Roles of Ozone

Monitoring and Management‎ > ‎4. The Atmosphere‎ > ‎ ‎

Describe ozone as a molecule able to act both as an upper atmosphere UV radiation shield and a lower atmosphere pollutant

  • Ozone is found at very low levels in the lower atmosphere (0.02 ppm in clean air).
  • The formation of ozone through the interaction between sunlight, hydrocarbons and nitrogen oxides can lead to higher concentrations in the lower atmosphere or troposphere.
  • Ozone is poisonous to humans (as well as most other organisms), with its effects in larger than normal concentrations including:
    • Breathing problems.
    • Aggravation of respiratory problems.
    • Headaches.
    • Fatigue.
    • Death (in high concentrations).

Structure of ozone
  • Ozone exists at much higher levels in the upper atmosphere or stratosphere (ranging from 2 ppm to 8 ppm in the stratosphere), where 90% of the atmosphere’s oxygen is found.
  • In the upper atmosphere both oxygen gas and ozone gas absorb different parts of the ultraviolet radiation range in a cycle of ozone formation and decomposition:
    • Oxygen absorbs high energy ultraviolet radiation as part of the ozone formation process:
    • Ozone absorbs medium energy ultraviolet radiation as part of the ozone decomposition process:
    • In both of these sets of reactions, the second reaction is exothermic, which accounts for the increasing temperature in the stratosphere.
  • Thus, the ozone layer acts as a radiation shield in the upper atmosphere by absorbing medium and high energy ultraviolet radiation.

Distribution of atmospheric oxygen
Ozone-induced discolouration of a leaf