A molecule made of three oxygen atoms, also called trioxygen, O3.

Ozone is present in relatively small amounts in the atmosphere, but plays an important role. Ozone at ground level is considered a pollutant and is, at high concentrations, harmful to humans. In the upper atmosphere the ozone, however, acts as a filter, shielding life on Earth from the Sun's dangerous high frequency ultraviolet radiation. This is the ozone layer.

The ozone is produced by a reaction with ultraviolet light (UV). The reaction takes place in the stratosphere, which is a layer of the atmosphere that begins at a height of about 8-10 km.

UV-light hits an oxygen molecule and splits it into two oxygen atoms:
O2 + UV → 2O

The free oxygen atoms combine with another oxygen molecule to form an ozone molecule:
O + O2 → O3

The ozone absorbs UV-light and the molecule is split into an oxygen atom and a molecule:
UV + O3 → O + O2

The free oxygen atom may then combine with an oxygen molecule and create ozone again:
O + O2 → O3, or combine with another oxygen atom:
O + O → O2

The absorption of the UV-energy by ozone releases heat, causing an increase in atmospheric temperature.

The ozone production in the stratosphere is highest above the tropics, because the UV-light is most intense there due to the Sun's position. The ozone produced in the tropics is distributed by wind to the southern and northern hemispheres, and the thickness of the ozone layer is actually thinner in the tropics.

In addition to the production of ozone in the upper part of the atmosphere, where it is beneficial, ozone is also produced closer to the Earth's surface. It is formed in a reaction between highly polluted air and the UV-light during the day. Ozone is harmful for humans, and can lead to a range of health problems.

The ozone in the lowest part of the atmosphere is also a concern because it acts as a greenhouse gas. Precisely how much effect it has as a greenhouse gas is difficult to estimate, but it could be substantial.

Low-lying ozone is also an important component of photochemical smog. Thus it is a health hazard and some scientists believe that
it interferes with negatively with photosynthesis.