Surprising satellite data reveals that the amount of ozone-depleting chlorine in the stratosphere is not decreasing as quickly as expected.

As the first group of scientists to compare data sets from both ground-based instruments and satellites, the international team - including Lancaster University - found an unexpected discrepancy.

The international team included Professor of Atmospheric Chemistry Ryan Hossaini from Lancaster Environment Centre with Dr Susann Tegtmeier and Dr Kimberlee Dube from the University of Saskatchewan in Canada, among others.

Their research published in Nature Communications Earth and Environment examined the amount of chlorine in the stratosphere using satellite data and compared this to the expected amount from ground-based measurements of now banned chlorine-containing gases, such as chlorofluorocarbons (CFCs).

The stratosphere is the second closest layer of atmosphere to the Earth’s surface and is one of five distinct layers of the atmosphere which protects life on Earth. But certain chemicals released into the air can damage ozone in this layer.

Increasing levels of chlorine in the upper atmosphere can compromise its integrity, leading to “holes” or areas of less ozone that can leave Earth more vulnerable to the sun’s rays. For decades, researchers have been focused on preventing and reversing this damage.

Dr Susann Tegtmeier from the University of Saskatchewan said: “In the 1980s, when it was discovered that the ozone layer was beginning to thin, researchers set out to understand the process, which eventually led to the creation of the Montreal Protocol, a policy which limited the use of certain chemicals that damage the ozone layer. Chlorine is one of those chemicals that destroy ozone and we’ve been measuring its level in the atmosphere to see the rate it’s decreasing.”

Over the past few decades, a reduction in chlorinated long-lived ozone-depleting substance emissions due to the regulations imposed by the Montreal Protocol has led to a global decrease in stratospheric chlorine. At the same time, emissions of chlorinated Very Short-Lived Substances, which are unregulated, have increased.

Scientists rely on a ground-based measurement network to monitor the lower atmosphere and satellites to measure higher layers like the stratosphere. While the ground-based measurements show the expected decrease in chlorine, the satellites have found chlorine levels in the stratosphere to be higher than expected and decreasing at a slower rate.

Satellite observations of stratospheric chlorine species from the Atmospheric Chemistry Experiment—Fourier Transform Spectrometer during 2004–2020 revealed that the observed decrease in inorganic stratospheric chlorine is 25%–30% smaller than expected based on trends of long-lived ozone-depleting substances alone. At mid-latitudes in the lower stratosphere, this can be explained by the chlorinated Very Short-Lived Substances increase, which offsets the long-term reduction of stratospheric chlorine by up to 30%.

Professor Ryan Hossaini from Lancaster Environment Centre said: “Our new findings provide further compelling evidence that uncontrolled emissions of halogenated Very Short-Lived Substances are offsetting some of the Montreal Protocol’s benefits. A timely recovery of the ozone layer should not be taken for granted and a concerted international effort is now needed to establish the sources of these gases and to identify whether action is required.”

Professor Hossaini and his team performed simulations with a global atmospheric model to interpret the satellite data in this new study. His work has previously warned that growing emissions of Very Short-Lived Substances, including the common solvent dichloromethane, may delay recovery of the ozone layer.