The Ozone Layer

The Solar Spectrum

To understand the importance of ozone in the atmosphere, we first must understand the nature of the energy that comes to us from the sun. The sun and other stars are very hot and so radiate energy. Specifically, we call this energy electromagnetic energy.

We can characterize electromagnetic radiation according to its wavelength. Remember that the energy of the radiation is inversely proportional to the wavelength.

E = hc/

The shape of the intensity vs. wavelength curve depends on the temperature of the emitting object. The higher the temperature, the higher is the maximum intensity and the lower the wavelength of this maximum.

The maximum intensity of the sun's output of radiation is in the visible light region.
At the top of the atmosphere, approximately 7% of the radiation is in the ultraviolet, 41% in the visible, and 52% in the infrared.

The intensity of electromagnetic radiation that reaches the surface of our planet is considerably less than that at the top of the atmosphere. Something in the atmosphere filters out energy. The highest energy light is most effectively filtered out.

This is very important because high energy light (gamma rays, x-rays, UV-radiation) penetrates the cells of living things and causes damage to them.

Molecular Filters

Consider the figure at right. The magenta filter blocks some wavelength of light, the cyan and yellow filter block others. When the 3 filters are combined, all wavelengths of visible light are blocked and the color appears black.

The atmosphere consists of many molecular and atomic filters that each block a certain range of wavelengths of light. With the combined effect of all of these, most of the harmful radiation from the sun is blocked before it reaches the plants and animals on the surface of the Earth.
  • In the highest region of the atmosphere, the ionosphere, atoms such as N absorb very high energy light and lose electrons.

    N + energy N+ + e-

  • Below that, in the mesosphere, strongly-bonded molecules such as N2 absorb energy and break apart.

    N2 + energy 2 N

  • Below that, in the stratosphere, ozone absorbs nearly all the ultraviolet radiation (between 200 and 310 nm).

  • In the lowest region of the atmosphere, the troposphere, colored molecules absorb some visible light and other molecules (H2O, CO2) absorb infrared or heat energy.

Ozone in the Stratosphere

Ozone has a low concentration throughout the atmosphere but there is a region in the stratosphere where its concentration is higher than anywhere else. About 90% of the ozone in the atmosphere is located within a band centered at 25 km above the surface.

Ozone's concentration within the ozone layer is 0.2 to 0.4 ppm. Typical concentrations in unpolluted air closer to the surface is 0.03 ppm.

The ozone in the ozone layer absorbs 97-99% of all the ultraviolet light that passes into the stratosphere.

The Ozone Hole

Until recently, the concentration of ozone in the stratosphere was nearly constant. However, man-make chemicals such as the FREON used in refrigerators and air conditioners destroy it.

Over the course of the winter, reactive molecules build up in stratospheric clouds over the poles. The light and heat of the early spring causes these molecules to combine with ozone, resulting in a hole in the ozone layer.

UV radiation can reach the Earth's surface under this hole in the protective ozone layer.

International treaties are now limiting the amount of ozone-damaging chemicals that are released.

Professor Patricia Shapley, University of Illinois, 2012