Some reactions of NO _ 2 relevant to photochemical smog formation...

When fossil fuels are burnt, a variety of pollutants are emitted into the earth’s troposphere. Two of the pollutants that are emitted are hydrocarbons (unburnt fuels) and nitric oxide (NO). When these pollutants build up to sufficiently high levels, a chain reaction occurs from their interaction with sunlight in which NO is converted into nitrogen dioxide ${NO}_{2}$. This ${NO}_{2}$ in turn absorbs energy from sunlight and breaks up into nitric oxide and a free oxygen atom.

$N{O}_{2} ( g ) \overset{hν}{\to}NO ( g ) + O ( g )$

Oxygen atoms are very reactive and combine with the ${O}_{2}$ in air to produce ozone.

$O ( g ) + {O}_{2} ( g ) ⇌ {O}_{3} ( g )$

Hence, $X = O, Y = NO, A = {O}_{2}, B = {O}_{3}$