Tropospheric ozone is formed from reactions involving precursors in the presence of sunlight. Which combination of precursors is most directly associated with its formation?

Tropospheric ozone is formed from reactions involving precursors in the presence of sunlight. Which combination of precursors is most directly associated with its formation?

• A. Nitrogen oxides and volatile organic compounds
• B. Ozone and water vapor
• C. Sulfur dioxide and ammonia
• D. Carbon dioxide and methane

Answer: (A)

In sunlight-driven photochemical smog, tropospheric ozone forms most directly from reactions involving nitrogen oxides and volatile organic compounds. The process starts when sunlight splits NO2 into NO and an oxygen atom; that oxygen quickly combines with O2 to produce ozone. VOCs help by reacting with radicals to generate peroxy radicals, which convert NO to NO2 without consuming ozone. This keeps the NOx–NO2 cycle releasing more ozone under the influence of sun, especially in sunny, polluted urban areas. Water vapor can participate in radical chemistry but isn’t the direct source of the ozone itself, while sulfur dioxide and ammonia tend to form different aerosols, and carbon dioxide and methane are greenhouse gases not primary precursors to ozone formation. So NOx and VOCs in the presence of sunlight best explain the formation of tropospheric ozone.

In sunlight-driven photochemical smog, tropospheric ozone forms most directly from reactions involving nitrogen oxides and volatile organic compounds. The process starts when sunlight splits NO2 into NO and an oxygen atom; that oxygen quickly combines with O2 to produce ozone. VOCs help by reacting with radicals to generate peroxy radicals, which convert NO to NO2 without consuming ozone. This keeps the NOx–NO2 cycle releasing more ozone under the influence of sun, especially in sunny, polluted urban areas. Water vapor can participate in radical chemistry but isn’t the direct source of the ozone itself, while sulfur dioxide and ammonia tend to form different aerosols, and carbon dioxide and methane are greenhouse gases not primary precursors to ozone formation. So NOx and VOCs in the presence of sunlight best explain the formation of tropospheric ozone.