Stratospheric Ozone (DP IB Environmental Systems & Societies (ESS))
Revision Note
Stratospheric Ozone
Ozone is a molecule composed of three oxygen atoms (O3)
It is mainly found in the Earth's stratosphere
This is a layer of the atmosphere located approximately 10 to 50 kilometres above the Earth's surface
Ozone plays a very important role in protecting life on Earth
This is because it absorbs a significant portion of the Sun's harmful UV radiation
This significantly reduces the amount of UV radiation that reaches the Earth's surface
Types of UV radiation:
UVA:
Longest wavelength
Least harmful but can cause skin aging and contribute to skin cancer
UVB:
Medium wavelength
Can cause skin burns and direct DNA damage
Mostly absorbed by stratospheric ozone, but some reaches the Earth's surface
UVC:
Shortest wavelength
Most harmful
Completely absorbed by stratospheric ozone
Ozone Equilibrium
The amount of ozone in the stratosphere remains relatively constant over long periods
This is due to a steady state of equilibrium
Equilibrium is maintained between the processes of ozone formation and destruction
When UV radiation from the Sun interacts with ozone molecules, some of the ozone absorbs the energy and breaks apart
This results in the formation of an oxygen molecule (O2) and a free oxygen atom (O)
This process of ozone destruction occurs naturally in the stratosphere
Under normal conditions, the free oxygen atom (O) can combine with another oxygen molecule (O2) to form ozone (O3) again
This ozone destruction and reformation creates a dynamic equilibrium in the stratosphere
There is a continuous cycle of ozone molecules being broken apart and reformed
This dynamic equilibrium ensures that the concentration of ozone in the stratosphere remains relatively stable over time
The rate of the forward reaction equals the rate of the backward reaction in the system, so the concentrations of the reactants and products remain relatively constant
Ozone Depletion
Ozone-depleting substances
Ozone-depleting substances (ODSs) are chemicals that cause stratospheric ozone depletion
These substances cause the destruction of ozone molecules
In other words, they enhance the natural ozone breakdown process (beyond natural levels)
ODSs are commonly used in various human activities and products:
Sources of Ozone Depleting Substances
Source | Details |
|---|---|
Aerosols | Chlorofluorocarbons (CFCs) were previously used as propellants in aerosol products like sprays, foams, and deodorants When released into the atmosphere during spraying, these substances can eventually reach the stratosphere and contribute to ozone depletion |
Gas-blown plastics | ODSs were also used as blowing agents in the production of foamed plastics These agents help create air pockets within the plastic material, making it lightweight During manufacturing or disposal of these products, ODSs can be released into the atmosphere |
Pesticides | Some pesticides, e.g. those containing methyl bromide, have been used in agricultural practices for soil fumigation When applied, these substances can vaporise and enter the atmosphere, where they can contribute to ozone depletion |
Flame retardants | Some flame retardants contain halogen atoms and have been used in various products to reduce their flammability When these products degrade or are disposed of, the halogenated compounds can be released into the atmosphere |
Refrigerants | ODSs were widely used as refrigerants in cooling systems, such as air conditioners and refrigerators The most well-known examples are CFCs When these refrigerants leak or are improperly disposed of, they can reach the stratosphere and contribute to ozone depletion |
Imbalance in equilibrium
When ozone formation and destruction rates are unequal, equilibrium is disrupted
This leads to increased ozone depletion
Increased UVB radiation reaches the Earth's surface
Affects ecosystems and human health
Causes increased rates of skin cancer and cataracts
Reduces terrestrial and marine productivity
Ozone holes
Ozone depletion affects the entire Earth's stratosphere
However, ozone holes are most prominent at the poles
Ozone holes are areas of low stratospheric ozone
These holes appear every spring due to ODSs and seasonal weather patterns
Exam Tip
The use of chemical symbols, formulae or equations for the equilibrium of ozone is not required, so you do not need to learn the chemical equations relating to the formation and destruction of ozone.
Did this page help you?