A balance disturbed by industrial gases

This balance was disturbed by mankind in the 1950s. The main culprits were chlorofluorocarbons (CFC). In the 1950s, this family of molecules synthesized by the chemical industry was seen as a miracle product. Cheap to produce, non-toxic for humans and particularly stable, it was extensively used in aerosol cans, air-conditioning systems, insulation, refrigerant liquids, dry cleaning etc. The production of CFC gases increased dramatically. But these compounds are too stable. “They do not break down and in the 1970s it was discovered that they are to be found everywhere in the atmosphere and in the oceans. Mario Molina and Frank Rowland imagine that the CFC molecules are transported into the stratosphere. Here under the energizing effect of UV rays, the CFC molecules still break down by means of photolysis. The chlorine atoms are released. By rebinding with hydrogen atoms they form hydrogen chlorine, HCl, which is redistributed throughout the stratosphere”, the researcher points out.

Certainly, the observers of the time detected a high concentration of HCl in the atmosphere but its origin could have been partly natural.  The impact of human activity and the causal relation with CFC still needed to be proved. This was an easy task! When broken down under UV rays, CFC does not only release chlorine, it also releases fluorine which when linked with hydrogen forms hydrofluoric acid. The origin of this gas cannot be natural, at least not in large quantities in any case.  Its presence in the atmosphere points to a common origin with hydrogen chlorine; it is undeniably of human origin, and directly points an incriminating finger at CFC gases. “In 1974, Rodolphe Zander, who supervised my thesis, oversaw a mission to send a balloon into the stratosphere in order to study its components. The balloon detected the presence of hydrofluoric acid. It was of human origin and therefore so was the HCl”.

But why is HCl dangerous for the ozone layer? It is dangerous because it contains chlorine and the catalytic cycle of this element destroys ozone. But in order to do this it first has to separate from the hydrogen atom because when it descends to the poles, chlorine is still trapped inside a stable molecule. “HCl is a sort of reservoir”, explains Emmanuel Mahieu. “It does not in itself destroy ozone, but it is transported throughout the atmosphere. At the poles, when it is very cold, stratospheric clouds of ice particles form at an altitude of fifteen kilometers and this activates the conversion of HCl. Also, when the light returns in the spring, the photons destroy the molecule. The chlorine atom is released and it breaks up the ozone molecules to bond with oxygen atoms to form chlorine monoxide CIO. It may be thought that this is not serious because chlorine is more abundant than ozone, but the problem is that this is a cyclical mechanism, where chlorine is continually separated from oxygen to decompose a new ozone molecule. This cycle continues in a loop. At the poles where chlorine is released, a small concentration of chlorine can destroy all the ozone”.