Polar stratospheric clouds (PSCs) play a key role in stratospheric ozone depletion in the polar stratosphere during winter and spring seasons. Surface- catalysed reactions on PSC particles generate chlorine compounds that photo- lyze readily to yield chlorine radicals, which in turn destroy ozone very efficiently.
The most prevalent PSCs form at temperatures several degrees above the ice frost point and probably consist of HN03 hydrates. Result indicate that the background stratospheric H2SO/H2O aerosols provide an essential link in the mechanism involved (Molina et al., 1993).
These liquid aerosols absorb significant amounts of HNO, vapor, leading most likely to the crystallization of nitric acid trihydrate (NAT). The frozen particles then grow to form PSCs by condensation of additional amounts of HNO3 and H2O vapor.
The chlorine radical precursors are formed readily at polar stratospheric temperatures not just on NAT and ice crystals, but also on liquid H2SO4 solutions and on solid H2SO4 hydrates. These results imply that the chlorine activation efficiency of the aerosol particles increases rapidly as the temperature approaches the ice frost point regardless of the phase or composition of the particles (Molina et al., 1993).
Research also shows that since 1979 ozone has declined by some five per cent over Antarctica throughout the year (as opposed to merely in springtime, when the hole appears). Indeed, there is now evidence that the ozone layer is thinning all over the world. It seems that since 1970 the protective ozone layer has already shrunk by about four per cent in winter and one per cent in summer over the northern hemisphere from 64 to 30 degrees.
If emissions of CFCs and halons continue to grow as in the past, the ozone layer will be depleted by about 20 per cent within the lifetimes of today’s children. Even one half of this loss in die protective shield would cause one and a half million extra deaths from skin cancer and five million extra cataracts in the United States alone (UNEP, 1989).
The CFCs and halons also cause much damage through their contribution to the Greenhouse Effect. Every molecule of CFCs 11 and 12 is more than 10,000 times as effective at warming up the earth than a molecule of carbon dioxide. They are responsible for about 20 per cent of the warming of the climate, and their contribution is rising.
Chlorofluorocarbons are industrially useful compounds that are extremely stable, inert and long-lived near the ground level where they do not harm anything, including ozone. However, some of them leak and diffuse slowly upward, reaching the stratosphere where in the presence of intense sunlight and strong ultraviolet radiation, they become reactive and their chlorine can become catalytic, converting ozone to oxygen by knocking out one atom of oxygen from the ozone molecule, thereby destroying ozone. One molecule of reactive chlorine can destroy thousands of molecules of ozone in the stratosphere.
It is instructive to compare the role of CFCs and ozone in the troposphere and stratosphere. CFCs are useful and valuable near ground level and have been responsible for much of the industrial progress in the developed world. It is only when they drift into the stratosphere that they start doing mischief by destroying ozone.
In contrast, ozone is a poison near the ground but becomes a blessing and extremely useful away from the ground. An ideal situation (wishful thinking!) would be for all the ozone in the troposphere to move up to the stratosphere whereas CFCs should remain confined within the bounds of the troposphere.
In addition, the destruction of ozone can itself upset the earth’s climate. The layer is affected differently at different heights. Generally speaking, it is likely to get thinner above about 25 km, with particularly large damage at about 40 km up. But levels of ozone may actually grow between about 20 km and ground level. The strata of the ozone layer will be grossly distorted, and this is sure to have strong effects on the world’s climate (UNEP, 1989).
In the early 1979s, nobody suspected any threat to the ozone layer from CFCs. Instead, the concern was on the damage that might be done to the ozone layer by supersonic aircraft, space shuttle flights and the nitrous oxide from nitrogenous fertilisers. By the mid-1970s all these turned out largely to be false alarms.
But in the autumn of 1973 two scientists at the University of California at Berkeley began investigating the role of CFCs. Professor Sherwood Rowland and his young assistant Mario Molina, realising that all the long-lived CFCs ever released still remained in the atmosphere, decided to find out what happened to them. They soon worked out that the gases would drift unchanged up to the stratosphere and could have a catastrophic effect on ozone. Their view was at that time generally not accepted by the industry and many others, but it did start a debate and controversy.
During the next few years, it became generally accepted that a potential problem did exist, and some countries soon took action to control CFCs.
In 1977-78, U.S.A., Canada, Norway and Sweden started phasing out or banning some uses of CFCs (e.g. in some aerosol cans). The emissions of CFCs 11 and 12 fell for a few years but began increasing again at the start of the 1980s as non-aerosol uses, such as foam blowing and refrigeration, increased. The use of CFCs in aerosols fell after the mid-1970s as a result of bans and other regulatory action in several countries. But other uses of the chemicals rose to compensate. By 1984 emissions were back to their 1977 levels and increasing by five per cent a year.
In 1980 the UNEP could produce convincing assessments of potential depletion caused by CFCs. These showed that there was, indeed, a serious threat to human health and the well-being of the planet.
As it happened, a scientific paper that would transform both scientific and political perceptions of the problem was awaiting publication while the delegates were meeting. In October 1982 colleagues from the British Antarctic Survey, taking measurements from Halley Bay, found that much of the ozone over-head had disappeared.
This was incredible because no one had ever predicted ozone depletion on this scale. This finding was substantiated by examining the data and measurements recorded by an American weather satellite, Nimbus 7. Thus the existence and extent of the ozone hole had been established by 1982-83. However, even then, many scientists were not sine about the cause of the ozone hole. As a precautionary measure, some countries agreed to freeze the production of certain CFCs to the levels being produced at that time.