Explanation

(b)
Photochemical smog is initiated by the photochemical dissociation of $NO_{2}$ and the resulting secondary reactions involving unsaturated hydrocarbons, other organic compounds and free radicals, lead to the formation of organic peroxides and ozone.
$NO_{2}(g)\overset{UV\ light\ \ \ \ \ }{\rightarrow}NO(g) + \lbrack O\rbrack$
$O_{2}(g) + \lbrack O\rbrack \rightarrow O_{3}(g)$
$O_{3} + NO\ \ \rightarrow NO_{2}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ + O_{2}$
Brown gas
(In high concentration
Forms haze)
Hydrocarbons $+ O_{3},O_{2},O,NO_{2},NO\$peroxides, peroxyacetyl nitrate, formaldehyde, ozone aldehyde, acrolein, etc. oxidised hydrocarbons and ozone in the presence of humidity cause photochemical smog, which dissipates at night.