How is Photochemical Smog formed ? What are its ill effects ?

Explanation

When fossil fuels are burnt, a variety of pollutants are emitted into the earth's troposphere. The hydrocarbons and nitric oxide after accumulated to sufficiently high leavels, a chain reaction occurs from their interaction with sunlight. NO is converted into nitrogen dioxide `(NO_2)`. This `NO_2` again breaks up into NO and Oxygen by absorbing energy from sunlight.
`NO_2 to NO+O`
Oxygen atoms are very active and combine with the `O_2` in air to produce Ozone `(O_3)`.
`O+O_2 to O_3`
The `O_3` thus formed react with NO forming `NO_2`.
The `O_3` and `NO_2` are strong oxidising agents and can react with hydrocarbons present in air forming toxic chemicals such as formaldehyde, acrolein and peroxyacetyl nitrate (PAN) . These all contribute to photochemical smog.