Is there a new threat to the ozone layer?

After the alarm was raised in the 1980s, the ozone layer, which up to then had been destroyed by industrial activity, began to improve slightly. However, on the initiative of Emmanuel Mahieu, a researcher at the University of Liege, an international team observed that between 2007 and 2011, the trend of chlorine – the main destructive element affecting the ozone layer – had reversed, showing an increase in the Northern Hemisphere  (1). This new imbalance was temporary. Moreover, it was not due to human activity but to an unexpected variation in atmospheric circulation which the researchers were able to model. The observation proves that the recovery of the ozone layer does not follow a neat rectilinear curve in sync with our environmental policies which seem to be working overall. Other factors come into consideration. The study has attracted the interest of the scientific community and has been published in the journal Nature.  

You don’t need to be a well-informed environmental activist to have heard about the ozone layer and its disintegration due to certain gases produced by industrial and domestic activity over the last sixty years. Chlorine, bromine and nitrogen are all identified as causes. For more than thirty years, the problem has been center stage because the ozone layer protects us from highly destructive UV rays and its disappearance from the atmosphere would have had catastrophic repercussions. There is no need for panic, however, because for the last twenty years there has been a global reduction in these destructive gases. Even better news, is the fact that the most recent reports conclude that in 2050, the concentration of ozone should have reached its pre-1980s levels.

However, Emmanuel Mahieu, a researcher in the Atmospheric and Solar Physics Infrared Group (GIRPAS) of the University of Liege, has observed a strange phenomenon at the Jungfraujoch, station in Switzerland. Although chlorine compounds which are destructive to the ozone layer have been progressively disappearing from the troposphere, the quantity of chlorine has increased in the stratosphere between 2007 and 2011, in the Northern hemisphere. This unexpected phenomenon was not linked to human activity but to atmospheric circulation. This interesting study underlines the success of the policies put in place at the end of the 1980s but it is also worth making a slight detour in order to better understand the chemical properties that influence the composition of the ozone layer.

The formation of the ozone layer

Ozone is a molecule that is made up of three atoms of oxygen (O₃). It is a relatively dynamic gas which is constantly being formed and destroyed. In tune with a natural cycle, it maintains a constant protective layer around our planet. It is formed in great masses over the tropics thanks to the effect of UV rays. This is a chemical process called photolysis.

The energy in UV rays breaks up oxygen molecules (O₂) which are naturally present in the atmosphere. Once the oxygen atoms are liberated they reassemble again either as oxygen, which doesn’t change anything, or as ozone. If they remained in the region of the tropics, these molecules would also be destroyed and reconstructed in a never-ending cycle. This is because the energy that makes it possible to separate the oxygen atoms also makes it possible to separate the ozone atoms. “But the dynamics of the atmosphere cause the molecules to migrate to the poles and the ozone is stored during the polar nights which last for several months every year. There is no light to destroy the ozone so it accumulates becomes concentrated and the poles act as actual reservoirs. When light returns in the spring, the dynamics of the atmosphere start up again and the ozone is redistributed across the two hemispheres”, explains Emmanuel Mahieu.

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”.

The turning point of 1987

For more than ten years, research on the massive destruction of the ozone layer increased dramatically and alarm bells began to ring. Not only does chlorine destroy our best protection against the sun, but it is mainly of human origin. More precisely, it is a by-product of industry. In 1987, experts and the public authorities of several nations gathered in Quebec and signed the Montreal Protocol. The treaty put in place a schedule for the elimination of CFC gases, and more broadly, industrial products made up of chlorine and bromine gases whose catalytic cycles destroy the ozone layer. The ambition is to re-establish a maximum concentration of ozone.  But the first measures were not sufficient. Up to 2007, the protocol was amended or adjusted several times. Other products were added to the list of prohibited substances.

“Mankind has turned off the tap”, says Emmanuel Mahieu by way of illustration. “As long as we no longer allow the emission of these gases, it is quite logical to expect that there will be a reduction in the presence of HCl in the stratosphere”. And this is exactly what has happened. Among the obligations imposed by the protocol, an assessment of the situation must be made every four years(2). A complete report aimed in equal measure at scientists, decision-makers and a curious public. The international scientific community must task itself therefore with the development of the ozone layer, and has recorded a decrease in HCl of approximately 1% per year since 1995. This may not seem like a lot, but because CFC gases are very stable, they can remain in the atmosphere for several decades and continue to create hydrogen chloride for many years before disappearing completely.

The unexpected increase in HCl in the Northern Hemisphere

It is in this general context that, against all expectations, Emmanuel Mahieu has observed a new increase in HCl in the Northern Hemisphere. “This was quite contradictory”, the chemist explains. “While there was a decrease in the emission of the underlying causes of HCl in the atmosphere, its concentration increased in a particular place. By comparing this data with other data from other observatories, we were able to apply this trend to the Northern Hemisphere alone. In the south, no site had recorded this phenomenon”. The first reaction of the researcher was to contact a network of observers around the globe to see if they had detected an increase in the sources of this chlorine emission. “This was my first theory. Since the Montreal Protocol, all countries have to account for the chlorine gas they are still using.  But it is possible to imagine that there are rogue emissions that the authorities are unaware of, the clandestine use of an unexhausted stock of the gases or even the processing of new compound. We immediately thought of the polluting countries of the Northern Hemisphere”. But the results were unequivocal. Nobody was able to account for an increase in the sources of chlorine gases. Emmanuel Mahieu then concentrated on the second theory. The phenomenon must be linked to a variation in atmospheric circulation.

A synergy between several observers

To verify his hypothesis, Emmanuel Mahieu called upon a network of researchers who enthusiastically joined the research programme. “We needed to verify this data with other instruments. After comparing them with the results from the network of sites on the ground, we wanted to use space instruments. By chance, three satellite missions were responsible for measuring these parameters. These were; HALOE which stands for HALogen Occulation Experiment, MLS for Microwave Limb Sounder and ACE for Atmospheric Chemistry Experiment. The first two are American and the third is Canadian, but with a Belgian involvment.  The data from the three instruments had just been calibrated and compared at the Jet Propulsion Laboratory of the California Institute of Technology. I contacted them and asked them if they wanted to join us. They accepted and sent us their data. The data confirmed the trends we had observed from the ground with the same level of precision. There was therefore no instrument-based bias”.

The final stage to be completed was modelling. Once again, Emmanuel Mahieu knew whom he could depend on and he contacted Martyn Chipperfield at the University of Leeds and Thomas Reddmann of the Karlsruhe Institute of Technology in Germany. Both developed a 3D model representing the circulation and behavior of the atmosphere. These models take account of the gases, the interactions between the component parts of the air and solar radiation; they include meteorological data, wind temperatures and atmospheric pressure etc. Both at Leeds and at Karlsruhe, by taking account of the observed scenarios, the researchers were able to simultaneously reproduce the increase in HCl in the stratosphere and its decrease in the troposphere. 

Other meteorological conditions included in the models

In order to better understand this phenomenon and to try to isolate the causes, Martyn Chipperfield had the idea of carrying out a particular experiment. Now that he had modelled all the factors leading to the phenomenon observed by Emmanuel Mahieu, he could explain the different ways, to observe what could change and therefore determine the causes of the two climatic phenomena.  He therefore modelled the developments of HCl for each year between 2000 and 2011 by imposing on each of the years in question the meteorological conditions of the year 2000.  This initiative should make it possible to see if under other atmospheric conditions the level of HCl continued to drop as opposed to increasing in 2007. “And indeed, by doing this, we found that there was no longer an increase in HCl, but rather a monotonous decrease similar to the one we had expected. By isolating the dynamic factor, Martyn Chipperfield showed that it was indeed due to a change in atmospheric circulation that was responsible for the increase in hydrogen chloride”. This conclusion made it possible to confirm that the Montreal Protocol was a success. Not only that, in the Southern Hemisphere, HCl continued to decrease while in the Northern hemisphere it was to be expected that the recent increase would be cancelled out. On a global scale, HCl would continue to show a decrease in the long-term.

What has changed in the atmospheric circulation?

It was not enough to identify the variations in atmospheric circulation as being responsible for the phenomenon. It still remained to pin down what exactly had happened. By comparing the state of the atmosphere between 2007 and 2011, they observed that the zones characterized by an increase in HCl showed a slowing down of the circulation of air and therefore an increase in the transportation times of compounds. There was therefore a direct correlation between a longer transportation time and a better conversion of source gases into HCl. The reason for this was very simple. As there was a slowing down, the CFC gases stayed at higher altitudes for a longer time. They were therefore subjected to UV rays for a longer period. A greater number of CFC molecules could be separated. A greater quantity of hydrogen chloride was produced before descending to the lower layers of the atmosphere. This is why, without a greater volume of emission of chlorine gases from the surface, the stratosphere suddenly underwent a new invasion of this destructive atom.

“We stopped there, ends the researcher. We could have gone further and tried to understand why this slowing of atmospheric circulation occurred. This is also a mystery. With regard to global warming, all the specialists tend to think that, conversely, the atmospheric circulation and air transportations of the compounds should accelerate. We therefore observed the opposite phenomenon to what was expected. Each expert should concentrate on his area of expertise. We are interested in the chemistry of the atmosphere and not its dynamics. But our research was sufficient to understand the origin of an increase of chlorine in the atmosphere in a given place while it should have gradually disappeared. Our research also made it possible to highlight a dynamics origin that was both unexpected and particular, but temporary. This was reassuring because one of our initial fears was that the phenomenon might be only the beginning, and might then spread to the rest of the globe after transportation and this homogenization of the atmosphere. But this is not on the agenda. Overall, the concentration of chlorine continues to drop”.

(1) Mahieu, E. et al., Recent Northern Hemisphere stratospheric HCl increase due to atmospheric circulation changes, Nature, 6 novembre 2014, doi :10.1038/nature13857 .

(2) Scientific assessment of ozone depletion, the last dates back to 2010, and the 2014 assessment is still under embargo.