Which pollutants primarily drive the formation of photochemical smog?

• A. NOx and VOCs
• B. CO2 and CH4
• C. SO2 and NH3
• D. Particulate matter and ozone

Answer: (A)

Photochemical smog forms when sunlight drives chemical reactions between nitrogen oxides and volatile organic compounds released by cars, factories, and solvents. NOx (especially NO2) reacts with sunlight to produce reactive oxygen atoms that combine with O2 to form ozone. VOCs supply reactive radicals that shuttle the chemistry, converting NO to NO2 without consuming it and keeping ozone production going. Without both NOx and VOCs, the chain reactions needed to build up ozone don’t proceed effectively, so smog doesn’t form as strongly.

Greenhouse gases like CO2 and CH4 aren’t the primary players in this daytime, sunlight-driven smog chemistry. Sulfur compounds (SO2) and ammonia (NH3) contribute to other air pollutants and aerosols, not the main photochemical ozone smog. Particulate matter can accompany smog, but the key precursors that drive its formation are the combinations of NOx and VOCs.