An ozone-destroying chemical long thought to be on the decline in Earth's upper atmosphere is making an unexpected comeback, an international team of scientists has found.

Backed by years of global observations including key contributions from a Canadian satellite, researchers reported in the journal Nature on Wednesday that concentrations of hydrogen chloride in the Northern Hemisphere have been edging upward since 2007.

The effect is pronounced enough to slow the recovery of the ozone layer, which helps to screen out harmful ultraviolet radiation from the sun. Ozone thinned dramatically in the past because of the use of chlorofluorocarbons (CFCs), which were phased out after international adoption of the Montreal Protocol in 1987.

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The study reveals that hydrogen chloride – a byproduct of CFC release – began diminishing around 1998 because of the impact of the protocol. But seven years ago the trend reversed.

The result came as "a complete surprise," said the study's lead author, Emmanuel Mahieu, an atmospheric scientist at the University of Liège in Belgium.

"Of course, we were anxious that unreported or rogue emissions of chlorine-bearing source gases were responsible for it," Dr. Mahieu said.

The reversal is seen only in the Northern Hemisphere at high altitudes, peaking in a zone that lies about 20 kilometres above the surface and 50 degrees north of the equator.

After an analysis of high-altitude winds and their effects on hydrogen chloride, the team ruled out rogue emissions and instead concluded that a slow down in air circulation in the stratosphere is the likely culprit. The effect of the change is that it concentrates "old air" that still carries a higher proportion of hydrogen chloride from years gone by.

Simply put, "the engine that's driving our atmosphere is changed in this one area," said Kaley Walker, an atmospheric physicist at the University of Toronto and a co-author on the study.

Dr. Walker added that it is not clear why the circulation has slowed. Climate change seems an unlikely culprit, at least directly, because some climate models predict the opposite effect. The result could simply be due to natural variability in which case the stratosphere is showing a broader range of behaviour than was previously expected.

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Other experts not involved with the study said the team's explanation for the hydrogen chloride increase was plausible, if not definitive.

Thomas Duck, an atmospheric scientist at Dalhousie University, added that if the circulation change persists, then researchers might expect to see more Arctic ozone holes in the future.

"We will have to keep a watchful eye," he said.

The heightened concentration, while a concern, is not expected to be permanent since CFCs and related chlorine-bearing chemicals are no longer being emitted in large quantities. When present in the upper atmosphere, chlorine can react with and disrupt ozone molecules.

"Our results do not change the expectation that the ozone layer will recover from the effects of depletion over the next 50 years or so," said Martyn Chipperfield, a team member and chemist at the University of Leeds in Britain.

Dr. Walker said that the result underscores the need for continued monitoring of atmospheric constituents. She is deputy mission scientist for SCISAT, a Canadian satellite launched in 2003 that provided a significant share of the space-based observations used in the study.

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"We do still need to keep making measurements to understand," she said.

The Canadian Space Agency is expected to approve an extension of SCISAT's mission. Dr. Walker and others have also proposed a more advanced satellite that would conduct detailed atmospheric measurements. The federal government has not yet weighed in on whether it supports the project.