Which secondary pollutant forms when chemicals from the combustion of fossil fuels interact in the presence of sunlight at warm temperatures?

• A. Smog
• B. Ozone
• C. Acid Deposition
• D. Particulate Matter

Answer: (A)

When fossil fuel combustion releases nitrogen oxides and volatile organic compounds, sunlight and warm temperatures drive photochemical reactions in the lower atmosphere. In these conditions, NO2 can photolyze and liberate reactive oxygen atoms that quickly combine with O2 to form ozone (O3). This ozone is a secondary pollutant because it isn’t emitted directly; it’s created by these atmospheric reactions from the emitted precursors. Smog describes the broader polluted-air phenomenon that includes ozone plus other components, but the specific pollutant formed by this sunlight-driven chemistry is ozone. Acid deposition and particulate matter involve different processes, so they aren’t the primary product of this photochemical sequence.