UV Radiation & Ozone (SL IB Environmental Systems & Societies (ESS))

• UV radiation is a form of electromagnetic radiation emitted by the Sun
• It is invisible to the human eye and is characterised by having shorter wavelengths than visible light

Ozone

• Ozone is a molecule composed of three oxygen atoms (O₃)
  • It is primarily found in the Earth's stratosphere, a layer of the atmosphere located approximately 10 to 50 kilometres above the Earth's surface

• Ozone plays a crucial role in protecting life on Earth by absorbing a significant portion of the Sun's harmful UV radiation
• When UV radiation from the Sun interacts with ozone molecules, some of the ozone absorbs the energy and breaks apart, resulting in the formation of an oxygen molecule (O₂) and a free oxygen atom (O)
  • This process of ozone destruction occurs naturally in the stratosphere due to the presence of UV radiation
  • However, 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, where 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, as 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 destruction and reformation creates a dynamic equilibrium in the stratosphere - there is a continuous cycle of ozone molecules being broken apart and reformed

Ozone Depleting Substances (ODSs)

• Ozone-depleting substances (ODSs) are chemicals that cause stratospheric ozone depletion by breaking down ozone molecules
• ODSs, including halogenated organic gases like chlorofluorocarbons (CFCs), are commonly used in various human activities and products:

   Aerosols

• • ODSs such as 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
  • However, during the manufacturing process or disposal of these products, ODSs can be released into the atmosphere

   Pesticides

• • Some pesticides, particularly those containing methyl bromide, a halogenated compound, have been used in agricultural practices for soil fumigation
  • When applied, these substances can volatilise and enter the atmosphere, where they can contribute to ozone depletion

   Flame retardants

• • Certain flame retardants, including polybrominated diphenyl ethers (PBDEs), 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 the chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs)
  • When these refrigerants leak or are improperly disposed of, they can reach the stratosphere and contribute to ozone depletion

• Halogen atoms (e.g. chlorine and bromine) from ODSs enter the stratosphere through these various human activities
• Once in the stratosphere, these atoms can undergo a repetitive cycle of ozone destruction
  • The released halogen atoms react with ozone molecules, breaking them apart and reducing the concentration of ozone in the stratosphere
  • After the ozone molecules are destroyed, the halogen atoms can be regenerated and participate in further ozone-depleting reactions, perpetuating the cycle
  • This repetitive cycle of ozone destruction by halogen atoms increases the overall depletion of ozone in the stratosphere, resulting in an increasingly thinner ozone layer
  • With a thinner ozone layer, more ultraviolet radiation from the Sun can penetrate the Earth's atmosphere, reaching the surface and potentially causing harmful effects on living organisms