It comprises high concentrations of ozone (O3) in connected other parts of the atmosphere.
Stratospheric Ozone is not harmful, but its presence on land is harmful.
Montreal Protocol on Substances That Deplete the Ozone Layer
It is an international pact intended to defend the ozone layer by phasing out production of numerous Ozone Depleting Substances (ODSs) that are accountable for ozone depletion which was agreed on 26 August 1987 in Montreal, Canada.
Letter it was came in to force on 26 August 1989 and then first meeting was held in Helsinki, May 1989.
This international agreement has helped a lot for recovering the ozone hole in Antarctica.
Vienna Convention for the Protection of the Ozone Layer
Vienna Convention for the Protection of the Ozone Layer (one of the most successful agreements) is a Mutual Environmental Agreement which was approved in the 1985 at Vienna Conference and came into force in 1988.
It was approved by 197 states (all UN members as well as the Niue, Holy See and the Cook Islands) along with European Union.