Question

how photochemical reaction is responsible for destroying a protective layer in the atmosphere?

Answer 1

Photochemical reactions in biological systems : probing the effect of the environment by means of hybrid quantum chemistry/molecular damage to protective layer of atmosphere by trapping harmful chemical reactions

Answer 2

Photochemical reactions are the chemical reactions that take place in presence of light.
Examples are photosynthesis by green plants, photochemical reaction in retina of eye so that we can see, production of Vitamin D by skin cells on exposure to sunlight is also photochemical reaction.
During spring season, near poles, sun provides energy to photochemical reactions.
Also the stratospheric clouds melt in this heat and releases considerable amount of ClO. This process leads to ozone (protective layer in the atmosphere) hole mechanism.