The ozone layer in the stratosphere is vital for life on earth because ozone (O3) absorbs the sun's high energy UV-C radiation (100-280 nm) and some UV-B radiation before it reaches earth. This UV radiation can damage life on earth by causing:
- DNA mutation by breaking hydrogen bonds
- skin cancer (due to dna mutation)
- Cataracts
- Crop failure (due to dna damage and interaction with plant growth regulators)
- Sunburn/ Epidermal hypoplasia
Human activity, in particular since the industrial revolution, has caused an increase in ozone depleting compounds in the atmosphere, including CFCs and NOx .
Chlorofluorocarbons are compounds which are highly unreactive in the troposphere due to their very strong C-Cl and C-F bonds (polar bonds with high enthalpy). This makes them useful in things like: aerosols, refrigerant coolants, electrical circuit cleaning solvents etc.
However, it is because they are so unreactive that makes them so harmful!
The C-Cl bond has too high an enthalpy to be broken down in the troposphere, but when they reach the stratosphere, uv light can break this bond homolytically which produces chlorine radicals.
The chlorine radicals can then go on to catalyse the destruction of ozone (it is a homogeneous catalyst because it provides an alternative route for the reaction to take place and is regenerated).
It is estimated that one chlorine atom can destroy over 100,000 ozone molecules before it is removed from the stratosphere by termination.
This means that more UV light is not absorbed by the thinner ozone layer and reaches earth.
Why is there a hole over the Antarctic?
Ozone is depleted over the antarctic in it's spring because sunlight falls on the polar stratospheric clouds (containinh chlorine radical reservoir compounds eg. HCl and ClONO2.) which are formed and trapped in the cold air (-80'C) in the polar
vortex in the winter. When sunlight falls on these clouds, ozone destruction is able to
happen, which forms a hole in the ozone during the spring.
Can the ozone ever recover?
Since the Montreal Protocol in 1987, many industrialized countries have fazed out the
use of ozone depleting compounds such as CFCs (apart from vital uses such as asthma
inhalers) and stratospheric chlorine levels have been continually dropping. Ozone can
replenish itself however this takes time; moreover, it takes decades for CFC compounds
to be completely destroyed so it will take time before the ozone layer completely recovers.
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| catalytic destruction of ozone |
It is estimated that one chlorine atom can destroy over 100,000 ozone molecules before it is removed from the stratosphere by termination.
This means that more UV light is not absorbed by the thinner ozone layer and reaches earth.
Why is there a hole over the Antarctic?
Ozone is depleted over the antarctic in it's spring because sunlight falls on the polar stratospheric clouds (containinh chlorine radical reservoir compounds eg. HCl and ClONO2.) which are formed and trapped in the cold air (-80'C) in the polar
vortex in the winter. When sunlight falls on these clouds, ozone destruction is able to
happen, which forms a hole in the ozone during the spring.
Can the ozone ever recover?
Since the Montreal Protocol in 1987, many industrialized countries have fazed out the
use of ozone depleting compounds such as CFCs (apart from vital uses such as asthma
inhalers) and stratospheric chlorine levels have been continually dropping. Ozone can
replenish itself however this takes time; moreover, it takes decades for CFC compounds
to be completely destroyed so it will take time before the ozone layer completely recovers.
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