Despite recent good news concerning the recovery of the ozone layer, a new study has revealed that ozone levels in the heavily populated lower latitudes don't seem to be recovering as well as regions near the poles. Although there isn't yet a clear explanation as to why this is happening, blame may still rest on short-lived ozone-destroying chemicals, such as those found in paint strippers and degreasing agents.

The ozone layer acts as a protective barrier between us and harmful UV radiation emitted from the Sun. In the 1970s, it was discovered that chloroflurocarbons (CFCs) used in refrigeration and aerosols were the main cause of ozone reduction in the stratosphere, eventually leading to their phase out under the Montreal Protocol in 1987.

While this means the ozone "hole" that formed over Antarctica is on its way to recovery, the bottom part of the ozone layer over latitudes from 60° N to 60° S is showing no signs of healing. These latitudes cover heavily populated areas spanning from Russia all the way to the Southern Ocean below Australia.

"The potential for harm in lower latitudes may actually be worse than at the poles," says co-author Joanna Haigh, Co-Director of the Grantham Institute at Imperial College London. "The decreases in ozone are less than we saw at the poles before the Montreal Protocol was enacted, but UV radiation is more intense in these regions and more people live there."

Using new algorithms to merge data from various satellite missions conducted since 1985, the team created a long time series that revealed a longer term trend of ozone decrease at lower latitudes and altitudes in the stratosphere between 10 and 50 km (6.2 to 31 mi). Although they currently can't provide any definitive reason for this, the researchers have proposed a couple of theories.

One is that a shift in atmospheric circulation patterns caused by climate change may be carrying more ozone away from the tropics. Another is that very short-lived substances (VSLSs), that were not thought to stay in the atmosphere long enough to reach the stratosphere and affect the ozone layer, may be hanging around longer than expected.

"The finding of declining low-latitude ozone is surprising, since our current best atmospheric circulation models do not predict this effect," says William Ball from ETH Zurich and PMOD/WRC Davos. "Very short-lived substances could be the missing factor in these models."

Ian Lowe, Emeritus professor at Griffith University, says that although the amount of ozone-depleting substances has been reduced dramatically, it has not yet reached zero.

"Since we have known for more than forty years that a group of chemicals weakens the ozone layer, which protects all life from damaging ultra-violet radiation, phasing these chemicals out completely should be a high priority," Lowe says.

The team's paper can be found in the European Geosciences Union journal, Atmospheric Chemistry and Physics.