Arctic Ozone

Ozone Depleting Substances and Their Impact on the Arctic Ozone Layer

Depletion of the ozone layer is occurring because human activities have introduced excessive amounts of chlorine, bromine, and other ozone-destroying chemicals into the stratosphere. Chlorine is the predominant ozone-destroying substance in the stratosphere,and the surplus comes mostly from various types of chlorofluorocarbons (CFCs). These are highly versatile chemicals that have been widely used as refrigerants and spray propellants as well as in an extensive variety of industrial applications. CFCs and other ozone-destroying substances (such as halons, carbon tetrachloride, methyl chloroform, and methyl bromide) are stable compounds, and most of them can survive for many years in the atmosphere before they eventually reach the stratosphere. Once in the stratosphere, however, these compounds gradually rise above the ozone layer where they are broken down by the intense ultraviolet radiation of the upper stratosphere and their chlorine or bromine is released. As a result of the release of these chemicals, the concentration of chlorine in the stratosphere is now about four times the natural level.

Chlorine and bromine are powerful ozone destroyers because they act catalytically. That is, they take part in reactions that destroy ozone, but they are not themselves consumed in the process and are therefore free to take part in these reactions again. Consequently, a single molecule of chlorine or bromine can destroy thousands of ozone molecules before it returns to the troposphere and is removed by other chemical reactions.

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Over polar regions, two additional factors make ozone destruction brutally efficient for several weeks in the spring. The first of these is the polar vortex, a nearly closed circulation system that develops over the poles with the onset of winter. Without sunlight and without warmer air flowing in from lower latitudes, the polar stratosphere becomes extremely cold, with temperatures falling to -80°C or lower. At these temperatures, the second factor comes into play - the formation of polar stratospheric clouds (PSCs), made up of ice, nitric acid, and sulphuric acid. In the absence of PSCs, most of the chlorine and bromine in the stratosphere is locked up in compounds that under ordinary conditions would be quite stable and therefore harmless to the ozone layer. PSCs cause these compounds to break down, however, leaving the chlorine and bromine atoms in less stable compounds. When sunlight returns in the spring, these compounds are broken apart by solar radiation, and chlorine and bromine are released. Ordinarily, the reactions that destroy ozone require relatively strong sunlight. However, with the cold temperatures and the especially high concentrations of chlorine that are present in the vicinity of PSCs, an entirely different set of reactions takes place that is actually far more effective in destroying ozone. With the resupply of fresh ozone from lower latitudes blocked by the vortex, ozone amounts drop rapidly and deeply as these reactions proceed (Figure 3). In some layers of the stratosphere, the ozone may be almost completely destroyed.

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Over the Antarctic, these processes commonly lead to the formation of a massive ozone hole (defined as an area in which total ozone amounts are less than 220 DU). These do not fill in until the winter vortex dissipates (allowing the return of ozone-rich air from the tropics) and warmer temperatures prevent the formation of PSCs. Over the Arctic, the same processes intensify depletion but, at least so far, they have not done so to the same degree as in the Antarctic. Why the difference? A major factor is that the greater variability of the atmospheric circulation in the Northern Hemisphere makes the Arctic vortex much less stable than its southern counterpart. As a result, it is frequently penetrated by stratospheric winds bringing ozone and warmer air from the south. Because of these events, known as sudden stratospheric warmings, the Arctic stratosphere is often too warm for PSC formation (Figure 4).

Consequently, PSCs do not form as often in the Arctic nor do they last as long as they do over the South Pole. For that reason, there have been no massive ozone holes over the arctic. Nevertheless, any PSC formation can greatly accelerate the pace of depletion.

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