Press Release: Ozone depleting substance to be on rise

A study published (6 November 2014) in Nature reports of a recent increase in atmospheric hydrogen chloride (HCl), a signature of the presence of ozone-destroying chemicals. The study shows that this unexpected increase has occurred in the Northern hemisphere since 2007, as a result of a temporary but prolonged anomaly in atmospheric circulation. This circulation change has led to a transient 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. However, the expected evolution of ozone-destroying chemicals in the atmosphere is shown to be more complex than a smooth decline.

Background

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. As a success of 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 (see new WMO O3 assessment).

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 load is a key element in the coordinated effort to monitor the success of the Montreal Protocol in reducing the level of ozone-destroying chemicals.

An international team of scientists now present observations showing a recent and unexpected upturn of the stratospheric HCl concentrations. The findings are based on measurements by a network with stations in Spitsbergen, Greenland, Sweden, Switzerland, Japan, Tenerife, Australia and New Zealand. They are backed-up by satellite observations and model simulations. The recent increase in HCl concentrations was only observed in the Northern Hemisphere, while in the Southern Hemisphere HCl continues to decrease as expected in line with the Montreal Protocol.

The scientists found that the increase of HCl levels above the Northern Hemisphere is related to an anomaly in atmospheric circulation, changing the balance between the CFCs and their breakdown product HCl. One of the authors of the study points out: “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.

An example of the continued susceptibility to ozone loss, more than 20 years after the adoption of the Montreal Protocols was the winter of 2011 where over much of the Arctic more than 40% of the ozone was catalytically destroyed by Cl.