Although ozone is found dispersed throughout the atmosphere, it is mostly concentrated 13 to 30 km (9 to 18 miles) above earth’s surface. This triatomic form of ozone protects the earth’s surface from the ultraviolet rays emitted from the sun. Even a small decrease in the concentration of ozone can increase the risk UV poses to the DNA of all living forms. Thus this toxic gas concentrated in the stratosphere actually protects us.
Ozone depletion is caused by chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), which are also known as halocarbons. These substances usually contain chlorine or bromine and were used in propellants in the past. CFCs have been banned in many countries and replaced by HCFCs, which do deplete the ozone layer but not as quickly as CFCs.
The ozone depletion reactions require cold, isolated darkness to begin and then light to complete the reaction. The halocarbons react with the ozone to create two different gases: oxygen and chlorine monoxide. One of the three atoms of the ozone molecule combines with chlorine to create the chlorine monoxide and the other two atoms form the oxygen.
The North and South Poles create ideal conditions for ozone depletion, especially the Antarctic. The water surrounding the single land mass of Antarctica isolates the air while the long twilight of polar winter provides the cold darkness. When summer brings sunlight, the reaction occurs quickly until the stratospheric polar clouds are dispersed when ozone depletion slows once more.
The atmosphere above the Antarctic has a lower concentration than other areas and this is what is referred to as the hole in the ozone layer. This hole is monitored from various stations including the Cape Grim Baseline Air Pollution Station in Tasmania, Australia. The hole varies seasonally because of the way the light and temperature affect the ozone depletion.
Ozone is measured using ultraviolet wavelengths, which are affected differently by the level of ozone in the atmosphere, or using an electrochemical reaction, which is how the Bureau of Meteorology’s balloon-borne sondes measure ozone. In the United States, the National Oceanic and Atmospheric Administration monitor ozone while the Bureau of Meteorology performs this function in Australia.
In addition to allowing UV rays to filter through the atmosphere and damage DNA, halocarbons are also powerful greenhouse gases. Like carbon dioxide, these gases contribute to global warming by trapping heat near the surface. While the halocarbons deplete the earth’s protective ozone layer, they also create a new, damaging layer.
The Montreal Protocol is an agreement between countries to control ozone depleting substances. It was ratified by the United States of America in 1988 and Australia in 1989. This is one measure that authorities are taking to defend the earth’s natural sunscreen—the ozone layer.
Australian Bureau of Meteorology: http://www.bom.gov.au/climate/
Cape Grim Baseline Air Pollution Station: http://www.bom.gov.au/inside/cgbaps/
National Oceanic and Atmospheric Administration: http://www.noaa.gov/wx.html
Union of Concerned Scientists: Global Warming: http://www.ucsusa.org/global_warming/science_and_impacts/science/global-warming-faq.html
United Nations Environment Programme: Ozone Secretariat: http://ozone.unep.org/