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.

OzoneOzone 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 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 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

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: 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

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

fertilizers

Ozone depletionOzone depletion: equations catalytic depletion:   

NO + O3 NO2 + O2

  NO2 + O NO + O2

When added:

O3 + O 2O2

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

CCl2F2 CClF2 + Cl catalytic depletion:   

Cl + O3 ClO + O2

  ClO + O Cl + O2

OzoneOzone: 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)

OzoneOzone: evidence of depletion

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

                                                                           

        

                                                                           

        

                                                                           

        

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*