CFCs escape into the atmosphere from refrigeration and propellant devices and processes. In the lower atmosphere, they are so stable that they persist for years, even decades. This long lifetime allows some of the CFCs to eventually reach the stratosphere. In the stratosphere, ultraviolet light breaks the bond holding chlorine atoms Cl to the CFC molecule. A free chlorine atom goes on to participate in a series of chemical reactions that both destroy ozone and return the free chlorine atom to the atmosphere unchanged, where it can destroy more and more ozone molecules.
For those who know the story of CFCs and ozone, that is the part of the tale that is probably familiar. Most of the roaming chlorine that gets separated from CFCs actually becomes part of two chemicals that—under normal atmospheric conditions—are so stable that scientists consider them to be long-term reservoirs for chlorine.
So how does the chlorine get out of the reservoir each spring? Under normal atmospheric conditions, the two chemicals that store most atmospheric chlorine hydrochloric acid, and chlorine nitrate are stable. But in the long months of polar darkness over Antarctica in the winter, atmospheric conditions are unusual. An endlessly circling whirlpool of stratospheric winds called the polar vortex isolates the air in the center.
Because it is completely dark, the air in the vortex gets so cold that clouds form, even though the Antarctic air is extremely thin and dry. So if you care about whales and tuna, you should care about plankton too. Smithsonian National Museum of Natural History. Poor Plankton Plankton tiny plants and animals in the ocean grow poorly or die when exposed to excessive UV radiation. Against the background of a mature sea urchin Strongylocentrotus droebachiensis are two sea urchin embryos.
One is normal, and one was deformed by over-exposure to UV radiation. 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.
The ozone hole has developed because people have polluted the atmosphere with chemicals containing chlorine and bromine. The primary chemicals involved are chlorofluorocarbons CFCs for short , halons, and carbon tetrachloride. CFCs in particular have been used for a wide range of applications, including refrigeration, air conditioning, foam packaging, and making aerosol spray cans.
Because these chemicals are so inert, they are able to stay in the atmosphere long enough to be carried upwards to the stratosphere where they can damage the ozone layer. The actual processes by which CFCs and other ozone depleting chemicals destroy ozone are complex and require certain weather conditions to exist. A simplified description of the process involving CFCs is as follows:.
The fact that most of the ozone depletion happens over Antarctica also requires some explanation. CFCs and other ozone depleting gases may come from anywhere, but it is in the south polar stratosphere where the conditions become most favourable for ozone destruction.
The key factor is the presence of stratospheric clouds and the lack of atmospheric mixing between the south polar latitudes and air from elsewhere during the austral winter and early spring. Normally there are no clouds in the stratosphere because there is so little water vapour present. As long as it remains dark, nothing happens; but when spring arrives, UV radiation from the Sun reaches the Antarctic Circle and starts the process of chlorine release and ozone destruction.
This continues until the stratospheric clouds disappear due to warming of the south polar atmosphere as summer approaches. By summertime, stratospheric air from lower latitudes is able to penetrate the polar latitudes, and thereby replenish the ozone layer above Antarctica. Hence, there is a seasonal cycle to the ozone hole over Antarctica with the lowest ozone levels recorded in late September and early October. The ozone layer protects life from harmful UV-B radiation which can cause cancer and stunt the growth of plants.
As UV radiation can penetrate into the surface of the ocean, marine organisms especially phytoplankton can also be damaged. If there was no ozone layer at all, photosynthesis by plants would be impaired and ecosystems could not function as they do today — so it is clearly in our interest to make sure we do not damage the ozone layer.
In an historic international agreement was signed the Montreal Protocol which came into force in and set deadlines for reducing and eliminating the production and use of ozone depleting substances.
It also promotes research and development into finding ozone safe substitute chemicals for the uses to which CFCs, etc. It has since been ratified by countries, has been revised several times, and has been described as one of the most successful international treaties.
Through its various mechanisms, the treaty has brought down worldwide emissions of CFCs and other ozone depleting chemicals sharply. However, due to the long residence time of many of these gases in the atmosphere for example CFC resides in the atmosphere for approximately years , the ozone layer will not fully recover until around
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