In addition to volatile organic compounds, which components must be present to form photochemical smog?

• A. NOx and UV light
• B. CO2 and water vapor
• C. Methane and sunlight
• D. SO2 and heat

Answer: (A)

Photochemical smog forms when reactive organic compounds (VOCs) react with nitrogen oxides in the presence of sunlight. Sunlight drives the photolysis of NO2, generating atomic oxygen that rapidly combines with O2 to make ozone. VOCs supply reactive radicals (like OH and RO2) that keep the chemical chain reactions going, converting NO to NO2 and allowing ozone to build up rather than being destroyed by NO. So, along with VOCs, the essential ingredients are nitrogen oxides from combustion sources and sunlight (UV light) to drive the photochemistry.

Other options don’t fit this pathway. CO2 and water vapor don’t participate in the photochemical reactions that produce ozone. Methane and sunlight lack the necessary NOx component to sustain the cycle, and sulfur compounds with heat lead to a different, sulfurous form of smog rather than photochemical smog.