How does photochemical smog form and which pollutants drive it?

How does photochemical smog form and which pollutants drive it?

• A. In sunlight, NOx and VOCs react to form ozone and other oxidants.
• B. In darkness, CO2 reacts with water to form smog.
• C. Photochemical smog is formed by sulfuric acid reactions in rain.
• D. Ozone forms directly from CO alone.

Answer: (A)

Photochemical smog on sunny days forms from sunlight-driven reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs). When sunlight is present, NO2 can be split by light to produce NO and an oxygen atom; that oxygen then combines with O2 to form ozone (O3). However, ozone can be limited or enhanced depending on the balance of precursors. VOCs interact with radicals like OH, forming peroxy radicals (RO2, HO2) that convert NO to NO2 without consuming ozone, allowing ozone to accumulate. This creates a mix of ozone and other oxidants—the hallmark of photochemical smog. The main pollutants driving this process are NOx and VOCs from vehicle exhaust, fuel use, and industrial solvents. The other options describe processes that don’t produce photochemical smog: smog formation in darkness, a reaction of CO2 with water, sulfuric acid chemistry in rain (which relates to acid rain, not photochemical smog), and ozone arising directly from CO alone (ozone formation requires NOx and VOCs under sunlight, not CO by itself).

Photochemical smog on sunny days forms from sunlight-driven reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs). When sunlight is present, NO2 can be split by light to produce NO and an oxygen atom; that oxygen then combines with O2 to form ozone (O3). However, ozone can be limited or enhanced depending on the balance of precursors. VOCs interact with radicals like OH, forming peroxy radicals (RO2, HO2) that convert NO to NO2 without consuming ozone, allowing ozone to accumulate. This creates a mix of ozone and other oxidants—the hallmark of photochemical smog. The main pollutants driving this process are NOx and VOCs from vehicle exhaust, fuel use, and industrial solvents.

The other options describe processes that don’t produce photochemical smog: smog formation in darkness, a reaction of CO2 with water, sulfuric acid chemistry in rain (which relates to acid rain, not photochemical smog), and ozone arising directly from CO alone (ozone formation requires NOx and VOCs under sunlight, not CO by itself).