Facts about the Ozone Layer

The ozone layer is a layer in the Earth´s atmosphere that protects the Earth from the ultraviolet rays (UV) of the Sun. The ozone layer protects the Earth from UV radiation by absorbing light at UV wavelengths. Ozone forms naturally in the stratosphere by the mixing of molecular oxygen (O2) with atomic oxygen (O). The ozone layer is found in the lower region of the stratosphere at approximately 15-30 km (9-19 miles) from the Earth´s surface. The thickness of the ozone layer varies geographically and seasonally. The ozone layer absorbs nearly 99 percent of the Sun´s ultraviolet light. If exposed to UV radiation, life forms on Earth could be seriously damaged.

Where is the ozone layer located?

The stratosphere is a layer of the Earth´s atmosphere that lies on top of the troposphere at approximately 10-50 km (6-31 miles) from the Earth´s surface. In the stratosphere, there exists an intense temperature inversion, as the air temperature increases rapidly with altitude. The temperature inversion stems from the ultraviolet radiation that is absorbed by the ozone gas at wavelengths ranging from 200-315 nanometers. The ozone layer is most dense at an altitude of 25 km (16 miles). The concentration of ozone at this altitude is of 12 ozone molecules for every million air molecules. The ozone layer is important, as it protects the Earth from UV radiation.

How the ozone layer is created

The ozone (O3) layer, which lies in the Earth´s stratosphere, is created when ultraviolet light interacts with oxygen molecules (O2). When UV light hits molecular oxygen, it splits them into individual atoms (atomic oxygen). Individual atoms of oxygen then combine with molecular oxygen that was not broken by UV light to produce ozone (O3). The ozone molecule, which is unstable, is broken down into a molecule of O2 and a molecule of atomic oxygen when hit by UV light. This is an ongoing process and is known as the ozone-oxygen cycle. The concentration of ozone gas is greatest between 20-40 km (12-24 miles), where the ozone concentrations range from 2-12 parts per million.

Ozone concentrations

The ozone layer varies greatly seasonally and geographically. Generally, the ozone layer is thinner at the equator and thicker at the poles. The ozone layer is also thicker during the spring and thinner during autumn in the northern hemisphere. Most ozone is found in the mid to high latitudes at the northern and southern hemispheres, and the highest ozone concentrations are usually found during the spring season. The stratosphere over Antarctica is known to have one of the world´s highest ozone concentrations. Most of the ozone gas in the stratosphere of Antarctica is produced in the tropics and is carried to the Antarctic by stratospheric winds.

Depletion of the ozone layer

Chemicals, including nitrous oxide, nitric oxide, chlorine, bromine and hydroxyl, are known to be depleting the ozone layer. When nitrous oxide emitted from fertilizers and chlorofluorocarbons (CFC) reaches the stratosphere, it´s broken down, releasing atomic chlorine and destroying ozone. A single atom of chlorine can remove as many as 10,000 ozone molecules before it combines with other substances. An increase in the concentrations of chlorofluorocarbons is a serious threat to the concentration of ozone, since the average lifetime of the CFC molecule is approximately 50-100 years. The concentration of CFC in the stratosphere and the use of bromine compounds destroy the ozone layer at a higher rate than chlorine compounds.

In the present, it´s believed that chlorine and bromine are deteriorating the ozone layer. Such deterioration allows great amounts of ultraviolet B radiation to reach the surface of the Earth. UV light can produce skin cancer, eye cataracts; suppress the immune system; and reduce oceanic phytoplankton. In the year 2009, nitrous oxide was the largest ozone-depleting substance stemming from human activities. According to research.noaa.gov, in 1987, an international panel of several countries established a treaty known as the Montreal Protocol on substances that deplete the ozone layer. Since its foundation of the protocol, there have been periodic assessments and updates.