Since the discovery of the ozone hole over Antarctica, after nearly ten years of research, scientists have finally come to a consistent conclusion: the destruction of the ozone layer and the emergence of the ozone hole are caused by human behavior, that is, people produce and use a large number of "ozone depleting substances (ODS)" in production and life, and emit a large amount of waste gas into the air.
ODS mainly includes the following substances: CFCs, halons, carbon tetrachloride, methyl chloroform, methyl bromide and so on.
Uses of ODS: used as refrigerant, spray, foaming agent, cleaning agent, etc.
Exhaust gas: mainly automobile exhaust gas, supersonic aircraft exhaust gas, industrial exhaust gas, etc.
Of all the above substances, chlorofluorocarbons and halons are the most destructive (or "culprits"). Chlorofluorocarbons, which we are all familiar with, are used most in our lives.
Chlorofluorocarbons are part of freon. It is a chlorofluorocarbon refrigerant developed and produced by DuPont in the 1930s, and its brand is "Freon". Now people are used to calling refrigerants "Freon".
According to some data, from the early 1930s to the 1950s and 1960s, human beings produced150,000 tons of chlorofluorocarbons.
The development of chlorofluorocarbons has improved people's lives, but it has brought huge environmental problems-the destruction of the ozone layer.
Mechanism of ozone layer destruction
(1), waste gas destroys the ozone layer.
Exhaust gas contains a large number of nitrogen oxides (such as NO and NO2), which can destroy a large number of ozone molecules, thus causing the destruction of the ozone layer.
(2) Destruction of ozone layer by chlorofluorocarbons and halons
Maurines and Rowland, American scientists, proposed that some synthetic chlorine-containing and bromine-containing substances are the culprit of ozone layer destruction, the most typical ones are chlorofluorocarbons (CFCs) and bromine-containing compound halons.
Chlorofluorocarbons and halons always leak in the process of production and use, and first enter the troposphere of the atmosphere after leakage. These substances are chemically inert in the troposphere, which means that they are very stable in the troposphere and can exist for decades or even hundreds of years. But these substances can't always exist in the troposphere, and they will eventually enter the stratosphere through the polar atmospheric circulation and the hot air flow in the equatorial belt. Then, under the action of wind, it is transported from low latitude to high latitude and mixed evenly in the stratosphere. In the stratosphere, intense solar ultraviolet radiation decomposes CFC and halon molecules, releasing highly active chlorine and bromine free radicals. Chlorine free radicals and bromine free radicals are the main substances that destroy the ozone layer, and their chemical mechanisms of destroying ozone are as follows:
R-Cl→R + Cl
Cl +O3→Cl0 + O2
C 10 +O3→Cl + 2O2
Bromine atom radicals also destroy ozone in the same process. It is estimated that a chlorine atom radical can destroy 104- 105 ozone molecule before inactivation, and the destructive power of bromine atom radical released by halon is 30-60 times that of chlorine atom. Moreover, there is a synergistic effect between chlorine and bromine, that is, when they exist at the same time, the ability to destroy ozone is greater than their simple addition.
Of course, in addition to the above chemical processes, the formation of ozone holes is also a heterogeneous catalytic reaction process involving aerodynamic processes and special temperature changes in polar regions, which is why ozone holes appear at the poles and mostly in spring.
Long-term destruction of the ozone layer
What worries scientists and all walks of life is that the atmospheric lifetime of chlorofluorocarbons and halons is long, and it is difficult to remove them once they enter the atmosphere, which means that even if human beings stop producing and using these substances, their damage to the ozone layer will last for a long time. However, through the efforts of all mankind, the destruction of the ozone layer will become smaller and smaller, and will eventually return to its original state.