Is there a new threat to the ozone layer?
11/5/14

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.

Ozone formation

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.

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

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