Correct Option
Photochemical smog is a type of air pollution formed when sunlight reacts with nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) in the atmosphere. Ozone (O₃) is a significant secondary pollutant produced during this process. Ground-level ozone is not directly emitted but is formed through a series of photochemical reactions. For instance, nitrogen dioxide (NO₂), a primary pollutant, absorbs sunlight and dissociates into nitric oxide (NO) and an atomic oxygen (O) radical. This atomic oxygen then combines with molecular oxygen (O₂) to form ozone (O₃).
NO₂ + sunlight → NO + O
O + O₂ → O₃
Thus, ozone is a characteristic product of photochemical smog and contributes substantially to its adverse environmental and health impacts.
Incorrect Options
- Hydrocarbons (or volatile organic compounds) are primary pollutants and precursors that react in the presence of sunlight and nitrogen oxides to form photochemical smog. They are not produced during the formation process but are consumed.
- Nitrogen oxides (NOₓ), such as NO and NO₂, are also primary pollutants emitted from sources like vehicular exhaust and industrial activities. They act as reactants in the photochemical reactions that lead to smog formation, rather than being a product of the smog formation itself.
- Methane (CH₄) is a potent greenhouse gas and a type of hydrocarbon. While it can contribute to atmospheric chemistry, it is not considered a primary product or a major direct participant in the specific photochemical reactions that lead to the formation of ozone and other secondary pollutants characteristic of photochemical smog.