Which region of the atmosphere is the domain of all living organisms?

Peroxyacetyl nitrate $(PAN)$,an eye irritant,is produced by:

Which of the following statements are correct?
a) $F^{-}$ ion concentration above $2 \ ppm$ causes brown mottling of teeth
b) The maximum limit of nitrate in drinking water is $400 \ ppm$
c) Depletion of ozone layer leads to cataract and skin cancer
d) The irritant red haze in the traffic is due to oxides of sulphur

The chlorine free radical reacts with ozone to form a ..... free radical.

What do you mean by green chemistry? How will it help decrease environmental pollution?

The enamel present on teeth becomes much harder due to the conversion of $[3 Ca_3(PO_4)_2, X]$ into $[3 Ca_3(PO_4)_2, Y]$. What are $X$ and $Y$?