It only exists in the atmosphere in trace quantities less than 0. 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.
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 and 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.
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.
Ozone is measured as the total amount that is present in a column of overlying atmosphere in Dobson units. One Dobson unit can be thought of as the amount of ozone that would be present if it formed a layer 0. A typical Dobson reading for the ozone layer is about Dobson units, meaning that the ozone layer would only be about 3mm thick if brought down to sea-level.
The Dobson unit is so named because of the Dobson spectrophotometer which is used to make the measurements. It works by comparing the ratio of two different wavelengths of UV radiation — one being more strongly absorbed by ozone than the other — and using the observed ratio to calculate the amount of ozone overhead. This method has been used to measure the ozone layer at Halley Research Station since In , British Antarctic Survey scientists published results showing a steep decline in the levels of ozone over Halley since the s, particularly during the austral spring, and the existence of the ozone hole was revealed.
Since then, the extent of the ozone hole has been monitored continuously using both ground-based and satellite-based techniques. The image from the link above shows the size and shape of the ozone hole as measured in October Notice that the hole where values are only around Dobson units covers most of Antarctica, and areas of depleted ozone under units extend beyond the continent.
Chemical reactions take place that could not take place anywhere else in the atmosphere. These unusual reactions can occur only on the surface of polar stratospheric cloud particles, which may be water, ice, or nitric acid, depending on the temperature.
The frozen crystals that make up polar stratospheric clouds provide a surface for the reactions that free chlorine atoms in the Antarctic stratosphere. These reactions convert the inactive chlorine reservoir chemicals into more active forms, especially chlorine gas Cl 2. When the sunlight returns to the South Pole in October, UV light rapidly breaks the bond between the two chlorine atoms, releasing free chlorine into the stratosphere, where it takes part in reactions that destroy ozone molecules while regenerating the chlorine known as a catalytic reaction.
A catalytic reaction allows a single chlorine atom to destroy thousands of ozone molecules. Bromine is involved in a second catalytic reaction with chlorine that contributes a large fraction of ozone loss.
The ozone hole grows throughout the early spring until temperatures warm and the polar vortex weakens, ending the isolation of the air in the polar vortex. As air from the surrounding latitudes mixes into the polar region, the ozone-destroying forms of chlorine disperse. When temperatures high up in the atmosphere stratosphere start to rise in late Southern Hemisphere spring, ozone depletion slows, the polar vortex weakens and finally breaks down, and by the end of December ozone levels have returned to normal.
Skip to main content. Tags: Ozone. Published 6 October Share this page. Climate change increases threats in South West Pacific. The special meteorological conditions in Antarctica cause these gases to be more effective there in depleting ozone compared to anywhere else. Human emissions of chlorofluorocarbons CFCs and halons bromine-containing gases have occurred mainly in the Northern Hemisphere.
Gases such as CFCs and halons, which are insoluble in water and relatively unreactive, are mixed within a year or two throughout the lower atmosphere.
The CFCs and halons in this well-mixed air rise from the lower atmosphere into the stratosphere mainly in tropical latitudes.
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