Which of the following is a secondary pollutant that is essential for the formation of photochemical smog?
- ANitric Oxide (NO)
- BVolatile Organic Compounds (VOCs)
- CPeroxyacetyl Nitrate (PAN)Correct
- DCarbon Monoxide (CO)
Solution
Concept
Secondary pollutant formation in photochemical smog
Formula
Hydrocarbons + NOx + Sunlight -> PAN + O3
Step-by-step solution
1. Identify primary pollutants: NO, VOCs, and CO are emitted directly from combustion and industrial exhausts. 2. In the presence of sunlight (UV radiation), primary pollutants react to generate secondary oxidants like Ozone and PAN (Peroxyacetyl Nitrate). 3. Therefore, PAN is the essential secondary pollutant characteristic of photochemical smog. Answer: C
Explanation
Peroxyacetyl Nitrate (PAN) is a powerful secondary air pollutant formed through photochemical reactions between nitrogen oxides and volatile organic compounds driven by sunlight. Unlike primary pollutants that are emitted directly from sources, PAN is generated within the atmosphere and is a defining hallmark of photochemical smog.
Why the other options are wrong
A) Nitric Oxide is a primary pollutant emitted directly from vehicles. B) VOCs are primary precursor emissions from unburnt hydrocarbons and solvents. D) Carbon Monoxide is a primary pollutant resulting from incomplete combustion.
Shortcut
Primary pollutants = emitted directly (NO, CO, VOC); Secondary = formed in air (PAN, O3, Acid Rain).
Common mistake
Classifying VOCs or NO as secondary pollutants rather than primary precursors.
Correct answer: C) Peroxyacetyl Nitrate (PAN)