Which substances are the primary precursors to the formation of tropospheric (ground-level) ozone in urban smog?

• A. NOx and VOCs
• B. CO2 and CH4
• C. Ozone and PM
• D. SO2 and NOx

Answer: (A)

Ground-level ozone in urban smog forms through photochemical reactions that require both nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. NOx, released from vehicle exhaust and industry, and VOCs—ranging from solvents to hydrocarbons from fuels and plants—react with radicals produced by sunlight-driven chemistry. This network of reactions converts NO to NO2 and then, when NO2 photolyzes, frees an oxygen atom that combines with O2 to make ozone. The process relies on the continuous interaction between NOx and VOCs, so having both as precursors is essential for substantial ozone production. Greenhouse gases like CO2 and CH4 are important for climate but don’t drive the daytime photochemical production of tropospheric ozone in smog. Ozone is the end product of these reactions, not a precursor, and particulate matter is a different pollutant category. Sulfur dioxide can influence atmospheric chemistry and aerosol formation, but it isn’t the primary driver of ozone formation without the concurrent presence of VOCs.