Solution:

• September 16 is observed as the International Day for the Preservation of Ozone Layer by the UN General Assembly. The day marks the signing of the Montreal Protocol on Substances that deplete the Ozone Layer. The day is observed to highlight the importance of the ozone layer and the ways it can be preserved.

• In 1994, the UN General Assembly proclaimed 16 September the International Day for the Preservation of the Ozone Layer, commemorating the date of the signing, in 1987, of the Montreal Protocol on Substances that Deplete the Ozone Layer.

• The closure of the hole in the ozone layer was observed 30 years after the protocol was signed.Due to the nature of the gases responsible for ozone depletion their chemical effects are expected to continue for between 50 and 100 years.