Photochemical smog is a resultant of the reaction among

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UPSC Prelims is a competitive examination conducted in India. This question is from the 2013 UPSC Prelims Environment & Ecology paper.

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This UPSC Prelims 2013 Environment & Ecology question has a medium difficulty level and is marked as an important question.

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Question

Photochemical smog is a resultant of the reaction among

Accepted Answer

Correct Answer: A — NO₂, O₃ and peroxyacetyl nitrate in the presence of sunlightStep-by-step Solution:Photochemical smog (also called Los Angeles-type smog or summer smog) is a type of air pollution formed through a series of photochemical reactions involving primary pollutants (mainly emitted from vehicle exhausts and industrial emissions) in the presence of sunlight.Formation mechanism: Primary pollutants (NOₓ — nitric oxide NO and nitrogen dioxide NO₂, and Volatile Organic Compounds/VOCs from unburnt hydrocarbons) are emitted mainly by vehicle exhausts. In sunlight: Sunlight causes photolysis of NO₂: NO₂ + UV light → NO + O (atomic oxygen); Atomic oxygen reacts with O₂: O + O₂ → O₃ (ozone — highly reactive); Ozone reacts with VOCs (unburnt hydrocarbons from vehicle exhaust): O₃ + VOCs → Aldehydes + PAN (Peroxyacetyl Nitrate) + other oxidants; The secondary pollutants formed — O₃, PAN, aldehydes — are the components of photochemical smog.The three key chemical species in photochemical smog are therefore: NO₂ (primary precursor/reactant), O₃ (ozone — a major product and component), and PAN (Peroxyacetyl Nitrate) — a secondary pollutant formed in the photochemical chain. Sunlight is the essential catalyst for these reactions.Why other options are incorrect:Option B (CO, O₂, PAN + sunlight): CO is not a primary precursor of photochemical smog in the same way NOₓ is. CO does not directly participate in the photochemical smog-forming reactions as a precursor. ✗Option C (CO, CO₂, NO₂ at low temperature): Photochemical smog requires sunlight (hence 'photochemical') and forms in warm conditions, not low temperatures. London smog (classical/reducing smog) forms in cold, foggy conditions. Also, CO and CO₂ are not the primary components of photochemical smog. ✗Option D (NO₂, O₃, CO in the evening): Photochemical smog forms during DAYTIME when sunlight is intense — not in the evening. Also, CO is not a primary component of photochemical smog. ✗Key Concept: Photochemical smog (summer smog, LA smog): requires sunlight; forms on hot, sunny, still days; pollutants: O₃, PAN, aldehydes, peroxides; brown/yellowish haze; major health effects: eye irritation (from aldehydes, PAN), respiratory damage (from ozone), reduced visibility; damages plants (ozone and PAN cause leaf damage); major cities affected: Los Angeles, Mexico City, Delhi, Beijing, Mumbai. Vs Classical/London smog (winter smog, reducing smog): formed on cold, foggy, still days; involves SO₂ + particulate matter + fog → sulphuric acid particles; black/grey; primarily from coal burning; London 1952 Great Smog killed ~4,000 people. Both types affect Indian cities — Delhi specifically faces photochemical smog in summer (vehicles + sunlight) and also severe PM2.5 pollution in winter (crop stubble burning + meteorological inversion layer trapping pollutants).

Photochemical smog is a resultant of the reaction among

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Photochemical smog is a resultant of the reaction among

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