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University of Toronto physicist has helped solve the scientific mystery behind the recent increase in ozone-depleting chemicals in the lower stratosphere of the Northern Hemisphere, despite a 25-year old ban on chlorofluorocarbons (CFCs).

Employing data from various sources, including the , an international team of scientists used numerical simulations to determine that the recently observed increase in hydrogen chloride (HCl) in the lower stratosphere is due to reduced atmospheric circulation in the Northern Hemisphere.

“Because the circulation has slowed, the old air with higher amounts of HCI is taking longer to move through the stratosphere,” said Walker. “CFCs produced before the was in place, as well as ozone-destroying substances that were phased out over the past two decades, are still in the atmosphere and causing damage.”

CFCs are responsible for the depletion of the ozone layer that protects Earth from ultraviolet radiation. They break up in the stratosphere and release chlorine atoms that then form HCl, the major chlorine reservoir in the Earth's upper atmosphere.

It is this chlorine that thins the ozone layer over the globe and causes the springtime holes in the ozone layer over Antarctica.

According to Walker, the SCISAT instrument provided the reliable hydrogen chloride measurements in the lower stratosphere that were essential to solve the mystery.The satellite was launched in 2003 to study ozone and molecules involved in ozone production and destruction. Its measurements contribute to improving our understanding of atmospheric chemistry and air quality over Canada and the Arctic, including air pollution caused by forest fires.

The study, co-authored by Walker, appeared in on November 6.