We have known for some years that a number of man-made, chlorine-based compounds were responsible for creating gaping holes in the ozone layer over the Earth's poles. Now scientists at University College Cork are looking at how a number of different particles, including volcanic ash and mineral dust, can combine with chlorine to open another route for ozone depletion in the stratosphere.
Prof John Sodeau, head of the physical chemistry department at UCC, and his team have recently completed research that shows inert chlorine particles, which normally would not do any harm to the ozone layer, will become threats after they have reacted with ice particles, ash or dust in the stratosphere.
Chlorine atoms most frequently enter the earth's atmosphere through chlorofluorocarbons, or CFCs, which have been used for decades in refrigeration units and aerosol cans. The CFC molecules float through the atmosphere and are broken down by sunlight, causing the release of a chlorine atom. Most of the time the chlorine will then float to the stratosphere and attack the ozone layer.
But once in every thousand billion times, the released chlorine atom will react with water and become hydrochloric acid, a compound that is not a threat to ozone. The UCC team believes, however, that when this hydrochloric acid settles on the surface of an ice crystal or dust particle, it changes chemically and becomes a compound that can attack ozone.
"That may not sound like it happens very much," Prof Sodeau said. "But as chlorine builds up in the atmosphere, the chance that this is happening increases geometrically, and also increases the amount of chlorine that is attacking the ozone layer."
Prof Sodeau and the other members of the UCC research group have determined that the reactions between chlorine and ice crystals can only take place in a specific atmospheric condition.
"When most people think of the atmosphere, they think of gases and particles banging up against one another," Prof Sodeau said. "But over the Antarctic, during the period of darkness, or winter, there is a considerable amount of calmness and stability. Then a polar vortex can be formed."
A polar vortex causes a stable mass of very cold air above the pole. It sucks in a variety of chemicals and remains stable for a long time, only to be broken up by the arrival of sunlight when the Antarctic spring approaches.
During the stable period, the chlorine has the opportunity to settle on ice crystals and dust. This gives the two the time to interact and create compounds that attack the ozone.
"A thermos - it attracts and condenses gases, that's why you get condensation on the outside of a cold thermos," Prof Sodeau explained. "It's the same in the stratosphere. The ice crystals attract these other molecules, and then release them as compounds that can directly attack the ozone."
Scientists have discovered that these reactions are also taking place over the North Pole. "The northern hemisphere is very different than the southern," Prof Sodeau explained. "It isn't nearly as calm and stable, and at most you would have three weeks or so of calm, which, with low levels of chlorine, couldn't produce these reactions. But now, with the enormous amount of chlorine available in the system, three weeks is enough to allow these reactions to take place."
Thus, the chlorine threat to the ozone layer is continuing to grow. And since chlorine atoms are regenerated through this catalytic process over and over again, they can continue to attack ozone. The only way for the chlorine to be removed is for it to be rained out.
"What this work has shown is that we have to be even more careful about not getting chlorine into the atmosphere," Prof Sodeau said.