

O3 + O 2O2

Ozone depletion: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

Ozone depletion: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

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