Which one of the following is an extremely short-lived and unstable air/water pollutant?

The correct option is Polycyclic Aromatic Hydrocarbons (PAHs).

To understand why Polycyclic Aromatic Hydrocarbons (PAHs) are considered short-lived and unstable in the context of air and water pollution, we can analyze the characteristics and environmental behavior of each option:
• SMOG: Smog is a complex mixture of primary and secondary pollutants (such as ozone, nitrogen oxides, and particulate matter). It represents an atmospheric condition rather than a single chemical substance, and its components can linger in the air for extended periods depending on weather conditions.
• Volatile Organic Compounds (VOCs): These are organic chemicals that easily vaporize at room temperature. While some react quickly, many VOCs can persist in the atmosphere for days or even weeks, contributing to long-term chemical reactions.
• Persistent Organic Pollutants (POPs): As their name suggests, POPs are highly stable chemical compounds that resist environmental degradation through physical, chemical, or biological processes. They remain in the environment for years or decades and bioaccumulate in the food chain.
• Polycyclic Aromatic Hydrocarbons (PAHs): PAHs are organic compounds composed of multiple carbon-containing rings. In the atmosphere and surface waters, they are highly susceptible to rapid photo-oxidation, photolysis (breakdown by sunlight), and reactions with ozone or hydroxyl radicals. Consequently, in active atmospheric and aquatic environments, many PAHs degrade very quickly, giving them a short chemical lifetime and making them unstable compared to highly persistent pollutants.