The Atmosphere

The Composition and Structure of the Atmosphere

  • The atmosphere is essentially composed of 78% nitrogen, 21% oxygen, 1% argon, and a tiny quantity of other gases including carbon dioxide and ozone.
  • The concentration of gases does not remain constant with altitude. It varies depending on the height from the surface of the Earth.
  • The atmosphere can be divided into several layers: the troposphere, stratosphere, mesosphere, thermosphere, and exosphere.
  • The layer closest to the Earth is the troposphere, which contains approximately 75% of the total mass of the atmosphere and it is where weather events occur.
  • Above the troposphere is the stratosphere, this is where the ozone layer is located and it extends from 10 to 50km above the Earth’s surface.

Ozone in the Atmosphere

  • Ozone is vital in the stratosphere layer of the atmosphere as it absorbs harmful ultraviolet (UV-B) radiation from the Sun, thereby protecting life on Earth.
  • Ozone, O3, is a pale blue gas composed of three oxygen atoms. It has a distinctive sharp, almost acrid odour.
  • The concentration of ozone in the atmosphere is extremely low. It typically ranges between 1 to 10 parts per million.

  • Ozone is produced by the reaction of UV-C radiation with the oxygen molecule, O2.

    O2 + UV-C –> 2O

    O + O2 –> O3

Threat to the Ozone layer

  • Chlorofluorocarbons (CFCs) are the main substances that cause destruction of ozone. CFCs are stable, non-toxic and non-flammable chemicals used in a variety of applications such as refrigerants and aerosol propellants.
  • CFCs can take many years to reach the stratosphere but once there, they are broken down by sunlight releasing chlorine atoms.

  • The chlorine atoms act as a catalyst in the ozone depletion reaction, leading to thinning of the ozone layer, known as the “Ozone Hole”.

    Cl + O3 –> ClO + O2

    ClO + O –> Cl + O2

Protection and Legislation

  • The depletion of the ozone layer led to the introduction of the “Montreal Protocol” in 1987, an international treaty designed to protect the ozone layer by phasing out ozone-depleting substances (ODS).
  • CFCs have now largely been replaced with more environment-friendly alternatives such as hydrogen, oxygen, nitrogen and carbon compounds (HFCs).
  • Hydrofluorocarbons (HFCs) do not contain chlorine and therefore do not deplete ozone. However, they are powerful greenhouse gases and contribute to global warming. This has necessitated the search for even safer alternatives.