Ozone layer depletion

The ozone layer is a thin layer of oxygen-related gases in the part of the atmosphere known as the stratosphere, about 25 kilometres above the earth. Absorption of solar ultraviolet radiation by stratospheric ozone to a large extent determines the temperature, structure and dynamic processes in the stratosphere. Stratospheric ozone also plays an important ecological role since it filters out most biologically harmful ultraviolet radiation.

The stratospheric ozone layer is threatened by the emissions of fluorocarbons and other halocarbons used as aerosol propellants, blowing agents in foam production, solvents and refrigerants; and by nitrous oxide emissions from both organic and inorganic nitrogen fertilizers. Collectively these are known as ozone depleting substances (ODSs); the principal ODSs are chlorofluorocarbons (CFCs), other halons (compounds of chlorine, bromine and fluorine), carbon tetrachloride, methyl chloroform and nitrous oxide. Is is believe that volcanic eruptions may also result in ozone depletion.

It has been estimated that a reduction of 1 percent in total ozone produces a 2-4 percent increase in biological effects; and that a 1 percent decrease in the ozone layer could result in a 4 to 6 percent increase in certain kinds of skin cancer, and contribute to eye damage, skin infections and reduced immunity to disease. For example, a 10% decrease in total stratospheric ozone is predicted to result in between 1.6 and 1.75 million additional cases of cataract per year worldwide. Potential effects on food crops and fish may well prove to be more significant a problem.

Changes in the ozone layer can also change the climate and the circulation of the atmosphere. Ozone depletion and increasing aerosol concentrations in the lower stratosphere and troposphere have a cooling effect, which may be partially offsetting, and hence masking, the full extent of the enhanced greenhouse effect.

The Antarctic ozone hole was discovered in 1985 by Joe Farman, a scientist working with the British Antarctic Survey.

The stratosphere is the upper layer of the atmosphere, between approximately 15-50 kilometres above the earth's surface. It is underlain by the troposphere. The tropopause is the zone of transition.

Ozone is a gas with molecules composed of three oxygen atom, consequently its chemical designation is O₃. Ozone is removed from the atmosphere naturally. Microorganisms in the soil produce a gas called nitrous oxide (N₂O). In the stratosphere, N₂O is broken down into the nitrogen oxides NO and NO₂. These react with ozone but are not themselves consumed. Human enhanced production of nitrogen oxides has resulted in an accelerated reduction of ozone content. In addition synthetic chemicals, such as chlorofluorocarbons (CFCs), also cause depletion of the sensitive ozone layer.

Current and future changes in the atmospheric ozone profile will have significant effects on the vertical temperature profile of the atmosphere, and hence on atmospheric circulation as well as surface temperatures. The strongest radiative effects of ozone changes are in the region of the tropopause. Decreases in ozone concentrations about 30 km cause surface warming because of increased penetration of solar radiation into the lower atmosphere while decreased ozone in the lower stratosphere and/or the troposphere causes surface cooling.

The principal reason for the greater stratospheric ozone depletion in the southern hemisphere is that there are more frequent meridional exchanges of air in the north that do not allow temperatures in the lower stratosphere to fall as low as in the south. Persistently low temperatures cause stratospheric clouds to form, with dehydration and denitrification that together favour ozone destruction. The concentration of chlorine seems to be similar over both polar regions. There are not enough systematic ozone observations from the tropical belt, but 12 years' satellite observations suggest insignificant changes there.

Model studies imply that dry deposition of ozone in summer may also be a significant sink, while downward fluxes from the stratosphere may be an important source of ozone in the upper troposphere in winter and early spring. Exhaust emissions from high flying aircraft, a significant portion of which takes place within the lower stratosphere, may contribute substantially to upper troposphere ozone chemistry in mid to high latitudes. However atmospheric measurements in several regions suggest there must be significant sources of tropospheric nitrogen oxides (NOx) other than fossil fuel combustion and downward stratospheric flux.

Recent unexpected developments in concentrations of ozone and its precursors within the troposphere suggest that much of the chemistry is not yet understood. Trends towards major increases in tropospheric nitrogen oxide (NOx) concentrations over eastern North America, Europe and China and Japan suddenly ceased around 1994, while carbon monoxide concentrations have declined and methane and tropospheric ozone concentrations appear to have stabilized. Rising OH concentrations caused by increased UVb radiation penetration into the troposphere may be an important contributing factor, as may the stalled East European economy and 1991 eruption of Mount Pinatubo. Global warming can also be expected to further alter related tropospheric chemistry.

In 1988, scientists believed that the amount of ozone surrounding earth decreased by 5 to 6 percent between 1979 and 1986, notably over the poles and tropics (observed ozone cover varies in other areas according to the season and weather systems). Fairly stable "holes" of thinning and localized reductions of well over 10 percent ozone have been present over both poles for several years. It was then estimated that over the previous decade, an average decline of 3 percent had occurred at the populated mid-latitudes in the Southern Hemisphere, possibly accompanying the appearance of the Antarctic ozone hole. Although there is more meteorological variability in the mid-latitudes of the Northern Hemisphere, there were indications in 1988 that a smaller decline had also occurred there. It was warned that such depletions could become much larger if the halocarbon release rates were to continue, and may anyway due to latent effects already initiated.

In 1993, it was reported that ozone values ranging from 9 to 20 percent below normal were found above the middle and higher latitudes of the northern hemisphere during the 1992-93 winter. It was the second winter in succession that such an extreme attenuation of the northern ozone layer had been observed and set new minimum limits for ozone depletion in the northern hemisphere. When the past two winter seasons were taken into account, ozone has been cumulatively reduced by more than 14 percent between 1969/70 and 1993 over continental parts of the northern middle latitudes. The overall decline has been steady and is likely to continue until around 2005 before the situation starts to improve. It has been estimated that it may take another 50-70 years before the ozone layer returns to 1979 levels.

Above Europe, the amount of ozone in the stratosphere declined by 5% between 1975 and 1995, allowing more ultraviolet B radiation to enter the lower atmosphere and reach the Earth's surface. The total ozone over the North Pole fell to 40% below the normal level in March 1997.