Ozone in the Atmosphere
Ozone in the Lower Atmosphere (troposphere)
About 10% of all ozone (O3) in the atmosphere is found in the troposhere (up to 16km above the earth's surface).
Ozone in the troposhere has harmful effects on many living things because it is toxic.
In humans, ozone causes eye irritation, compromised lung functions, aggrevation of respiratory conditions like asthma, and increases the susceptibility to infection.
Ozone pollution in the troposphere is often linked to photochemical smog.
Ozone in the lower atmosphere is formed during electrical discharge from high voltage appliances as shown in the equations below:
O2(g) → 2O(g)
O2(g) + O(g) → O3(g)
Ozone in the Upper Atmosphere (stratosphere)
About 90% of all ozone (O3) in the atmosphere is found in the stratosphere (16 to 32 km above the earth's surface).
In the stratosphere ozone acts as the primary UV radiation shield, short wavelength UV radiation from the sun (<240nm) provides the energy to split oxygen molecules into oxygen atoms:
|O2(g)||UV radiation |
Oxygen atoms then react with oxygen molecules to form ozone:
O(g) + O2(g) → O3(g)
Ozone can absorb harmful UV-B and UV-C radiation, preventing it from reaching the earth's surface:
|O3(g)||UV radiation |
| O2(g) + O(g)|
The constant formation and destruction of ozone maintains a balance over time.
Human acitivities, such as the release of chlorofluorocarbons in to the atmosphere, have disturbed this balance.
Chlorofluorocarbons (CFCs) and Halons
- CFCs and halons belong to the haloalkanes.
- Chlorofluorocarbons (CFCs) are compounds containing only carbon, chlorine and fluorine (no hydrogen).
Halons are compounds containing only carbon, bromine and other halogens (no hydrogen).
- Chlorofluorocarbons (CFCs) are sold under the trade name of Freons.
- CFCs are used as working fluids in refrigerators and air conditioners because they are gases at room temperature wich can be easily liquified by compression and because they are stable and non-toxic
- CFCs are used as foaming agents in the production of polystyrene and polyurethane foam plastics used for insulation and packing materials
- CFCs are used as a propellant in spray cans for paint, insect repellants, deodorants
- Halons are used in fire extinguishers because they are dense, non-flammable liquids.
Bromochlorodifluoromethane, CF2ClBr, is commonly used in halon fire extinguishers.
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Chemistry of Ozone Depletion by CFCs
- CFCs destroy the ozone in the stratosphere (15 - 20 km above the earth's surface)
(Ozone concentrations are measure in Dobsen units, 1 Dobsen unit represents 1 molecule of O3 for every 1 billion air molecules)
Ozone loss is greatest over Antarctica where the ozone depletion has been recorded and is commonly referred to as the "ozone hole".
- Ozone (O3), an allotrope of oxygen, is poisonous to humans if breathed in, but is important to life in that it filters out or absorbs short wavelength ultraviolet radiation (u.v) in the 280 - 320nm range which can cause serious sunburn, skin cancer and eye disorders.
- The inertness and lack of water solubilty of CFCs mean they are not destroyed nor are they dissolved in rain water so they stay in the atmosphere for a very long time and diffuse up to the stratosphere
- In the stratosphere, CFCs come into contact with short wavelength ultraviolet radiation which is able to split off chlorine atoms from the CFC molecules
- These chlorine atoms destroy the ozone layer
|Cl(g)||+||O3(g)|| → ||ClO(g)||+||O2(g)|
- There are significant numbers of oxygen atoms in the stratosphere (since ozone undergoes a natural photochemical decomposition producing oxygen atoms and molecules) which leads to the regeneration of chlorine atoms in the stratosphere.
So, 1 CFC molecule can destroy many ozone molecules.
|ClO(g)||+||O(g)|| → ||O2(g)||+||Cl(g)|
Substitutes for CFCs
- The only long term solution to solve the problem of depletion of the ozone layer is to phase out the use of CFCs
(Montreal Protocol of 1987 and subsequent modifications)
- Some CFCs can be replaced by HCFCs (hydrochlorofluorocarbons), compounds containing at least 1 H atom.
The C-H bond makes these compounds more reactive in the atmosphere so they are destroyed more quickly and so are less able to diffuse into the stratosphere
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