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Ozone destroying substances on the rise - but Montreal Protocol on track

Posted on 5 November 2014

An international team of scientists, including a researcher from the University of York, has reported a recent increase in atmospheric hydrogen chloride (HCI), an ozone destroying substance.

ACE satellite in orbit (credit T. Doherty, Bristol Aerospace)

The findings, reported today in the journal Nature, are based on measurements by a network with stations in Spitsbergen, Greenland, Sweden, Switzerland, Japan, Tenerife, Australia and New Zealand. These are backed up by satellite observations and model simulations. The increase in HCl concentrations was only observed in the Northern Hemisphere, while in the Southern Hemisphere HCl continues to decrease.

The team, which included Peter Bernath, an Emeritus Professor with the University of York’s Department of Chemistry, found that the increase of HCl levels is related to a temporary but prolonged anomaly in atmospheric circulation, which is changing the balance between chlorofluorocarbons (CFCs) and their breakdown product HCl.

The research was led by the University of Liège, Belgium, and involved scientists from the United States, Japan, Germany, Australia, New Zealand and Canada, as well as the Universities of York and Leeds.

The study demonstrates that this unexpected increase has occurred in the Northern hemisphere since 2007 and that the circulation change has led to a temporary reversal in the decline of HCl which would be expected under the Montreal Protocol.

The Montreal Protocol is an international treaty for the protection of the ozone layer which banned the production of major ozone-depleting substances worldwide. By explaining the HCl increase through a change in circulation and refuting the possibility that rogue emissions of ozone-depleting substances are at play, the study confirms that the Montreal Protocol remains a success and that the ozone layer will likely fully recover during the second half of this century.

Professor Bernath provided data from the Atmospheric Chemistry Experiment (ACE) satellite mission which confirmed ground-based measurements of an increase in atmospheric hydrogen chloride. The satellite data also indicated that the HCl increase was in the lower stratosphere.

Professor Bernath said: “Our observations do not challenge the general view that the Montreal Protocol is working. They rather show that atmospheric variability and perhaps climate change can significantly modify the path towards full recovery. It will be a bumpy ride rather than a smooth evolution.

“The recovery of ozone-depleting chemicals in the atmosphere is a slow process and will take many decades. During this time the ozone layer remains vulnerable.”

The ozone layer shields the biosphere from harmful UV radiation and is an essential part of the climate system. Solving the problem of ozone depletion depends on the success of the Montreal Protocol, which has essentially banned the production of chlorofluorocarbons (CFC) and similar compounds globally. These substances are responsible for the ozone depletion observed over the last 30 years. Thanks to the Montreal Protocol, their atmospheric burden has decreased during the last decade and scientists are optimistic that ozone levels will fully recover during the second half of this century.

Further information:

  • Hydrogen chloride (HCI) and the Montreal Protocol: In the stratosphere – the atmospheric layer between about 15-45 km altitude – CFCs break up and release chlorine atoms. These then form hydrogen chloride (HCl), the major reservoir of chlorine in the stratosphere. Chemical processes in polar winter release the chlorine atoms from this reservoir, which then destroy ozone in polar spring. Observing the long-term evolution of the stratospheric HCl loading is a key element in the coordinated effort to monitor the success of the Montreal Protocol in reducing the level of ozone-destroying chemicals.
  • The paper ‘Recent Northern Hemisphere stratospheric HCl increase due to atmospheric circulation changes’ is published in Nature at
  • For more information on the University of York’s Department of Chemistry visit

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