|Background information and Summaries 1995-2003 about Antartic Ozone hole by Andreas Fischer
Background information about the Antarctic ozone hole
by Andreas Fischer
Formation of the ozone hole
Ozone is a form of oxygen and is destroyed by ultraviolet radiation (UV) and by chemical reactions. Certain manmade chemicals when released into the atmosphere, ultimately result in the chemical destruction of ozone in the stratosphere, including many compounds that contain chlorine (e.g. CFCs) and bromine (e.g. Halons). Although these compounds are not themselves reactive, when they reach the stratosphere, they are brobken down into various molecules by intense UV, including reactive ones. Under some conditions, the presence of a single reactive molecule can result in the repeated destruction of ozone molecules. However for this repeated “catalytic” process to occur, other photochemical reactions must also occur.
The Antarctic ozone hole was discovered in the mid-1980s and quickly opened a field of intense research. At the start, both dynamical and chemical theories were proposed to explain this phenomenon, but measurements over Antarctica showed that the hole was a result of unusual chemistry on the surfaces of polar stratospheric clouds (PSCs) that activated chemical processes necessary for the catalytic destruction of ozone in the stratosphere.
The development of the Antarctic ozone hole is strongly influenced by meteorological conditions in the stratosphere. The low temperatures during winter over Antarctica lead to the formation of the polar vortex, a region with high velocity winds in the stratosphere that generally circle the Antarctic continent. This vortex somewhat limits the exchange of air between its interior and its exterior and leads to extremely low temperatures. It is these low temperatures that form the PSCs that activate chemical processes which in the presence of sunlight, result in the rapid ozone depletion that annually produces the ozone hole. Temperatures colder than -78°C can produce PSCs consisting of water and nitric acid and temperatures lower than -85°C can produce PSCs of nearly pure water-crystals. Both types of PSCs provide surfaces where reactions occur that convert less reactive molecules into much more reactive forms that readily destroy ozone.
Due to these requirements for the formation of the ozone hole (polar vortex with low temperatures, PSC formation and sunlight) the hole begins to appear in August or early September as the sun rises over Antarctica after the total darkness and extreme cold of winter. Maximum ozone loss occurs during September and October and the hole dissipates when the vortex disappears, usually in November or early December. The year to year changes in the area, depth and persistence of the ozone hole are dependent upon the meteorological conditions present during each ozone hole season. Often there are also rapid ozone decreases appearing over the edge of Antarctica in early August. Due to their small horizontal scales they are called mini-holes being a dynamical rather than a chemical phenomen.
Ozone is measured from satellites, ground based stations, balloons and aircrafts. Ground based stations and satellites measure the total column ozone, a measurement of the total amount of ozone in a column extending vertically from Earth's surface to the top of the atmosphere and reported in Dobson Units (DU). While ground based stations measure from a single location, satellites provide maps of the ozone distribution on global scales. Balloons, ground based stations and satellites all provide measurements of the detailed vertical distribution of ozone over a broad range of altitudes.
Ozone data over Antarctica are supplied by the WMO Global Atmosphere Watch network of ground based stations and by satellites such as TOMS, SBUV, GOME and TOVS.
Assessing the strength of the ozone hole
An evaluation of size, depth and persistence of the ozone hole is often useful for making year-to-year comparisons. The usual measure of the size of the ozone hole is the area where total column amounts are less than 220 DU, while the ozone mass deficit (OMD) has been used as a daily indicator of accumulated ozone depletion and considered a measure of the depth of the ozone hole. To evaluate the accumulated ozone depletion, it is necessary to compare the daily measured ozone values at a particular location with its historical pre-ozone hole values, usually considered to be the daily average column ozone at a particular location during the pre-ozone hole years 1964 – 1976. The OMD is then calculated by summing up the amount of ozone loss within the area more than 10% below pre-ozone hole norms and is expressed in millions of tons of ozone. The dates of the beginning and end of the ozone hole period each year is used to indicate the persistence.
To fully assess the Antarctic ozone hole it is also important to consider a variety of meteorological parameters such as the area and shape of the vortex, the minimum temperatures and the area of temperatures sufficiently low to produce PSCs. The meteorological data vary considerably with altitude and are therefore needed at a variety of levels.
To help the general public evaluating possible exposure to damaging UV radiation where they live, a value called the UV Index was defined that is related to the erythemal effects of solar UV on human skin. Index values of 1-3 mean low exposure, 4-6 medium, 7-9 high and 10 and more extreme exposure. This index is also used to evaluate daily UV exposure in the Antarctic and nearby populated regions.
Effects of the ozone hole
Since ozone is an effective absorber of UV, ozone depletion leads, on the average, to an increase in average UV at the ground. At any particular time, the amount of UV increase depends upon the elevation of the sun above the horizon, the amount of ozone overhead (column ozone) and local cloudiness and pollution. A portion of the UV spectrum can cause skin cancer and eye damage in humans and has the greatest impact on animals, marine organisms and plant life. Therefore the ozone hole has become a concern for the regions around Antarctica.
Ozone depletion is linked to greenhouse warming and therefore climate change in a number of ways. First, many of the gases related to ozone depletion are also greenhouse gases, such as CFCs. Second, both ozone depletion and greenhouse warming can reduce temperatures in the ozone layer and therefore increase the intensity of the ozone hole. Measurements have shown that over the past two decades there has been a global and annual-mean cooling of the stratosphere and it is believed to be due to stratospheric ozone depletion and increases in greenhouse gases and water vapor in the stratosphere. In addition, the lower temperatures in the stratosphere are believed to be partially responsible for the Antarctic polar vortex and the associated ozone hole persisting longer in recent years.
In 1987 the Montreal Protocol on Substances that Deplete the Ozone Layer was signed to reduce the global production of ozone-depleting substances. Since then several amendments have been made such as the London and Copenhagen Amendments. Without the Montreal Protocol and its Amendments, the continuing use of CFCs and other ozone-depleting substances could have increased the total stratospheric abundances of chlorine and bromine many times by now if compared with 1987 values. Although the total amount of chlorine and bromine containing compounds in the lower atmosphere peaked in 1995 and is beginning to decline, the stratosphere will recover more slowly. The stratospheric ozone depletion caused by human-produced chlorine and bromine compounds is expected to disappear by about the middle of this century if there is continued adherence to the international agreements that are presently in place.
Summaries to the Ozone Bulletins