The formation of photochemical smog requires several components, but one of them is not necessary for its formation. Let`s examine each option to understand why it is or is not required for the formation of photochemical smog:

1. Oxygen: Oxygen is required for the formation of photochemical smog. It plays a crucial role in the chemical reactions that occur between pollutants and sunlight.

2. Oxides of nitrogen: Oxides of nitrogen, such as nitrogen dioxide (NO2), are an essential component of photochemical smog. They react with sunlight and other pollutants to form ozone and other harmful compounds.

3. Carbon monoxide: Carbon monoxide is not required for the formation of photochemical smog. While it is a pollutant and can contribute to air pollution, it is not directly involved in the chemical reactions that lead to the formation of smog.

4. Sunlight: Sunlight is a necessary component for the formation of photochemical smog. It is responsible for initiating the chemical reactions between pollutants, such as oxides of nitrogen and volatile organic compounds, leading to the formation of smog.

Therefore, out of the given options, the one that is not required for the formation of photochemical smog is carbon monoxide (option 3).