THE OZONE LAYER

LEARNING OUTCOMES

• Explain that ozone is continuously being formed and broken down in the stratosphere by ultraviolet radiation.

• Describe, using equilibria, how the ozone concentration is maintained in the ozone layer, including the role of ultraviolet radiation.

• Outline the role of ozone in the absorption of harmful ultraviolet radiation and the resulting benefit for life on Earth.

© Pearson Education Ltd 2008This document may have been altered from the original

Structure of the AtmosphereSpace Shuttles

and Satellites

Aurora (ionosphere)

AirplanesCloudsMountains

Ozone Layer

• Greatest concentration of ozone occurs at altitudes 19 - 30km above the Earth's surface.

• Ozone layer is IN the stratosphere.

• Concentration of O3 in ozone layer is roughly 10 parts per million.

• A molecule compound of oxygen atoms

• Composed of three atoms of oxygen (O3)

• Colourless

• Has a harsh odour

• It has the ability to absorb most UV radiation

Ozone

Ozone – good or bad?

• Good – ozone in the upper atmosphere (stratosphere) protects living organisms by preventing harmful UV reaching us.

• Bad – ozone near the Earths’ surface (troposphere) is an air pollutant and has harmful effects on the respiratory systems of animals.

Absorption of UV radiation by the ozone layer

© Pearson Education Ltd 2008This document may have been altered from the original

Ozone – oxygen cycle

Ozone is continuously being formed and broken down in the stratosphere by the action of UV radiation.

The Ozone Cycle

1. O2 molecules absorb UV radiation (wavelength 240nM) to produce O atoms.

2. The O atoms react with the O2 molecules to form ozone and heat.

The Ozone Cycle

3. O3 absorbs UV with a wavelength of 240-310nM and breaks down into O2 and O.

4. The atomic O produced immediately reacts with another O2 molecule to reform ozone.

The Ozone Cycle

And so the cycle continues

The overal net effect is to convert UV radiation into heat with no loss of ozone.

Overal equation:

Removal of ozone

When an O and O3 combine they form O2

This reaction is so slow that it has little effect on the amount of ozone in the upper atmosphere. However human activity can have an effect…

Questions

1. Outline the filtering of UV radiation by the ozone layer

2. Outline, including equations, how the concentration of ozone is maintained in the atmosphere.

OZONE DEPLETION

Learning outcomes

• Understand that radicals from CFCs and NOx may catalyse the breakdown of ozone

• Explain that apparent benefits may be offset by unexpected and detrimental side effects.

• The thinning of the ozone layer

• It can happen naturally but human intervention is the main reason for ozone depletion

• Is brought about by ODC’s or Ozone Depleting Chemicals

• Chemicals have an ozone depletion potential (ODP) which is the relative amount of breakdown to the ozone layer caused by that substance.

Ozone Depletion

Natural Ozone depletion

Ozone in the stratosphere breaks down naturally: 2O3 —> 3O2

Ozone (trioxygen) can break up to give ordinary oxygen and an oxygen radical when it absorbs UV radiation: O3 —> O• + O2

Disruption of the natural equilibrium

The introduction of new compounds into the environment can lead to ozone loss by disrupting the natural equilibrium.

Radicals from CFCs and NOx catalyse the breakdown of ozone.

CFC

• A group of compounds known as Chlorofluorocarbons

• It is the main source of ozone depletion

• Very stable

• Insoluble in water (rain)

• As a result: it can travel from the lower atmosphere up until it reaches the ozone layer

NOxAnother chemical that destroys ozone is nitrogen oxide which comes from lightning or aircraft engines (and also laughing gas).

Five main CFC’s

• CFC-11 (trichlorofluoromethane - CFCl3)

• CFC-12 (dichloro-difluoromethane- CF2Cl2)

• CFC-113 (trichloro-trifluoroethane- C2F3Cl3)

• CFC-114 (dichloro-tetrfluoroethane- C2F4Cl2)

• CFC-115 (chloropentafluoroethane- C2F5Cl)

Sources of CFCs• CFCs were widely used as

coolants in refrigeration and air conditioners

• …used as solvents in cleaners (for electronic boards)

• …used as a blowing agents in the production of foam (i.e., frire extinguishers)

• …used as propellants in aerosols.

Mechanism of CFC breaking down ozone

1. Initiation – UV radiation strikes the CFC molecule in the stratosphere and the C-Cl bond breaks producing a Cl radical.

2. Radicals are extremely reactive. The Cl radical reacts with ozone, breaking it apart. This is a two step process…

Breakdown of ozone by chlorine radicals

1. Propagation step 1

2. Propagation step 2

This is similar to radical substitution of alkanes by chlorine

• Take note that the ozone depleting process is a cycle as Cl can react again for as much as 100,000 times.

• What do we call the Cl radical if it speeds up the rate of reaction but is itself un-changed?

Breakdown of ozone by nitrogen oxide radicals

1. Propagation step 1

2. Propagation step 2

Effects of Ozone DepletionMarine wild life and ecosystem – plankton are sensitive to UV.Proof: Damage to marine life - Smith et.al _Science_1992 found a 6-12% drop in phytoplankton productivity once their ship entered the area of the spring-time ozone hole.

Genetic damage - DNA absorbs UV-B light and the absorbed energy can break bonds in the DNA. This can lead to skin cancer.Fact:90% of the skin carcinomas are attributed to UV-B exposure

[R.P.Wayne, Chemistry of the Atmospheres 2nd ed. Oxford 1991)

Other reasons we need ozone:

1. Strong UV radiation from the Sun can cause inflammation of the cornea, the outer coating of the eyeball, leading to photokeratosis or "snow blindness… permanent damage to the cornea …dimming of the eye's lens and increased risk for cataracts…eye cancer, conjunctivitis and pterygium

2. 1% decrease in stratospheric ozone may result in 100,000 to 150,000 additional cases of blindness due to eye cataracts worldwide.

3. Material damages - Increased UV-B radiation can accelerate the deterioration of plastics, wood, paper, cotton and wool (Manins et. al., 2001).

4. Air pollution - as more UV reaches the lower troposphere, more high-energy EM waves will decompose oxygen and produce Ozone... Resulting to photochemical smog

5. By the way, Ozone is poisonous.

Question

1. Outline, including equations, how a single molecule of CFC can remove many ozone molecules from the ozone layer.

Uses of CO2 – Independent learning

Uses of CO2 – Foam

– Solvent

– Dry cleaning

– Toxic waste treatments

– Chemical synthesis

1. Why is CO2 used in these processes?

2. What use to be used?

3. Decaffeination 1. What is it?

2. What are the three different methods to decaffeinate coffee?

3. What is a supercritical fluid extraction?

4. What happens to the extracted caffeine?

Using the internet make notes on the following uses of CO2