Question

Photochemical smog always contains:
A. Ozone
B. Methane
C. Carbon monoxide
D. Carbon Dioxide

Answer 1

Hint: Photochemical smog is produced by the reaction of the primary pollutants and gives rise to the secondary pollutants. Ozone is one of such secondary pollutants.

Complete answer:
To know the correct answer to this question, we must first know about smog, especially the photochemical smog.
One of the main causes of air pollution is smog.
Smog is made up of two words- smoke and fog. When fog turns heavy and dark in colour when smoke gets mixed with it, then it is known as smog. Smog is of two types, i.e. classical smog and photochemical smog. Classical smog is more commonly known as the sulphur smog. It is so named because its main constituent is sulphur. Smoke, dust particles, Sulphur Dioxide (SO$_2$) and Hydrogen Disulphide (H$_2$S) are also present in classical smog.
Next is the Photochemical Smog. It is yellowish brown in colour. It is the opaque smog which is formed due to the photochemical reactions between hydrocarbons and oxides of nitrogen. Photochemical reactions lead to the production of secondary pollutants like ozone, aldehydes, phenols and Peroxyacyl Nitrates (PAN).

Therefore, the correct answer to this question is ozone. We all know that methane and carbon dioxide are the greenhouse gases.

Note:
The photochemical smog is also known as the Los Angeles Smog while the classical or the sulphur smog is also known as the London Smog. The names of the two smog must not be confused with each other.