The ozone layer plays an important role in our climate system, and it protects the biosphere from damaging ultraviolet radiation. An international team of researchers recently published data that reveal an increase of hydrogen chloride (HCl) in the stratosphere, the atmospheric layer at an altitude between 5 and 15 thousand feet. This substance plays an important role in depletion of the ozone layer. According to the Montreal Protocol, an international agreement on reducing the production of substances that deplete the ozone layer reached in 1989, no such increase should have been allowed to happen. The countries that signed up to the environmental agreement entered into a commitment to reduce emissions of the chlorine and bromine chemicals responsible for destroying stratospheric ozone, and eventually to ban them altogether. Researchers believe a transient – but nevertheless ongoing – abnormality in atmospheric circulation may be responsible for the increase. An environmental physicist at the University of Bremen, Professor Justus Notholt, coauthored the recent study containing these surprising results.

Background

Solar rays break down the hydrochlorofluoralcarbons (CFC) present in the stratosphere into chlorine atoms. These then form hydrogen chloride (HCl), the main reservoir of chlorine in the stratosphere. The chemical processes that take place during the polar winter lead to chlorine atoms being released from this reservoir. When the polar spring arrives, these atoms subsequently deplete the ozone. Whether the stratospheric ozone layer can remain intact depends on whether countries respect the Montreal Protocol, which essentially bans the production of CFCs and similar compounds. These substances are responsible for the depletion of the ozone layer observed over the past 30 years or so. It is generally recognized that over the past decade the Montreal Protocol has been successful in reducing the atmospheric pollution caused by CFCs, and scientists are optimistic that the stratospheric ozone layer will have completely recovered by the second half of this century.

Results of the study

In order to judge whether the Montreal Protocol is really curbing the production of substances that deplete ozone it is necessary to monitor the long-term developments taking place in the stratospheric CFC layer. The latest increase in HCl concentrations was only observed in the northern hemisphere. In the stratosphere of the southern hemisphere the concentration is being reduced in accordance with that permitted by the Montreal Protocol. Researchers have now discovered that the increase in HCl is connected with an abnormality in atmospheric circulation, which has slowed down the ascent of air masses in the stratosphere of the northern hemisphere. This has subsequently enabled the sun to release more chlorine from the CFCs, raising the concentration of HCl. The results are based on data partially systematically collected over decades via a network with weather stations in Spitsbergen, Greenland, Sweden, Switzerland, Japan, Tenerife, Australia and New Zealand. These ground-based measurements are verified by satellite observations and model simulations.

How effective is the Montreal Protocol?

By showing that the increase in HCl is attributed to a change in atmospheric circulation rather than an increase in the presence of new substances that lead to the deletion of ozone, the study proves that the Montreal Protocol continues to be effective. However, the predicted long-term reduction of ozone depleting substances in the atmosphere is turning out to be more complicated than previously thought. In the words of Professor Notholt: “Our observations don’t call the Montreal Protocol into question. Rather, they reveal that atmospheric variability, and maybe climate change too, can also have an effect on whether the ozone layer can fully recover. This is not going to be a uniform process, but rather an up and down one. Reducing the presence of ozone depleting chemicals in the atmosphere is going to take a very long time, during which the stratospheric ozone layer will continue to remain at risk.”

Publication in the science journal “Nature”

These spectacular research results have been published in the science journal “Nature”, a weekly English-language journal. This highly respected journal is the most cited interdisciplinary science journal.

If you would like to have more information on this topic, please contact:
University of Bremen
Institute for Environmental Physics
Prof.Dr. Justus Notholt
Phone: +49 421-218-62190
Cell: +49 176-30346533
e-mail: jnotholtprotect me ?!iup.physik.uni-bremenprotect me ?!.de