Exploring the ozone layer
In the 1980s monitoring of the ozone layer produced the first clear-cut evidence of the impact human activities have had on the global atmosphere.
A hole in the layer was discovered by the British Antarctic Survey. Further research showed conclusively that the ozone loss was related to chemical destruction that takes place following spring sunrise in the Antarctic polar region.
We have played a major role in monitoring and investigating the state of the ozone layer and how the loss affects our weather and climate.
Ozone is a naturally occurring gas in our atmosphere and is found mostly in a layer in the stratosphere, at heights between eight and 35 km.
This stratospheric ozone acts as a shield by absorbing potentially harmful ultraviolet (UV) radiation from the sun which would otherwise reach the Earth's surface
Ozone is also found in the lower atmosphere (the troposphere) where, in large quantities, it can be harmful to human health.
Although ozone plays a vital role in protecting life on Earth from harmful UV radiation, it has relatively low abundance in the atmosphere. If all the ozone in the atmosphere were brought to the surface it would form a layer less than one centimetre thick.
Southern hemisphere - the Antarctic ozone hole
In 1985 the British Antarctic Survey reported a hole in the ozone layer over Halley Bay, Antarctica. Satellite measurements later confirmed that the springtime ozone loss was a continent-wide feature that is now referred to as the 'Antarctic ozone hole'.
The main cause of the ozone destruction is chlorine, which comes from man-made chlorofluorocarbons (CFCs) released into the atmosphere. Although the CFCs themselves are not chemically reactive, they are broken down by solar ultraviolet radiation high up in the atmosphere, thereby releasing reactive chlorine.
The ozone hole forms every year in southern hemisphere spring (Sept-Nov).
During the dark Antarctic winter it is cold enough (less than -78 °C) for ice clouds to form in the lower stratosphere. The presence of these clouds is key for the chemical changes that produce the destructive chlorine.
When sunlight returns to the Antarctic in the spring, the combination of sunlight and active chlorine leads to rapid chemical ozone loss, causing the hole.
Northern hemisphere - ozone destruction
Enhanced rates of ozone depletion can also occur over the Arctic during the winter and spring, if winds and temperature in the stratosphere are right.
Although there are fewer ice clouds, which are key to the ozone destruction, the more disturbed nature of the Arctic winter circulation means that ozone-poor air can be transported away from the polar region to temperate zones, whereas in the Antarctic the ozone hole tends to be restricted to polar latitudes.
Total ozone is measured in Dobson Units (DU) - the higher the number, the more ozone there is and the less risk of damage from the Sun's UV rays. Total ozone levels over the UK vary with the seasons, nearly 400 Dobson units (DU) in the early spring and less than 300 DU in the autumn.
High quality measurements over the last 20 years suggest a long-term rate of decrease in total ozone over the UK of around 3% per decade. The major part of this decrease has occurred in the spring.
It is possible that the impact of the Montreal Protocol, which restricted the production of CFC gases, and climate change, could lead to a reversal of this trend in the future.
Very low in 2006
Met Office scientists noted a record low value of 177 DU over Reading in January 2006. This was partly explained by the transport of ozone-poor air from polar latitudes to the UK, but was also due to the presence in the lower atmosphere of anticyclonic conditions which led to a reduction in ozone concentrations at heights between around eight and 12 km, and a significant decrease in the total ozone.
The production of CFCs, the gases at the source of the problem, was restricted by global agreement in 1987 - the Montreal Protocol - and by subsequent revisions.
After 1995 most CFC, previously used in a range of applications from aerosol spray propellants to refrigerator coolants, ceased to be produced by the main signatories to the agreement.
The aim of the Montreal Protocol is a return to pre-1980s levels of Antarctic ozone by the middle of this century (21st). As CFCs have a long atmospheric lifetime - around 50-100 years -; predicting exactly when it will recover is difficult.
A report on the current state of the ozone hole can be found on the British Antarctic Survey website
Up-to-date ozone maps (derived from satellite data) can be found at NASA's Ozone Hole Watch
Ozone depletion and climate change are often confused. In fact, they are two distinct phenomena.
Climate change occurs due to the build up of greenhouse gases (GHGs) in the atmosphere, which traps outgoing solar radiation and causes the global average temperature to increase
Ozone depletion occurs due to the build-up of, primarily, chlorine- and bromine-containing compounds, which act to destroy ozone in the polar stratosphere
The two effects are linked. Ozone is a GHG, so increases in tropospheric ozone and decreases in stratospheric ozone contribute to climate change. This contribution is not nearly as significant as that from another GHG, carbon dioxide.
Conversely, climate change affects the temperature-dependent chemical reactions causing ozone depletion. Ozone-depleting gases are GHGs, so ozone depletion and climate change are also linked in this indirect way.
The depletion of polar ozone can also increase the level of solar UV at the Earth's surface, leading to a greater risk of overexposure to UV radiation and related health effects.
The amount of UV radiation reaching the surface depends not only on total ozone, but also on the elevation of the sun and filtering by clouds and aerosols. This means that the level of UV radiation can undergo large and rapid changes at any location.
The Met Office measures total ozone daily (when conditions allow), at Lerwick in the Shetland Islands (60.1°N, 1.2°W). Universities also supply observations from sites at Reading and Manchester.
During the winter/spring these measurements are sent to the World Meteorological Organization's (WMO) Ozone Mapping Centre in Greece. They are then combined with data from other stations to produce maps showing the daily distribution of total ozone over the northern hemisphere. Ozone measurements from satellites are used in data-sparse areas.