Bruce Bernasconi, of Dunedin, asks :-

Does burning coal help produce the ozone hole over New Zealand and will the hole repair itself?

Greg Bodeker, an atmospheric physicist at the National Institute for Water and Air at Lauder in Central Otago where ozone levels are monitored, responded.

There is no ozone hole above New Zealand. The ozone hole is confined primarily to Antarctica, but occasionally covers the southern tip of South America.

Burning coal has no direct effect on the ozone layer. Coal combustion produces carbon dioxide (CO2), carbon monoxide (CO), sulphur dioxide (SO2), nitrous oxides (NOx) and particles. CO2 and CO are greenhouse gases associated with global warming, while SO2 and NOx contribute to acid rain. It is chlorine and bromine that are primarily responsible for destroying ozone. For these chemicals to destroy ozone, they must be released within very stable compounds, such as the CFCs, (the old working fluid of refrigerators), that are transported into the stratosphere where they are broken down to release the chlorine or bromine that destroys ozone. This journey can take years. If these chemicals are released in more reactive forms, they will react with gases close to the ground and will be removed from the atmosphere. It is for this reason that pool chlorine is not harmful to the stratospheric ozone layer.

The international protocols implemented to reduce the production and release of ozone depleting substances have been very successful and levels of CFCs in the atmosphere have started to decrease. CFCs have very long lifetimes and will remain in the atmosphere for decades to come, but as their concentration falls, global ozone levels are expected to recover. Current studies estimate that by 2070 the ozone layer will have recovered to 1970 levels, though this recovery may be delayed a decade or two as a result of greenhouse gas increases. These gases warm the troposphere, but cool the stratosphere, and a cold stratosphere is more conducive to ozone depletion.