Natural ozone depletion long before industrialisation
Ozone reduction probably already happened at pre-industrial times because of bromine and chlorine emitted during explosive volcanic eruptions.
In the 1980s the scientific community, polititians, the population, etc. became aware that parts of the stratospheric ozone layer were strongly reduced. The ozone depletion resulted from the injection of the halogens fluorine and chlorine into the atmosphere, where chemical reactions split the ozone molecules in a catalytic cycle influenced by solar radiation. In the course of industrialisation, the quantities of halogenated hydrocarbons that were released by industrial production and household devices strongly increased. The ozone depletion was therefore clearly related to human activities.
However, it is well possible that the atmosphere had experienced phases of ozone reduction long before industrialisation. There are natural sources for halogens. In particular, volcanic gases released during large explosive eruptions, where the eruptions columns reach heights of 10–40 km, have the power to strongly modify the stratosphere's composition. Halogens are very typical components of volcanic gases. It is well known that volcanoes emit large amounts of chlorine. In a recent study it was shown, however, that also the heavy halogen bromine is transported into the stratosphere by explosive eruptions. Bromine is much less abundant in magmas than chlorine. But because of its chemical highly-reactive behaviour, it is likely to play a major role in destroying ozone. Especially in the presence of related agents, it efficiently takes part in catalytic chain reactions, thus even small amounts of bromine can have an tremendous effect on depleting ozone.
Nicaragua was site of a series of large, highly-explosive plinian eruptions over the past 70,000 years. Using a newly-developed analytical method that employs synchrotron-X-rays, high bromine contents were detected in melt inclusions in those magmas. Together with chlorine, the bromine was carried into high altitudes in the stratosphere, where it attacked the ozone layer. In this way, natural “ozone holes” emerged – long before human intervention in the atmosphere's composition. Likewise, the solar irradiation changed, and therewith the climatic conditions.