Ninety percent of the ozone in the atmosphere sits in the stratosphere, the layer of atmosphere between about 10 and 50 kilometers altitude. The natural level of ozone in the stratosphere is a result of a balance between sunlight that creates ozone and chemical reactions that destroy it.
Ozone is destroyed when it reacts with molecules containing nitrogen, hydrogen, chlorine, or bromine. Some of the molecules that destroy ozone occur naturally, but people have created others, such as CFCs (chlorofluorocarbons) have led to increased rates of ozone destruction, upsetting the natural balance of ozone and leading to reduced stratospheric ozone levels. These reduced ozone levels have increased the amount of harmful ultraviolet radiation reaching the Earth’s surface. When scientists talk about the ozone hole, they are talking about the destruction of stratospheric, “good,” ozone.
Tropospheric OzoneAlthough ozone high up in the stratosphere provides a shield to protect life on Earth, direct contact with ozone is harmful to both plants and animals (including humans). Ground-level, “bad,” ozone forms when nitrogen oxide gases from vehicle and industrial emissions react with volatile organic compounds (carbon-containing chemicals that evaporate easily into the air, such as paint thinners). In the troposphere near the Earth’s surface, the natural concentration of ozone is about 10 parts per billion (0.000001 percent). According to the Environmental Protection Agency, exposure to ozone levels of greater than 80 parts per billion for 8 hours or longer is unhealthy. Such concentrations occur in or near cities during periods where the atmosphere is warm and stable. The harmful effects can include throat and lung irritation or aggravation of asthma or emphysema.
What is a Dobson Unit?The Dobson Unit (DU) is the unit of measure for total ozone. If you were to take all the ozone in a column of air stretching from the surface of the earth to space, and bring all that ozone to standard temperature (0 °Celsius) and pressure (1013.25 millibars, or one atmosphere, or “atm”), the column would be about 0.3 centimeters thick. Thus, the total ozone would be 0.3 atm-cm. To make the units easier to work with, the “Dobson Unit” is defined to be 0.001 atm-cm. Our 0.3 atm-cm would be 300 DU.
What is the Ozone Hole?The ozone hole is not technically a “hole” where no ozone is present, but is actually a region of exceptionally depleted ozone in the stratosphere over the Antarctic that happens at the beginning of Southern Hemisphere spring (August–October). 220 Dobson Units is the boundary of the region representing ozone loss. See The Ozone Hole Tour from the Centre for Atmospheric Science, Cambridge University
Latest Northern Hemisphere
Latest Southern Hemisphere
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Images Courtesy of Canada Environnement
Excellent site with an extensive ozone section
Latest Global Map
The average size of the hole in September–October 2013 was 21.0 million square kilometers (8.1 million square miles).
The average size since the mid 1990s is 22.5 million square kilometers (8.7 million square miles).
The largest single-day ozone hole ever recorded by satellite was 29.9 million square kilometers (11.5 million square miles) on September 9, 2000.
UK Ozone Ground StationsThe Met. Office measures ozone at Lerwick in the Shetlands using a Dobson spectrophotometer (until 2003 measurements were also made at Camborne in Cornwall) Stratospheric ozone monitoring is also undertaken by The University of Manchester at Reading University
|UK Ground Monitoring Ozone Stations|
Defra UK Ozone Monitoring Stations
Canada Environnement Excellent Ozone Information Site
The Ozone Hole Tour Centre for Atmospheric Science, Cambridge University
Stratosphere - SBUV/2 Total Ozone US National weather service
Defra UK Ozone Monitoring Stations
28/1/2014 Work in progress