Question

give reason
a)in the stratosphere ozone shield normally has steady conc. of ozone
b)among the ozone depleting substances cfcs are the most harmful

Answer 1

Normally, the production and destruction is balanced, so the amount of ozone in the stratosphere at any given time is stable. However, scientists have discovered that certain chemicals react with UV radiation in the stratosphere, which causes them to break apart and release chlorine or bromine atoms.

Answer 2

(B) Ozone depletion. Man-made compounds such as chlorofluorocarbons (CFCs), hydrofluorocarbons (HCFCs) and halons destroy ozone in the upper atmosphere (stratosphere). The stratospheric ozone layer makes life possible by shielding the earth from harmful ultraviolet (UV-B) rays generated from the sun.

Don't know the reason of (a)