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Difficulty: MediumAir Pollutants, Photochemical Smog, and Acid Rain

During peak daylight hours in heavily industrialized urban areas, unburnt hydrocarbons and nitrogen oxides undergo complex atmospheric reactions catalyzed by ultraviolet light. Which of the following substances is produced as a toxic secondary pollutant in this process and serves as a major component of photochemical smog?

  1. Peroxyacetyl nitrate (PAN)Answer
  2. B
    Carbon(IV) oxide (CO2CO_2)
  3. C
    Sulfur(IV) oxide (SO2SO_2)
  4. D
    Chlorofluorocarbon (CFC-11)

Answer

Peroxyacetyl nitrate (PAN) is formed as a secondary pollutant when hydrocarbons and nitrogen oxides react in the presence of sunlight during photochemical smog formation.
Peroxyacetyl nitrate (PAN) is a classic secondary pollutant produced when volatile hydrocarbons and nitrogen dioxide react in the lower atmosphere under the influence of ultraviolet sunlight, forming a major eye-irritating constituent of photochemical smog.

Step-by-Step Solution

1
Analyze the conditions given in the prompt.
The reaction takes place in sunlight (ultraviolet radiation) involving primary emissions of nitrogen oxides (NOxNO_x) and hydrocarbons.
Photochemical smog is driven by solar radiation acting on primary motor vehicle exhaust gases.
2
Distinguish between primary and secondary pollutants.
Primary pollutants (CO2CO_2, SO2SO_2, NONO) are emitted directly from sources, whereas secondary pollutants (such as ozone O3O_3 and peroxyacetyl nitrate) form via chemical interactions in the atmosphere.
The question requires identifying a secondary pollutant generated during light-driven reactions.
3
Identify the specific secondary product.
Peroxyacetyl nitrate (PAN) is a powerful lachrymator and phytotoxic secondary pollutant characteristic of photochemical smog.
PAN is produced when peroxyacyl radicals combine with nitrogen dioxide (NO2NO_2) under UV radiation.

Key Concept

Secondary Pollutants and Photochemical Smog
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Topics

Air Pollutants, Photochemical Smog, and Acid Rain

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