Photochemical smog is most likely to reach its peak intensity during
- AEarly morning before sunrise
- BMid-afternoon on a hot, sunny dayCorrect
- CLate evening after sunset
- DCold, foggy winter mornings
Solution
Concept
Diurnal peak of photochemical smog
Formula
NONE
Step-by-step solution
1. Morning vehicular traffic releases substantial quantities of primary emissions (NO and hydrocarbons). 2. Photochemical reactions require intense solar radiation to drive the conversion into secondary oxidants like Ozone and PAN. 3. Sunlight and atmospheric temperatures peak during the afternoon hours. 4. Hence, photochemical smog reaches its peak intensity during mid-afternoon on a hot, sunny day. Answer: B
Explanation
Photochemical smog formation relies on intense solar ultraviolet radiation to drive the dissociation of nitrogen dioxide into nitric oxide and reactive oxygen atoms. Consequently, smog intensity builds throughout the morning as precursor concentrations rise and peaks during the mid-afternoon when ambient temperature and solar insolation reach their daily maximums.
Why the other options are wrong
A) Early morning has peak primary precursor emissions but insufficient solar UV flux to complete photochemical reactions. C) Smog diminishes after sunset due to lack of activating solar radiation and chemical scavenging. D) Cold, foggy winter mornings are characteristic of reducing London-type sulphurous smog, not photochemical smog.
Shortcut
Photochemical = sunlight needed -> peak intensity at peak sun (mid-afternoon).
Common mistake
Confusing photochemical (Los Angeles) smog with reducing (London) winter morning smog.
Correct answer: B) Mid-afternoon on a hot, sunny day