Photochemical smog normally does not contain

The correct answer is Chlorofluorocarbons.

To understand why chlorofluorocarbons (CFCs) are not normally found in photochemical smog, let us break down what photochemical smog is, how it forms, and what its primary components are:
Photochemical smog is a type of air pollution that occurs in warm, sunny, and dry climates. It is formed through chemical reactions triggered by sunlight (hence "photochemical") between primary air pollutants, specifically nitrogen oxides (NO_x) and volatile organic compounds (VOCs) emitted from vehicles and industrial sources.

Let us look at the primary components of photochemical smog:
1. Ozone (O₃): Sunlight splits nitrogen dioxide (NO₂) into nitrogen monoxide (NO) and atomic oxygen (O). This atomic oxygen then reacts with atmospheric molecular oxygen (O₂) to form ozone. Therefore, ozone is a major secondary pollutant in photochemical smog.
2. Peroxyacetyl nitrate (PAN): PAN is formed when volatile organic compounds (VOCs) react with oxygen and nitrogen dioxide in the presence of sunlight. It is a powerful eye irritant and a key component of photochemical smog.
3. Acrolein: Acrolein is an unsaturated aldehyde (CH₂=CHCHO) that is produced during the photochemical reactions of hydrocarbons. Like PAN, it acts as a strong irritant in photochemical smog.

In contrast, Chlorofluorocarbons (CFCs) are synthetic organic compounds containing carbon, chlorine, and fluorine. They are highly stable, non-toxic, and non-flammable compounds that were widely used in refrigeration, air conditioning, and aerosol propellants. Because of their chemical inertness in the lower atmosphere, CFCs do not readily react with other pollutants in the troposphere and do not participate in the ground-level photochemical reactions that produce smog. Instead, they gradually drift up to the stratosphere, where they are broken down by UV radiation and contribute to the depletion of the ozone layer. Therefore, CFCs are not a constituent of photochemical smog.