Difficulty: Easy
Correct Answer: All of the above
Explanation:
Introduction / Context:Photochemical smog is a secondary air pollution phenomenon found in urban areas with heavy traffic and strong solar radiation. It involves sunlight-driven reactions of precursors from vehicle exhaust.
Given Data / Assumptions:
Concept / Approach:Sunlight catalyzes reactions of NOx and VOCs to form oxidants; these oxidants cause health and vegetation damage.
Step-by-Step Solution:
1) Vehicle exhaust emits NO, NO2, and hydrocarbons.2) Under sunlight, NO2 photolyzes, generating O atoms.3) O combines with O2 to form O3; VOC radicals sustain NO2 regeneration and ozone accumulation.4) Resulting oxidants cause eye irritation, asthma exacerbation, and plant damage.Verification / Alternative check:Observationally strongest on clear, sunny, stagnant days; declines with reduced NOx/VOC emissions and after sunset.
Why Other Options Are Wrong:
Formed by HC and NO in sunlight: correct.Appears only on sunny days: correct in practical terms—sunlight is required.Harmful to vegetation and causes irritation: correct.Mainly a winter fog phenomenon: incorrect—classical photochemical smog is summer/sunny-day related.Common Pitfalls:Confusing photochemical smog with reducing (London) smog; assuming fog is necessary; overlooking VOC controls.
Final Answer:All of the above
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