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