Photochemical formation of eye-irritating pollutants Direct reactions of unsaturated hydrocarbons with nitrogen oxides (NO or NO2) in sunlight commonly form which eye-irritant compound in photochemical smog?

Introduction / Context:
Urban air pollution episodes under sunny, stagnant conditions often involve reactions between volatile organic compounds (VOCs) and nitrogen oxides. The resulting oxidants and secondary pollutants cause eye irritation and respiratory stress. Environmental engineers must recognise key products to design control strategies targeting VOC and NOx precursors.

Given Data / Assumptions:

• Unsaturated hydrocarbons (alkenes, aromatics) are present from traffic and solvents.
• NO and NO2 are available from combustion sources.
• Sunlight drives radical chemistry forming secondary oxidants.

Concept / Approach:
Peroxyacetyl nitrate (PAN) forms via photochemical oxidation of hydrocarbons producing peroxyacetyl radicals, which then react with NO2. PAN is a potent eye irritant and a plant toxin, acting as a marker of photochemical smog severity. While “photochemical smog” is the name of the overall phenomenon, the specific eye-irritant compound commonly identified is PAN. Compounds like benzopyrene are carcinogenic polycyclic aromatics from incomplete combustion, not primary photochemical products of VOC + NOx reactions. Polymers like polyacrylonitrile are unrelated to atmospheric photochemistry.

Step-by-Step Solution:

Verification / Alternative check:
Field measurements in photochemical smog events consistently show PAN alongside ozone and aldehydes; PAN tracks with oxidant levels and eye irritation complaints.

Why Other Options Are Wrong:

• Photochemical smog: Describes the overall mixture, not a specific compound.
• Benzopyrene: Soot-phase carcinogen, not the main photochemical eye irritant.
• Polyacrylonitrile: A synthetic polymer; irrelevant to atmospheric formation.
• Formaldehyde only: An irritant and intermediate, but PAN is the classic eye-stinging oxidant cited in smog chemistry.

Common Pitfalls:
Equating general smog with a single compound; multiple species (ozone, PAN, aldehydes) co-occur, but PAN is especially noted for ocular irritation.

Final Answer:
Peroxyacetyl nitrate (PAN)