Question

What structural modification of oxygen is an air pollutant in the lower atmosphere but beneficial in the upper atmosphere?

Answer 1

Ozone ($O_{3}$) is the structural modification of oxygen ($O_{2}$) which is an air pollutant in the lower atmosphere (the troposphere) but at the same time benefecial in the upper atmosphere (the stratosphere).

It is an air pollutant in the troposphere because it is
i) harmful to breathe
ii) damages crops and other vegetation
iii) the main ingredient of urban smog

It is beneficial in the stratosphere because it protects life on earth by blocking harmful ultraviolet (UV) rays from the sun.