Ozone Depletion

Question 1

Explain stratospheric ozone and how it is in equilibrium.

Answer 1

In O3 form in the stratosphere around 12-24km above Earth’s surface. O3 is in dynamic equilibrium with of chemical reactions caused by absorption of ultraviolet light

Question 2

Explain the importance of stratospheric ozone?

Answer 2

It prevents most of the high-energy ultraviolet solar radiation from reaching Earth’s surface.

Question 3

Categorise UV light by its wavelength, with its characteristics.

Answer 3

UV A: 320-400nm and not absorbed by ozone or diatomic oxygen (O2).
UV B: 280-320nm and almost fully absorbed by ozone.
UV C: <280nm. Completely absorbed by ozone and diatomic oxygen (O2).

Question 4

Explain the formation of ozone in steps:

Answer 4

1, UV light splits a molecule of O2
2. Monatomic (O) reacts with Diatomic (O2) to form O3 (ozone).

Question 5

Explain the destruction of ozone in steps:

Answer 5

1. UV light splits an ozone molecule
2. O3 will be split into one O2 molecule and an O atom.

Question 6

How does UVB light effect living organisms.

Answer 6

If UVB is not absorbed in the atmosphere then it will reach the Earth’s surface and my be absorbed by living cells. The energy of UVB is absorbed and some is converted to chemical energy as its breaks up biological molecules causing skin damage, skin cancer, DNA damage, cataracts (cloudy eye), leaf tissue damage and reduced photosynthesis, and damage to marine organisms.

Question 7

What properties did made CFCs ideal to aircon units, aerosols, and refrigerants

Answer 7

1. Boiling points close enough to ambient temperatures that the gases can be liquefied with easily achievable pressures. So aerosol cans did not need to be very strong and compressors in fridges and AC units did not have to be powerful.
2. Ability to dissolve grease and oils but not damage electrical components.
3. CFCs are not flammable.
4. Most are non-toxic.

Question 8

What is the Rowland-Molina hypothesis:

Answer 8

In 1974, two American research scientists suggested that the chemical properties of CFCs could lead to ozone depletion. Their theory was based on the chemical behaviour of CFCs.
1. Persistence of CFCs
2. Dissociation by UV and the release of Chlorine.
3. Reaction of Chlorine and Oxygen

Question 9

How is ozone measured?

Answer 9

Dobson Units (DU), which estimates the total thickness of all the ozone in the atmosphere as if it existed as a layer of pure ozone at sea level.
100DU is equivalent to 1mm thick ozone layer, normal ozone levels are around 300DU.

Question 10

What level of DU is considered an Ozone hole

Answer 10

Around 220DU

Question 11

Explain ground-based data collection of ozone

Answer 11

The first evidence of ozone depletion was collected by the British Antarctic Survey at Halley Station in Antarctica. The detection of higher levels of UV at the ground level was evidence of ozone loss in the atmosphere.

Question 12

Explain Satellite surveys for ozone data

Answer 12

Satellites orbit Earths at greater altitudes than the stratosphere so UV passing downwards through the stratosphere cannot be measured. However reading of UV light reflected by the Earth were higher than expected, suggesting ozone levels were reduced.

Question 13

Explain air samples from the stratosphere

Answer 13

Ground-based and satellite surveys showed high UV levels as evidence of ozone depletion but could not confirm the cause, or detect any changes in the chemistry of the stratosphere. Air samples collected by helium balloons and high-flying research aircraft confirmed the chemicals that were causing depletion, especially chlorine and chlorine monoxide.

Question 14

Explain how ozone levels vary

Answer 14

Ozone concentration varies in different areas, at different times, and at different altitudes

Question 15

Where is ozone depletion most severe?

Answer 15

At altitudes of 12-24km where UV light splits ozone molecules, releasing monatomic oxygen that can react with chlorine, preventing to reformation of ozone.
Over Antarctica where levels have sometimes dropped below 100DU.
During September to December in Antarctica.

Question 16

Why is ozone depletion more severe over Antarctica?

Answer 16

1. Stratospheric temperatures over Antarctica are lower than anywhere else on the Earth, allowing the formation of stratospheric clouds and ice crystals. These provide surfaces on which chemicals reactions take place, producing Cl2 from the Chlorine released from CFCs.
2. Winds around Antarctica rotate to create a ‘polar vortex’ with little mixing between the air over Antarctica and the rest of the atmosphere.
3. Due to daylight cycle, when spring arrives sunlight splits the chlorine molecules which produce the chlorine free radicals that cause ozone depletion.

Question 17

What acts have been passed to protect the ozone layer?

Answer 17

Montreal Protocol (1987)

Question 18

Explain the Montreal protocol

Answer 18

1. Ban of CFCs
2. Use of HCFCs phased out by 2030.
3. Funds available to help countries implement the protocol

Question 19

Explain alternative processes to CFCs

Answer 19

Pump action sprays are now used, e.g. for domestic cleaning products.
Stick or roll-on deodorants

Question 20

Explain alternative materials to CFCs

Answer 20

1. HCFCs for refrigeration and air conditioning units. They are less chemically stable meaning they break down in the troposphere, and would not reach the stratosphere so easily meaning less ozone depletion.
2. HFCs have replaced HCFCs as they contain no chlorine and do not cause ozone depletion, but they are more expensive.
3. CFCs as aerosol propellants have been replaced by hydrocarbons such as propane and butane. Unlike CFCs these are flammable and pose a fire risk.
4. CFCs in asthma inhalers have been replaced with HFAs.
   4.

Question 21

Evaluate safe disposal of waste CFCs

Answer 21

Waste CFCs from old fridges and aircon units are drained and incinerated. The CFcs are broken down into C02 and acidic gases (HCl, HF) that can be neutralised with crushed lime.

Question 22

Evaluate how the Montreal Protocol has been successful:

Answer 22

1. International cooperation succeeded
2. Nearly every country had to sign up & take action.
3. Encouraged the development of less harmful alternative processes and materials, making ODS no longer necessary