The correct option is (a).

Explanation

Photochemical smog, also known as Los Angeles smog, is a type of air pollution that typically occurs in warm, dry, and sunny climates. Unlike classical smog (London smog), which is reducing in nature, photochemical smog is oxidizing in character due to the high concentration of oxidants like ozone and organic peroxides.

Mechanism of Formation

The formation of photochemical smog involves a complex series of reactions driven by solar energy:

Primary Precursors: Automobiles and factories emit Nitrogen oxides (NO_x) and hydrocarbons (unburnt fuels) into the atmosphere.
Role of Sunlight: Sunlight acts on these pollutants. Specifically, it causes the photo-dissociation of Nitrogen Dioxide (NO₂) into Nitric Oxide (NO) and nascent oxygen (O).
Secondary Pollutants: The nascent oxygen reacts with molecular oxygen (O₂) to form Ozone (O₃). The ozone then reacts with the hydrocarbons and NO to produce toxic chemicals such as Peroxyacetyl nitrate (PAN), formaldehyde, and acrolein.

Therefore, photochemical smog is the resultant of the reaction among NO₂, O₃, and Peroxyacetyl nitrate (PAN) in the presence of sunlight.

Analysis of Incorrect Options

Option (b): Carbon Monoxide (CO) is a pollutant, but the specific chain reaction defining photochemical smog relies primarily on NO_x and volatile organic compounds (hydrocarbons), not CO.
Option (c): This describes conditions suitable for Classical Smog (London Smog), which occurs in cool, humid climates and involves smoke and Sulphur Dioxide (SO₂). Photochemical smog requires high temperatures.
Option (d): Photochemical smog requires strong sunlight to initiate the photochemical reactions. Therefore, the concentration of oxidants peaks around noon, not in the evening.

Key Takeaway: Photochemical smog is an oxidizing smog caused by the interaction of sunlight with Nitrogen Oxides (NO_x) and Hydrocarbons, resulting in secondary pollutants like Ozone (O₃) and PAN.