Chlorofluorocarbons
CFCs are compounds made up of chlorine, fluorine and carbon. The most common CFCs are CFC-11, CFC-12, CFC-13, CFC-14 and CFC-15. They are colourless gaseous molecules, odourless or with a faint ether-like odour, non-flammable, chemically stable and without any toxic action. These gases were invented in the 1920s; since then they have been produced (by chemical synthesis starting from methane) and used in large quantities as: refrigerants for refrigeration systems and air conditioners, propellants for aerosol cans, foaming agents and solvents.
CFCs were once considered ideal substances for industrial uses because they were cheap, stable and inert, and therefore non-toxic; but it is precisely this lack of reactivity that makes them dangerous for stratospheric ozone.
Since 31 December 2000 the use of CFCs, including in the maintenance and recharging of existing refrigeration and air-conditioning systems, has been prohibited.
In the upper atmosphere, in fact, certain photolysis reactions release chlorine atoms which, through a catalytic chain reaction, destroy ozone (O3) by converting it into oxygen (O2). The atmospheric lifetime of these gases is in the order of hundreds of years, so their greenhouse effect is extremely dangerous and very long-lasting.
Although their concentration in the atmosphere is 0.001 ppm, their contribution to the greenhouse effect is very high. It is estimated that in tropical regions they are directly responsible for one fifth of the man-made greenhouse effect (0.25 W/m2). The World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) launched the debate on the thinning of the atmospheric ozone layer in the 1970s. The ozone hole, as it has commonly been called, reduces the ability of ozone to shield radiation that is harmful to our skin, with disastrous effects on the climate. With the Montreal agreement (1987) first, and then those of London (1991) and Copenhagen (1992), most industrialised countries took measures to eliminate the production of chlorofluorocarbons (CFCs) contained in spray cans and refrigerators. Fortunately, the measures taken have proved effective, as the ozone layer has thickened considerably and it is estimated that within a few years the situation will return to normal.
Previous studies used statistical analyses of changes in the size of the ozone hole to argue that ozone depletion was decreasing. This study is the first to use measurements of the chemical composition inside the ozone hole to confirm not only that ozone depletion is decreasing, but that the decrease is caused by the decline in CFCs.