The main purpose of setting up the Ozone Cell by the Union Ministry of Environment and Forests is to implement the Montreal Protocol. The Montreal Protocol is an international treaty that aims to protect the ozone layer by phasing out the production and consumption of ozone-depleting substances. The Ozone Cell is responsible for coordinating and facilitating the implementation of the Montreal Protocol in India.

Option 1, which suggests that the Ozone Cell assists the Central Pollution Control Board, is not the main purpose of its establishment. While the Ozone Cell may work in coordination with the Central Pollution Control Board, its primary objective is to implement the Montreal Protocol.

Option 2 states that the purpose of the Ozone Cell is to promote public awareness of ozone depletion. While the Ozone Cell may contribute to raising public awareness, its main focus is on implementing the Montreal Protocol.

Option 4 suggests that the Ozone Cell serves as a local unit of the office of Intergovernmental Panel on Climate Change. This is not correct, as the Ozone Cell specifically deals with the implementation of the Montreal Protocol, which is focused on protecting the ozone layer, rather than the broader issue of climate change.

Therefore, the correct answer is option 3, which states that the main purpose of the Ozone Cell