Introducing ozone: the basics
An ozone molecule consists of three oxygen atoms instead of the usual two (the oxygen we breathe, O2, makes up 21% of the atmosphere). It only exists in the atmosphere in trace quantities (less than 0.001%), but its effects are very important. Ozone molecules are created by the interaction of ultra-violet (UV) radiation from the Sun with O2 molecules: when an O2 molecule is split, the two free oxygen atoms will bond with other O2 molecules to form O3 molecules. Because UV radiation is more intense at higher altitudes where the air is thinner, it is in the stratosphere where most of the ozone is produced, giving rise to what is called the ‘ozone layer’. The ozone layer, containing over 90% of all atmospheric ozone, extends between about 10 and 40km altitude, peaking at about 25km. (Above this level the concentration of oxygen available to be converted into ozone declines so less ozone is formed despite the abundance of UV radiation.)
The ozone layer is very important for life on Earth because it has the property of absorbing the most damaging form of UV radiation, UV-B radiation which has a wavelength of between 280 and 315 nanometres. As UV radiation is absorbed by ozone in the stratosphere, it heats up the surrounding air to produce the stratospheric temperature inversion that is shown in the following diagram.
In the lower atmosphere, temperature declines with height because of the steep decline in pressure from sea-level upwards. This is referred to as an adiabatic temperature change – the air cools simply because of expansion with altitude (and conversely, air under more pressure is compressed and warms up). However above the troposphere, where atmospheric pressure is a small fraction of its sea-level value, the presence of ozone causes temperature to rise with height until the altitude of the stratopause is reached. Above the stratosphere, temperature declines with height in the mesosphere, but rises again in the thermosphere due to the effect of radiation and charged particles from the Sun on what little atmosphere is left near the boundary with space.
Learn more about how scientists in Antarctica collect data about how the atmosphere changes with height.
Most of the stratospheric ozone is produced at tropical latitudes, but high altitude winds spread it over the whole planet. It is continually forming and breaking down, and its distribution over the planet is not uniform or constant. Instead, there are seasonal and longer term variations in the quantity of stratospheric ozone in different parts of the world. However, over the long run the natural processes of formation and breakdown are balanced: it is only in recent decades that human activities have led to ozone being destroyed much faster than it can be formed, thereby creating the ozone hole that exists today.
Ozone can also be formed at ground level to produce ‘photochemical smog’; and, as ozone is a toxic gas, there is a health hazard when ozone reaches high levels. This problem occurs primarily in the summer in cities with a high amount of traffic when sunlight interacts with car exhaust fumes containing nitrogen oxides.
There is an irony that human activities have been destroying ozone high in the atmosphere where we want it, while creating ozone at ground level where we don’t want it!