What is smog and how it is formed?

Explanation

Smog is a mixture of smoke and fog in suspended droplet form. It is a dangerous form of atmospheric pollution caused by combustion of fossil fuels in internal combustion engine. Formation of smog
(i) Hydrocarbons and oxides of nitrogen, which are introduced by automobiles get accumulated in the atmosphere
(ii) `NO_(2)`, which is introduced to atmosphere absorb sunlight in blue and near UV region and decomposes into NO (nitric oxide) and O (atomic oxygen).
(iii) The decomposition of `NO_(2)` into NO and O followed by many reactions, converts NO to `NO_(2)` with simultaneous formation of aldehydes and ozone.
(iv) The hydrocarbons in the atmosphere undergo oxidation to form secondary pollutants like formaldehyde, acrolein, peroxyacetyl nitrates (PAN). These secondary form haze like aerosols which are constituents of photochemical smog called Los Angeles smog.
The chemical and photochemical reactions are given below in simplified manner.
`NO_(2) + hv rarr NO + overset(.)(O)`
`overset(.)(O) + O_(2) rarr O_(3)`
`O_(3) + NO rarr NO_(2) + O_(2)`
`underset(("Hydrocarbon"))(RH) + overset(.)(O) rarr R overset(.)(O)`
`R overset(.)(O) + O_(2) rarr R overset(.)(O)_(3)`
`R overset(.)O_(3) + RH rarr " Aldehydes + ketones"`
`R overset(.)O_(3) + NO rarr R overset(.)(O)_(2) + NO_(2)`
`R overset(.)O_(2) + O_(2) rarr R overset(.)(O)_(3) + O_(2)`
` R overset(.)O_(2) + NO_(2) rarr` Peroxyacetyl nitrate (PAN)