Final answer:
Secondary pollutants such as smog and ozone are primarily formed from the chemical reactions of primary pollutants like nitrogen dioxide and volatile organic compounds in the presence of sunlight.
Step-by-step explanation:
The agent mainly responsible for the formation of secondary pollutants is smog and ozone. Secondary pollutants are not emitted directly into the air but form when primary pollutants such as sulfur oxides, nitrogen oxides, and volatile organic compounds undergo chemical reactions in the atmosphere, often in the presence of sunlight. For instance, nitrogen dioxide (NO₂) and volatile organic compounds (VOCs) react with sunlight to create ground-level ozone (O₃), a key component of smog which can cause respiratory health issues and damage to the environment.
Nitrogen dioxide (NO₂) itself is a primary pollutant that contributes significantly to the formation of both nitrate particles, causing haze and reduced visibility, and ground-level ozone, which is harmful to human health. It is primarily emitted from fuel combustion in vehicles and industrial processes. Similarly, sulfur dioxide (SO₂) reacts in the atmosphere to form sulfuric acid and sulfate particles, which contribute to particulate pollution and acid rain.