Photochemical smog components — source of PAN Peroxyacyl nitrates (PAN) are secondary pollutants typically found where?

Introduction / Context:
Peroxyacyl nitrates (PAN) are eye-irritating components of photochemical smog formed by reactions of nitrogen oxides (NOx) with volatile organic compounds (VOCs) under sunlight. Urban traffic emissions provide the precursor mix that leads to PAN formation.

Given Data / Assumptions:

• Automobiles emit NOx and hydrocarbons (VOCs).
• PAN is not a primary pollutant; it forms in the atmosphere from precursors.
• Coal power and acid plants have different dominant pollutants (SOx, NOx) but not the VOC mix and conditions that favor PAN at the source.

Concept / Approach:
Photochemical smog requires sunlight, NOx, and reactive hydrocarbons. Urban road traffic supplies both NOx and VOCs; in sunlight, radical chemistry generates PAN and ozone. Industrial stacks may emit NOx or SOx but generally lack high VOC co-emissions and immediate photochemical conditions at the stack exit to create PAN right there.

Step-by-Step Solution:

Verification / Alternative check:
Air-quality monitoring in cities shows PAN peaks during sunny traffic periods, co-varying with ozone and NOx–VOC photochemistry.

Why Other Options Are Wrong:

Common Pitfalls:
Treating PAN as a primary pollutant; it is a secondary pollutant formed in the urban atmosphere.

Final Answer:
automobile exhaust.